[9fans] PDP11 (Was: Re: what heavy negativity!)

[9fans] PDP11 (Was: Re: what heavy negativity!)

[Lucio De Re]

On 10/8/18, Digby R.S. Tarvin <digbyt42@gmail.com> wrote:
>
> So the question is... is plan9 still lean and mean enough to fit onto a
> machine with a 64K address space? Doing a port would certainly provide
> plenty of opportunity to tinker with the lights and switches on front
> panel, and if it the port was initially limited to being a CPU server,
> there would be no need to worry about displays and mass storage.... just
> the compiler back end and low level kernel support.
>
You really must be thinking of Inferno, native, running in a host with
1MiB of memory. 64KiB isn't enough for anything other than maybe CPM.
Even MPM won't cut it, I don't think.

Lucio.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Digby R.S. Tarvin]

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A native Inferno port would certainly be a lot easier, but I think you
might be a bit pessimistic about would can fit into a 64K address space
machine. The 11/70 certainly managed to run a very respectable V7 Unix
supporting 20-30 simultaneous active users in its day, and I wouldn't have
thought plan 9  arriving about a decade later, would have been hugely
bigger than V7 Unix.
I recall a demo of Plan9 (I think it also included the source) being given
by Rob Pike at UNSW which he carried on a 1.44Mb floppy disc. By its open
source release in 2002 the distribution was 65MB

The smallest Linux system I have used recently had 256K RAM and 512K flash.
A rather stripped down busybox based system, but it did include a full
TCP/IP stack and a web server. Thats comparable to a PDP11 except for the
limitation on the largest individual process.

Bear in mind that 16 bit executables are smaller, and whilst the 11/70 had
a 64Kb address space, physical memory could be somewhat larger, and an
individual process could have 128K of memory is using separate instruction
and data space.

I am used to thinking of Plan9 as very compact, but I havn't really looked
to see if it has grown much since the 80s, and perhaps it is only next to
the astronomical expansion of other systems that it still looks small. It
would be an interesting exercise to find out.

It would be an interesting thing to try, if only to get a better feel for
how compact Plan9 actually is ...

DigbyT

On Mon, 8 Oct 2018 at 14:38, Lucio De Re <lucio.dere@gmail.com> wrote:

> On 10/8/18, Digby R.S. Tarvin <digbyt42@gmail.com> wrote:
> >
> > So the question is... is plan9 still lean and mean enough to fit onto a
> > machine with a 64K address space? Doing a port would certainly provide
> > plenty of opportunity to tinker with the lights and switches on front
> > panel, and if it the port was initially limited to being a CPU server,
> > there would be no need to worry about displays and mass storage.... just
> > the compiler back end and low level kernel support.
> >
> You really must be thinking of Inferno, native, running in a host with
> 1MiB of memory. 64KiB isn't enough for anything other than maybe CPM.
> Even MPM won't cut it, I don't think.
>
> Lucio.
>

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[hiro]

saving every bit of memory has costs in coding, the pressure wasn't as
strong any more.
the earned flexibility can be used for more elegant design.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Charles Forsyth]

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Ideally, anyway.

On Mon, 8 Oct 2018 at 11:20, hiro <23hiro@gmail.com> wrote:

> saving every bit of memory has costs in coding, the pressure wasn't as
> strong any more.
> the earned flexibility can be used for more elegant design.
>
>

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[hiro]

i should have said could, not can :)

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Digby R.S. Tarvin]

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Does anyone know what platform Plan9 was initially implemented on? My guess
is that there is no reason in principle that it could not fit comfortably
into the constraints of a PDP11/70, but if the initial implementation was
done targeting a machine with significantly more resources, it would be
easy to make design decisions that would be entirely incompatible.
Certainly Richard Millar's comment suggests that might be the case. If it
is heavily dependent on VM, then the necessary rewrite is likely to be
substantial.

I'm not sure how the kernel design has changed since the first release. The
earliest version I have is the release I bought through Harcourt Brace back
in 1995. But I won't be home till December so it will be a while before I
can look at it, and probably won't have time to experiment before then in
any case.

For what it is worth, I don't think the embarrassment of riches presented
to programmers by current hardware has tended to produce more elegant
designs. If more resources resulted in elegance, Windows would be a thing
of beauty.  Perhaps Plan9 is an exception. It certainly excels in elegance
and design simplicity, even if it does turn out to be more resource hungry
than I imagined. I will admit that the evils of excessively constrained
environments are generally worse in terms of coding elegance - especially
when it leads to overlays and self modifying code.

PDP11's don't support virtual memory, so there doesn't seem any elegant way
to overcome that fundamental limitation on size of a singe executable.  So
I don't think it i would be worth a substantial rewrite to get it going. It
is a shame that there don't seem to have been any more powerful machines
with a comparably elegant architecture and attractive front panel :)

It is sounding like Inferno is going to be the more practical option. I
believe gcc can still generate PDP-11 code, so it shouldn't be too hard to
try.

DigbyT

On Tue, 9 Oct 2018 at 04:53, hiro <23hiro@gmail.com> wrote:

> i should have said could, not can :)
>
>

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Dan Cross]

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On Mon, Oct 8, 2018 at 6:25 PM Digby R.S. Tarvin <digbyt42@gmail.com> wrote:

> Does anyone know what platform Plan9 was initially implemented on?
>

My understanding is that the earliest experiments involved a VAX, but
development quickly shifted to MIPS and 68020-based machines (the "gnot"
was, IIRC, a 68020-based computer).

My guess is that there is no reason in principle that it could not fit
> comfortably into the constraints of a PDP11/70, but if the initial
> implementation was done targeting a machine with significantly more
> resources, it would be easy to make design decisions that would be entirely
> incompatible.
>

I find this unlikely.

The PDP-11, while a respectable machine for its day, required too many
tradeoffs to make it attractive as a development platform for a
next-generation research operating system in the late 1980s: be it
electrical power consumption vs computational oomph or dollar cost vs
available memory, the -11 had fallen from the attractive position it held a
decade prior. Perhaps slimming a plan9 kernel down sufficiently so that it
COULD run on a PDP-11 was possible in the early days, but I can't see any
reason one would have WANTED to do so: particularly as part of the impetus
behind plan9 was to exploit advances in contemporary hardware: lower-cost,
higher-performance, RISC-based multiprocessors; ubiquitous networking;
common high-resolution bitmapped graphical displays; even magneto-optical
storage (one bet that didn't pan out); etc.

Certainly Richard Millar's comment suggests that might be the case. If it
> is heavily dependent on VM, then the necessary rewrite is likely to be
> substantial.
>

As a demonstration project, getting a slimmed-down plan9 kernel to boot on
a PDP-11/70-class machine would be a nifty hack, but it would be quite a
tour de force and most likely the result would not be generally useful. I
think that, as has been suggested, the conceptual simplicity of plan9
paradoxically means that resource utilization is higher than it might
otherwise be on either a more elaborate OR more constrained system (such as
one targeting e.g. the PDP-11). When you can afford not to care about a few
bytes here or a couple of cycles there and you're not obsessed with
scraping out the very last drop of performance, you can employ a simpler
(some might say 'naive') algorithm or data structure.

I'm not sure how the kernel design has changed since the first release. The
> earliest version I have is the release I bought through Harcourt Brace back
> in 1995. But I won't be home till December so it will be a while before I
> can look at it, and probably won't have time to experiment before then in
> any case.
>

The kernel evolved substantially over its life; something like doubling in
size. I remember vaguely having a discussion with Sape where he said he
felt it had grown bloated. That was probably close to 20 years ago now.

For what it is worth, I don't think the embarrassment of riches presented
> to programmers by current hardware has tended to produce more elegant
> designs. If more resources resulted in elegance, Windows would be a thing
> of beauty.  Perhaps Plan9 is an exception. It certainly excels in elegance
> and design simplicity, even if it does turn out to be more resource hungry
> than I imagined. I will admit that the evils of excessively constrained
> environments are generally worse in terms of coding elegance - especially
> when it leads to overlays and self modifying code.
>

plan9 is breathtakingly elegant, but this is in no small part because as a
research system it had the luxury of simply ignoring many thorny problems
that would have marred that beauty but that the developers chose not to
tackle. Some of these problems have non-trivial domain complexity and,
while "modern" systems are far too complex by far, that doesn't mean that
all solutions can be recast as elegantly simple pearls in the plan9 style.
Whether we like those problems or not, they exist and real-world solutions
have to at least attempt to deal with them (I'm looking at you, web x.0 for
x >= 2...but curse you you aren't alone).

PDP11's don't support virtual memory, so there doesn't seem any elegant way
> to overcome that fundamental limitation on size of a singe executable.
>

No, they do: there is paging hardware on the PDP-11 that's used for address
translation and memory protection (recall that PDP-11 kept the kernel at
the top of the address space, the per-process "user" structure is at a
fixed virtual address, and the system could trap a bus error and kill a
misbehaving user-space process). What they may not support is the sort of
trap handling that would let them recover from a page fault (though I
haven't looked) and in any case, the address space is too small to make
demand-paging with reclamation cost-effective.


> So I don't think it i would be worth a substantial rewrite to get it
> going. It is a shame that there don't seem to have been any more powerful
> machines with a comparably elegant architecture and attractive front panel
> :)
>

An attractive front panel for nearly any machine is just a soldering iron,
LEDs and some logic chips away. As far as elegant architectures, some are
very nice: MIPS is kind of retro but elegant, RISC-V is nice, 680x0
machines can be had a reasonable prices, and POWER is kind of cool. I know
I shouldn't, but I have a soft spot for ARM.

It is sounding like Inferno is going to be the more practical option. I
> believe gcc can still generate PDP-11 code, so it shouldn't be too hard to
> try.
>

Sounds like a nifty hack. Fitting Dis into a 64k/64k split I/D space is the
challenge.

        - Dan C.

On Tue, 9 Oct 2018 at 04:53, hiro <23hiro@gmail.com> wrote:
>
>> i should have said could, not can :)
>>
>>

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Bakul Shah]

On Mon, 08 Oct 2018 19:03:49 -0400 Dan Cross <crossd@gmail.com> wrote:
>
> plan9 is breathtakingly elegant, but this is in no small part because as a
> research system it had the luxury of simply ignoring many thorny problems
> that would have marred that beauty but that the developers chose not to
> tackle. Some of these problems have non-trivial domain complexity and,
> while "modern" systems are far too complex by far, that doesn't mean that
> all solutions can be recast as elegantly simple pearls in the plan9 style.

One thing I have mused about is recasting plan9 as a
microkernel and pushing out a lot of its kernel code into user
mode code.  It is already half way there -- it is basically a
mux for 9p calls, low level device drivers, VM support & some
process related code.  Such a redesign can be made more secure
and more resilient.  The kind of problems you mention are
easier to fix in user code. Different application domains may
have different needs which are better handled as optional user
mode components.

Said another way, keep the good parts of the plan9 design and
reachitect/reimplement the kernel + essential drivers/usermode
daemons.  This is unlikely to happen (without some serious
funding) but still fun to think about!  If done, this would be
a more radical departure than Oberon-7 compared to Oberon but
in the same spirit.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Christopher Nielsen]

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On Mon, Oct 8, 2018, 17:15 Bakul Shah <bakul@bitblocks.com> wrote:

> On Mon, 08 Oct 2018 19:03:49 -0400 Dan Cross <crossd@gmail.com> wrote:
> >
> > plan9 is breathtakingly elegant, but this is in no small part because as
> a
> > research system it had the luxury of simply ignoring many thorny problems
> > that would have marred that beauty but that the developers chose not to
> > tackle. Some of these problems have non-trivial domain complexity and,
> > while "modern" systems are far too complex by far, that doesn't mean that
> > all solutions can be recast as elegantly simple pearls in the plan9
> style.
>
> One thing I have mused about is recasting plan9 as a
> microkernel and pushing out a lot of its kernel code into user
> mode code.  It is already half way there -- it is basically a
> mux for 9p calls, low level device drivers, VM support & some
> process related code.  Such a redesign can be made more secure
> and more resilient.  The kind of problems you mention are
> easier to fix in user code. Different application domains may
> have different needs which are better handled as optional user
> mode components.
>
> Said another way, keep the good parts of the plan9 design and
> reachitect/reimplement the kernel + essential drivers/usermode
> daemons.  This is unlikely to happen (without some serious
> funding) but still fun to think about!  If done, this would be
> a more radical departure than Oberon-7 compared to Oberon but
> in the same spirit.
>

I've mused about that also. My problem has been finding the time. I think
it would be a worthwhile project.

Not entirely unrelated, I've been tinkering with seL4.

>

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Lucio De Re]

On 10/9/18, Bakul Shah <bakul@bitblocks.com> wrote:
>
> One thing I have mused about is recasting plan9 as a
> microkernel and pushing out a lot of its kernel code into user
> mode code.  It is already half way there -- it is basically a
> mux for 9p calls, low level device drivers, VM support & some
> process related code.  Such a redesign can be made more secure
> and more resilient.  The kind of problems you mention are
> easier to fix in user code. Different application domains may
> have different needs which are better handled as optional user
> mode components.
>
There are religious reasons not to go there and, perhaps not very
widely advertised, Minix-3 already does that, although I confess that
all my best efforts have not yet created the space for my own
experimentation with it.

You won't believe what kind of madnesses I need to deal with to
consume my few and short remaining years - I'm with Dan in cursing the
modern technological trends, but one of these days I'm going to lock
myself in someone's attic or basement (or a prison cell, if that's
what it takes, a monastery, whatever...) with my Galaxy S4 and a dated
Riff-box - is that really what this black object is called? - and
build an OS from the accumulated wisdom of the last forty years. It
will probably look more like MS-DOS, though! :-(

> Said another way, keep the good parts of the plan9 design and
> reachitect/reimplement the kernel + essential drivers/usermode
> daemons.  This is unlikely to happen (without some serious
> funding) but still fun to think about!  If done, this would be
> a more radical departure than Oberon-7 compared to Oberon but
> in the same spirit.
>
Surely, the targets for experimentation should be the ubiquitous
smart-mobile and the insane arithmetic power of GPUs? All neatly
networked over SDLC (or HDLC: AoH, anyone, for persistent storage?).

Lucio.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[hiro]

we already have a lot of user filesystems. feel free to add other useful ones.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Ethan Gardener]

On Tue, Oct 9, 2018, at 4:28 AM, Lucio De Re wrote:
> On 10/9/18, Bakul Shah <bakul@bitblocks.com> wrote:
> > One thing I have mused about is recasting plan9 as a
> > microkernel and pushing out a lot of its kernel code into user
> > mode code.
> >
> There are religious reasons not to go there

I'm trying to forget all the religious beliefs I once held with regard to computers, but I've had these lines in my head for a long time, and probably won't get a better opportunity to post them:

One day, Uriel met a man who explained very
convincingly that the Plan 9 kernel is a microkernel.
On another day, Uriel met a man who explained very
convincingly that the Plan 9 kernel is a macrokernel.
Uriel was enlightened.

Based on a true story. ;)


> You won't believe what kind of madnesses I need to deal with to
> consume my few and short remaining years - I'm with Dan in cursing the
> modern technological trends, but one of these days I'm going to lock
> myself in someone's attic or basement (or a prison cell, if that's
> what it takes, a monastery, whatever...) with my Galaxy S4 and a dated
> Riff-box - is that really what this black object is called? - and
> build an OS from the accumulated wisdom of the last forty years. It
> will probably look more like MS-DOS, though! :-(

I've started already, but I keep getting sidetracked by my need for entertainment, which often comes down to spending my energies on things which don't require such deep design work.  I'm hoping it'll get easier as my health improves; I'm still too stressed too often.  The trouble with this stress is I forget my goals, which are things I've learned from Plan 9 and other conclusions I've come to.

--
Progress might have been all right once, but it has gone on too long -- Ogden Nash

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Bakul Shah]

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> On Oct 9, 2018, at 2:45 AM, Ethan Gardener <eekee57@fastmail.fm> wrote:
> 
> One day, Uriel met a man who explained very 
> convincingly that the Plan 9 kernel is a microkernel.
> On another day, Uriel met a man who explained very 
> convincingly that the Plan 9 kernel is a macrokernel.
> Uriel was enlightened.

Exactly! No point in being scared by labels! I am really
only talking about distilling plan9 further. At least as a
thought experiment.

Isn’t it more fun to discuss this than all the “heavy
negativity”? :-)

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Ori Bernstein]

On Tue, 9 Oct 2018 10:50:08 -0700
Bakul Shah <bakul@bitblocks.com> wrote:

> Exactly! No point in being scared by labels! I am really
> only talking about distilling plan9 further. At least as a
> thought experiment.
> 
> Isn’t it more fun to discuss this than all the “heavy
> negativity”? :-)

It's much better with patches.

-- 
Ori Bernstein <ori@eigenstate.org>

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Giacomo Tesio]

Il giorno mar 9 ott 2018 alle ore 05:33 Lucio De Re
<lucio.dere@gmail.com> ha scritto:
>
> On 10/9/18, Bakul Shah <bakul@bitblocks.com> wrote:
> >
> > One thing I have mused about is recasting plan9 as a
> > microkernel and pushing out a lot of its kernel code into user
> > mode code.  It is already half way there -- it is basically a
> > mux for 9p calls, low level device drivers,
> >
> There are religious reasons not to go there

Indeed, as an heretic, one of the first things I did with Jehanne was
to move the console filesystem out of kernel.
Then I moved several syscalls into userspace. Or turned them to files
or to operation on existing files.
More syscall/kernel services will move to user space as I'll have time
to hack it again.

You know... heretics ruin everything!

I'm not going to turn Jehanne to a microkernel, but I'm looking for
the simplest possible set of kernel abstractions that can support a
distributed operating system able to replace the mainstream Web+OS
mess.
You know... heretics are crazy, too!


Giacomo

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Lyndon Nerenberg]

Bakul Shah writes:

> One thing I have mused about is recasting plan9 as a
> microkernel and pushing out a lot of its kernel code into user
> mode code.  It is already half way there -- it is basically a
> mux for 9p calls, low level device drivers, VM support & some
> process related code.

Somewhat related to this ... after reading some papers on
TCP-in-user-space implementations, I've been thinking about how an
interface that supported fast/secure page flipping between the
kernel and process address space would change how we do things.

E.g. right now Plan 9 suffers from a *lot* of data copying between
the kernel and processes, and between processes themselves.  If we
could eliminate most of that copying, things would get a lot faster.
Dealing with the security issues isn't trivial, but the programmer
time going into eeking out the last bit of I/O throughput of the
current scheme could be redirected.

If it works, this would reduce the kernel back to handling
process/memory management, and talking to the hardware.  Not a
micro-kernel, but just as good from a practical standpoint.

And no, this wouldn't get us to running on the 11/70.  But by taking
advantage of modern large virtual memory spaces by using page
flipping, we could cut down on physical memory usage in the kernel.


--lyndon

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[hiro]

> E.g. right now Plan 9 suffers from a *lot* of data copying between
> the kernel and processes, and between processes themselves.

Huh? What exactly do you mean? Can you describe the scenario and the
measurements you made?

> If we could eliminate most of that copying, things would get a lot faster.

Which things would get faster?

> Dealing with the security issues isn't trivial

what security issues?

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Lyndon Nerenberg]

hiro writes:

> Huh? What exactly do you mean? Can you describe the scenario and the
> measurements you made?

The big one is USB.  disk/radio->kernel->user-space-usbd->kernel->application.
Four copies.

I would like to start playing with software defined radio on Plan
9, but that amount of data copying is going to put a lot of pressure
on the kernel to keep up.  UNIX/Linux suffers the same copy bloat,
and it's having trouble keeping up, too.

--lyndon

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[erik quanstrom]

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Lyndon Nerenberg]

> I have been able to copy 1 GiB/s to userspace from an nvme device. I should
> think a radio should be no problem.

The problem is when you have multiple decoder blocks implemented
as individual processes (i.e. the GNU radio model).  Once you have
everything debugged, you can put it into a single threaded process
and eliminate the copy overhead.  But it's completely impractical
to prototype or debug real applications this way.  And it's the
prototyping case I'm interested in here.

So I'm *curious* to know if page flipping a 'protocol buffer' like
object between processes provides an optimization over copying
through the kernel.  Not so much for the speed aspect, but to free
up CPU cycles that can be devoted to actual SDR work.

Since when did curiosity become a capital crime?   Oh, wait, that
was January 20, 2017.  My bad.

--lyndon

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Kurt H Maier]

On Thu, Oct 11, 2018 at 10:54:22AM -0700, Lyndon Nerenberg wrote:
>
> Since when did curiosity become a capital crime?   Oh, wait, that
> was January 20, 2017.  My bad.

Turns out it's not, so you can climb down off your cross.  It's just
that it helps to be a little clearer about your meaning, that's all.
Otherwise you might do something embarassing, like posting SAS
controller code into an NVMe discussion.

khm

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[hiro]

> through the kernel.  Not so much for the speed aspect, but to free
> up CPU cycles that can be devoted to actual SDR work.

those 2x25kHz channels would hardly need many cycles. rather it's just
a matter of selecting the right CPU that can actually do the FFT with
some software floating point implementation :)

i don't see memory bandwidth or even random memory access latency
affecting this scenario in the slightest.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[hiro]

i meant without having to resort to some soft fp.

On 10/11/18, hiro <23hiro@gmail.com> wrote:
>> through the kernel.  Not so much for the speed aspect, but to free
>> up CPU cycles that can be devoted to actual SDR work.
>
> those 2x25kHz channels would hardly need many cycles. rather it's just
> a matter of selecting the right CPU that can actually do the FFT with
> some software floating point implementation :)
>
> i don't see memory bandwidth or even random memory access latency
> affecting this scenario in the slightest.
>

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[hiro]

We also have CPU extensions that can help make fast FFT, because it's
such a generic problem, and in the worst case you can use fpgas,
asics, in any case dedicated hardware.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Skip Tavakkolian]

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I was able to use dump1090 (same author as redis) to get ADSB data reliably
on RPi/Linux a while back.

On Thu, Oct 11, 2018, 10:54 AM Lyndon Nerenberg <lyndon@orthanc.ca> wrote:

> > I have been able to copy 1 GiB/s to userspace from an nvme device. I
> should
> > think a radio should be no problem.
>
> The problem is when you have multiple decoder blocks implemented
> as individual processes (i.e. the GNU radio model).  Once you have
> everything debugged, you can put it into a single threaded process
> and eliminate the copy overhead.  But it's completely impractical
> to prototype or debug real applications this way.  And it's the
> prototyping case I'm interested in here.
>
> So I'm *curious* to know if page flipping a 'protocol buffer' like
> object between processes provides an optimization over copying
> through the kernel.  Not so much for the speed aspect, but to free
> up CPU cycles that can be devoted to actual SDR work.
>
> Since when did curiosity become a capital crime?   Oh, wait, that
> was January 20, 2017.  My bad.
>
> --lyndon
>
>

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Lyndon Nerenberg]

> I was able to use dump1090 (same author as redis) to get ADSB data reliably
> on RPi/Linux a while back.

I have a pair of Flightbox ADS-B receivers I am using as references.
While mostly reliable, they can and do stutter along with the rest
of the alternatives on occasion.

--lyndon

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Skip Tavakkolian]

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I assumed you were using an RTL2832U (rtlsdr library).

On Thu, Oct 11, 2018, 12:40 PM Lyndon Nerenberg <lyndon@orthanc.ca> wrote:

> > I was able to use dump1090 (same author as redis) to get ADSB data
> reliably
> > on RPi/Linux a while back.
>
> I have a pair of Flightbox ADS-B receivers I am using as references.
> While mostly reliable, they can and do stutter along with the rest
> of the alternatives on occasion.
>
> --lyndon
>
>

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Lyndon Nerenberg]

Skip Tavakkolian writes:

> I assumed you were using an RTL2832U (rtlsdr library).

I'm pretty sure they all do, under the hood.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[hiro]

>> I assumed you were using an RTL2832U (rtlsdr library).
>
> I'm pretty sure they all do, under the hood.
>
>

don't you need sending ability, too for AIS?

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Lyndon Nerenberg]

hiro writes:

> don't you need sending ability, too for AIS?

No, a receive-only setup is very useful on a small boat.  Where I
would like to go with this is to take the decoded AIS data as input
for "ARPA" style collision plots.  I'm interested in the big boats
sailing through the straight.  They can't turn fast, and rarely
change course.  If I can derive their intentions, I can plot a path
between them that requires the least amount of tacking.

The big boats, in turn, have no interest in us little critters.
They actively filter out the "class B" (I think that's the term)
noise that are AIS transmissions from the small craft.  Even if we
hit them, we can't sink them, so they don't care about us.  Therefore
there is no incentive for small boats to transmit AIS.  Unless you're
trying to locate your buddies for a tie-up somewhere.  (That can
be a very valid reason for transmitting!)

--lyndon

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Ethan Gardener]

On Tue, Oct 9, 2018, at 8:14 PM, Lyndon Nerenberg wrote:
> hiro writes:
>
> > Huh? What exactly do you mean? Can you describe the scenario and the
> > measurements you made?
>
> The big one is USB.  disk/radio->kernel->user-space-usbd->kernel->application.
> Four copies.
>
> I would like to start playing with software defined radio on Plan
> 9, but that amount of data copying is going to put a lot of pressure
> on the kernel to keep up.  UNIX/Linux suffers the same copy bloat,
> and it's having trouble keeping up, too.

References, please.  Programmers are notoriously bad at determining the cause of performance problems.  Examining the source will help to see if "copy bloat" is the actual problem.

>
> --lyndon
>


--
Progress might have been all right once, but it has gone on too long -- Ogden Nash

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Lyndon Nerenberg]

hiro writes:

> > Dealing with the security issues isn't trivial

> what security issues?

Passing protocol buffer like objects around user space, that might
affect how the kernel talks to hardware.  E.g. IPsec offload into
hardware.  You don't want user-space messing with that sort of
context, but you want to tag it with the data buffer as it gets
passed up and down through the user/kernel gate.  Practical page
flipping needs a kernel-read-only context attached to the non-kernel
user data part of the page.  A quick solution is to pair pages, one
half of which the kernel owns, the other being the data payload.  But
that't just a start.  And that's all I'm saying: this might be an
approach to a better/faster I/O paradigm, but it needs interested
people to explore it ...


--lyndon

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[hiro]

from what i see in linux people have been more than just exploring it,
they've gone absolutely nuts. it makes everything complex, not just
the fast path.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[hiro]

also, if all you care about is throughput, i don't see how those 4
copies you identified makes a difference. especially with something
slow like USB.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Lyndon Nerenberg]

hiro writes:
> from what i see in linux people have been more than just exploring it,
> they've gone absolutely nuts. it makes everything complex, not just
> the fast path.

And those are the Linux folks doing thier thing.  The reading I'm
doing right now is related to the pessimizations page flipping throws
at the CPU caches.  It looks scary ...


--lyndon

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Dan Cross]

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On Tue, Oct 9, 2018 at 7:24 PM hiro <23hiro@gmail.com> wrote:

> from what i see in linux people have been more than just exploring it,
> they've gone absolutely nuts. it makes everything complex, not just
> the fast path.
>

To whom are you responding? Your email is devoid of context, so it is not
clear.

However your statement appears to be based on an unstated assumption that
there is a plan9 school of thought, and a Linux school of thought, and no
other school of thought. If so, that is incorrect.

        - Dan C.

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[hiro]

I was responding to lyndon's comment on certain "experiments" that
should have to be done here, 2 messages up.
But what he described sounded exactly like the zero-copying stuff that
linux is trying to shove into everything.
I have not made any statement about non-linux systems, and I'm not
even saying these experiments couldn't be done on plan9, it's just
that the linux people are way busier going down that path.

On 10/10/18, Dan Cross <crossd@gmail.com> wrote:
> On Tue, Oct 9, 2018 at 7:24 PM hiro <23hiro@gmail.com> wrote:
>
>> from what i see in linux people have been more than just exploring it,
>> they've gone absolutely nuts. it makes everything complex, not just
>> the fast path.
>>
>
> To whom are you responding? Your email is devoid of context, so it is not
> clear.
>
> However your statement appears to be based on an unstated assumption that
> there is a plan9 school of thought, and a Linux school of thought, and no
> other school of thought. If so, that is incorrect.
>
>         - Dan C.
>

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[erik quanstrom]

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Bakul Shah]

On Oct 9, 2018, at 3:06 PM, erik quanstrom <quanstro@quanstro.net> wrote:
> 
> with meltdown/Spectre mitigations in place, I would like to see evidence that flip is faster than copy.

If your system is well balanced, you should be able to
stream data as fast as memory allows[1]. In such a system
copying things N times will reduce throughput by similar
factor. It may be that plan9 underperforms so much this
doesn't matter normally.

But the reason I want this is to reduce latency to the first
access, especially for very large files. With read() I have
to wait until the read completes. With mmap() processing can
start much earlier and can be interleaved with background
data fetch or prefetch. With read() a lot more resources
are tied down. If I need random access and don't need to
read all of the data, the application has to do pread(),
pwrite() a lot thus complicating it. With mmap() I can just
map in the whole file and excess reading (beyond what the
app needs) will not be a large fraction.

The default assumption here seems to be that doing this
will be very complicated and be as bad as on Linux. But
Linux is not a good model of what to do and examples of what
not to do are not useful guides in system design. There are
other OSes such as the old Apollo Aegis (AKA Apollo/Domain),
KeyKOS & seL4 that avoid copying[2].

Though none of this matters right now as we don't even have
a paper design so please put down your clubs and swords :-)

[1] See: https://code.kx.com/q/cloud/aws/benchmarking/
A single q process can ingest data at 1.9GB/s from a
single drive. 16 can achieve 2.7GB/s, with theoretical
max being 2.8GB/s.

[2] Liedke's original L4 evolved into a provably secure
seL4 and in the process it became very much like KeyKOS.
Capability systems do pass around pages as protected
objects and avoid copying. Sort of like how in a program
you'd pass a huge array by reference and not by value
to a function.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[erik quanstrom]

> > with meltdown/Spectre mitigations in place, I would like to see evidence that flip is faster than copy.
>
> If your system is well balanced, you should be able to
> stream data as fast as memory allows[1]. In such a system
> copying things N times will reduce throughput by similar
> factor. It may be that plan9 underperforms so much this
> doesn't matter normally.

sure.  but flipping page tables is also not free.  there is a huge cost in processor stalls, etc.
spectre and meltdown mitigations make this worse as each page flip has to be accompanied
by a complete pipeline flush or other costly mitigation.  (not that this was cheap to begin with)

it's also not an object to move data as fast as possible.  the object is to do work as fast as possible.

> [1] See: https://code.kx.com/q/cloud/aws/benchmarking/
> A single q process can ingest data at 1.9GB/s from a
> single drive. 16 can achieve 2.7GB/s, with theoretical
> max being 2.8GB/s.

with my same crappy un-optimized nvme driver, i was able to hit 2.5-2.6 GiB/s
with two very crappy nvme drives.  (are you're numbers really GB rather than GiB?)
i am sure i could scale that lineraly.  there's plenty of memory bandwidth left, but
i haven't got any more nvme.  :-)

similarly coraid built an appliance that did copying (due to cache) and hit 1 million
4k iops.  this was in 2011 or so.

but, so what.  all this proves is that with copying or without, we can ingest enough
data for even the most hungry programs.

unless you have data that shows otherwise.  :-)

- erik

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Dan Cross]

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On Tue, Oct 9, 2018 at 5:28 PM hiro <23hiro@gmail.com> wrote:

> > E.g. right now Plan 9 suffers from a *lot* of data copying between
> > the kernel and processes, and between processes themselves.
>
> Huh? What exactly do you mean?


The current plan9 architecture relies heavily on copying data within a
process between userspace and the kernel for e.g. IO. This should be well
known to anyone who's rummaged around in the kernel, as it's pretty
evident. Because of the simple system call and VM interfaces, things like
scatter/gather IO or memory-mapped direct hardware access aren't really
options. `iowritev`, for example, coalesces its arguments into a single
buffer that it then pwrite()'s to its destination.

Can you describe the scenario and the
> measurements you made?
>

This is a different issue. I don't know if copying is as significant an
overhead as Lyndon suggested, but there are plenty of slow code paths in
plan9. For example, when we ported the plan9 network stack to Akaros, we
made a number of enhancements that combined sped things up by 50% or
greater. Most of these were pretty simple: optimizing checksum
calculations, alignment of IP and TCP headers on natural word boundaries
meaning that we could read an IP address with a 32-bit load (I think that
one netted a gigabit increase in throughput), using optimized memcpy
instead of memmove in performance critical code paths, etc. We went from
about 7Gbps on a 10Gbps interface to saturating the NIC. Those measurements
were made between dedicated test machines on a dedicated network using
netperf. Drew Gallatin, now at Netflix working on FreeBSD's network stack,
did most of the optimization work.

If that experience in that one section of the kernel is any indicator,
plan9 undoubtedly has lots of room for optimization in other parts of the
system. Lots of aspects of the system were optimized for much smaller
machines than are common now and many of those optimizations no longer make
much sense on modern machines; the allocator is slow, for example, though
very good at not wasting RAM. Compare to a vmem-style allocator, that can
allocate any requested size in constant-time, but with up-to a factor of
two waste of memory.

Lots of plan9 code is also buggy, or at least racy: consider the seemingly
random valued timeouts to "give other threads 5 seconds to get out" in
ipselffree() and iplinkfree() before "deallocating" an Iplink/Ipself.
Something like RCU, even a naive RCU, would be more robust here,
particularly under heavy load. Device drivers are atrophied and often
buggy, or at least susceptible to hardware bugs that are fixed by the
vendor-provided drivers. When I put in the plan9 networks to support Akaros
development, we ran into a bug in the i218 ethernet controller that caused
the NIC to wedge. We got Geoff Collyer to fix the i82563 driver and we sent
a patch to 9legacy, but it's symptomatic of an aging code base with a
shrinking developer population.

> If we could eliminate most of that copying, things would get a lot faster.
>
> Which things would get faster?
>

Presumably bulk data transfer between devices and the user portion of an
address space. If copying were eliminated (or just reduced) these would
certainly get fast*er*. Whether they would be sufficiently faster as to
make a perceptible performance different to a real workload is another
matter.

> Dealing with the security issues isn't trivial
>
> what security issues?
>

Presumably the bread-and-butter security issues that arise whenever the
user portion of an address space is being concurrently accessed by
hardware. As a trivial example, imagine scheduling a DMA transfer from some
device into a buffer in the user portion of an address space and then
exit()'ing the process. What do you do with the pages the device was
writing into? They had better be pinned in some way until the IO operation
completes before they're reallocated to something else that isn't expecting
it to be clobbered.

I wouldn't be surprised if the raft of currently popular speculative
execution bugs could be exacerbated by the kernel playing around with data
in the user address space in a naive way. It doesn't look like plan9 has
any serious mitigations for those.

        - Dan C.

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Bakul Shah]

On Tue, 09 Oct 2018 10:45:37 -0700 Lyndon Nerenberg <lyndon@orthanc.ca> wrote:
Lyndon Nerenberg writes:
> Bakul Shah writes:
>
> > One thing I have mused about is recasting plan9 as a
> > microkernel and pushing out a lot of its kernel code into user
> > mode code.  It is already half way there -- it is basically a
> > mux for 9p calls, low level device drivers, VM support & some
> > process related code.
>
> Somewhat related to this ... after reading some papers on
> TCP-in-user-space implementations, I've been thinking about how an
> interface that supported fast/secure page flipping between the
> kernel and process address space would change how we do things.
>
> E.g. right now Plan 9 suffers from a *lot* of data copying between
> the kernel and processes, and between processes themselves.  If we
> could eliminate most of that copying, things would get a lot faster.
> Dealing with the security issues isn't trivial, but the programmer
> time going into eeking out the last bit of I/O throughput of the
> current scheme could be redirected.

Funny you say this. I wrote I wanted memory mapping to avoid
having to copy data multiple times but then deleted it,
thinking it would detract from the main point.

Actually I want this even without any major redesign!

> If it works, this would reduce the kernel back to handling
> process/memory management, and talking to the hardware.  Not a
> micro-kernel, but just as good from a practical standpoint.

Some of this process/memory management can be delegated to
user code as well.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Lyndon Nerenberg]

Bakul Shah writes:

And funny you should mention this!

> Some of this process/memory management can be delegated to
> user code as well.

At $DAYJOB we would really like to have application process control
over the kernel scheduler, as this seems to be the only realistic
way to avoid the (kernel) resource starvation issues we run into.

Our back end servers don't go down often.  But when they do, it's for
reasons entirely out of our control.  Because those resource allocation
policies have been pushed into the kernel, and beyond our control.


--lyndon

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Digby R.S. Tarvin]

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On Tue, 9 Oct 2018 at 10:07, Dan Cross <crossd@gmail.com> wrote:

> My guess is that there is no reason in principle that it could not fit
>> comfortably into the constraints of a PDP11/70, but if the initial
>> implementation was done targeting a machine with significantly more
>> resources, it would be easy to make design decisions that would be entirely
>> incompatible.
>>
>
> I find this unlikely.
>
> The PDP-11, while a respectable machine for its day, required too many
> tradeoffs to make it attractive as a development platform for a
> next-generation research operating system in the late 1980s: be it
> electrical power consumption vs computational oomph or dollar cost vs
> available memory, the -11 had fallen from the attractive position it held a
> decade prior. Perhaps slimming a plan9 kernel down sufficiently so that it
> COULD run on a PDP-11 was possible in the early days, but I can't see any
> reason one would have WANTED to do so: particularly as part of the impetus
> behind plan9 was to exploit advances in contemporary hardware: lower-cost,
> higher-performance, RISC-based multiprocessors; ubiquitous networking;
> common high-resolution bitmapped graphical displays; even magneto-optical
> storage (one bet that didn't pan out); etc.
>

If you  mean that you find it unlikely that that development would have
been done on a PDP11, then I agree, for the reasons you mentioned.

Not sure that I can see why it wouldn't  have been feasible, but I can see
why it wouldn't have been desirable.

I thought there might have been a chance of an early attempt to target the
x86 because of its ubiquity and low cost - which could be useful for a
networked operating system. And those were 16 bit address constrained in
the early days. But its probably not an architecture you would choose to
work with if you had a choice.. 68K is what I would have gone for..


> Certainly Richard Millar's comment suggests that might be the case. If it
>> is heavily dependent on VM, then the necessary rewrite is likely to be
>> substantial.
>>
>
> As a demonstration project, getting a slimmed-down plan9 kernel to boot on
> a PDP-11/70-class machine would be a nifty hack, but it would be quite a
> tour de force and most likely the result would not be generally useful. I
> think that, as has been suggested, the conceptual simplicity of plan9
> paradoxically means that resource utilization is higher than it might
> otherwise be on either a more elaborate OR more constrained system (such as
> one targeting e.g. the PDP-11). When you can afford not to care about a few
> bytes here or a couple of cycles there and you're not obsessed with
> scraping out the very last drop of performance, you can employ a simpler
> (some might say 'naive') algorithm or data structure.
>
> I'm not sure how the kernel design has changed since the first release.
>> The earliest version I have is the release I bought through Harcourt Brace
>> back in 1995. But I won't be home till December so it will be a while
>> before I can look at it, and probably won't have time to experiment before
>> then in any case.
>>
>
> The kernel evolved substantially over its life; something like doubling in
> size. I remember vaguely having a discussion with Sape where he said he
> felt it had grown bloated. That was probably close to 20 years ago now.
>

I guess kernel size wasn't a priority. I did a bit of searching back
through the old papers, and whilst there is a lot of talk about lines of
code and numbers of system calls, I didn't find any reference to kernel
size or memory requirements.


> For what it is worth, I don't think the embarrassment of riches presented
>> to programmers by current hardware has tended to produce more elegant
>> designs. If more resources resulted in elegance, Windows would be a thing
>> of beauty.  Perhaps Plan9 is an exception. It certainly excels in elegance
>> and design simplicity, even if it does turn out to be more resource hungry
>> than I imagined. I will admit that the evils of excessively constrained
>> environments are generally worse in terms of coding elegance - especially
>> when it leads to overlays and self modifying code.
>>
>
> plan9 is breathtakingly elegant, but this is in no small part because as a
> research system it had the luxury of simply ignoring many thorny problems
> that would have marred that beauty but that the developers chose not to
> tackle. Some of these problems have non-trivial domain complexity and,
> while "modern" systems are far too complex by far, that doesn't mean that
> all solutions can be recast as elegantly simple pearls in the plan9 style.
> Whether we like those problems or not, they exist and real-world solutions
> have to at least attempt to deal with them (I'm looking at you, web x.0 for
> x >= 2...but curse you you aren't alone).
>
> PDP11's don't support virtual memory, so there doesn't seem any elegant
>> way to overcome that fundamental limitation on size of a singe executable.
>>
>
> No, they do: there is paging hardware on the PDP-11 that's used for
> address translation and memory protection (recall that PDP-11 kept the
> kernel at the top of the address space, the per-process "user" structure is
> at a fixed virtual address, and the system could trap a bus error and kill
> a misbehaving user-space process). What they may not support is the sort of
> trap handling that would let them recover from a page fault (though I
> haven't looked) and in any case, the address space is too small to make
> demand-paging with reclamation cost-effective.
>

>From what I recall, PDP11 hardware memory management was based on
segmentation rather than paging (64K divided into 16 variable sized
segments), and Unix did swapping rather than paging (a process is either
completely in memory or completely on disk). It does relocation and
protection, but I think it was limited in its ability to restart trapped
instructions. I suppose there was a sort of embryonic virtual memory in the
way the process stack was able to expand dynamically. I believe that was
handled by generating a trap when an address 'near' the top of the stack
was accessed. The instruction could complete, then a trap would allow the
operating system to add a bit more memory to the stack before returning to
allow the user process to continue.

Unix kept the 'per process data area' at the top of the 'process image' in
physical memory, but it was not mapped into the process address space. It
was, however. mapped into a fixed address in kernel space whenever the
corresponding process was the 'current' process.

But you are right - even is it could do virtual memory/paging, it wouldn't
help much because of limited size of the virtual address space.


> So I don't think it i would be worth a substantial rewrite to get it
>> going. It is a shame that there don't seem to have been any more powerful
>> machines with a comparably elegant architecture and attractive front panel
>> :)
>>
>
> An attractive front panel for nearly any machine is just a soldering iron,
> LEDs and some logic chips away. As far as elegant architectures, some are
> very nice: MIPS is kind of retro but elegant, RISC-V is nice, 680x0
> machines can be had a reasonable prices, and POWER is kind of cool. I know
> I shouldn't, but I have a soft spot for ARM.
>

I have thought about it, but there are a couple of problems (in addition to
my lack artistic talent when it comes to building physically attractive
enclosures)..  One is the sheer number of LEDs required to display all of
the address and data lines in a modern architecture.  Mainly an issue if I
want to use the old PDP11/70 front panel that I had saved for the purpose,
I suppose. The other problem is getting access to the all of the machine
state that was displayable on a mini computer console. Virtual addresses,
User/Kernel mode, register contents etc are all hard to get at. I have
toyed with using JTAG etc, but there always seems to be something that I
can't get to. So it is hard to do more than resort to a software controlled
front panel. I used to have a little box of LEDs and switches that I
plugged into the parallel port on PCs, and had my BSDi kernel modified to
update it as part of the clock interrupt. But now the parallel ports are
becoming rare and you can't update LEDs connected via USB in a single
instruction... :-/

Oh, and sure, you can find reasonable architectures. I spent a long time
using 680x0 after learning on PDP11s, and found it equally comfortable for
those occasions when I had to work in assembly language (I dread having to
disassemble Intel binaries). Its just that packaging is so uninteresting
now, it gives no indication of what is going on inside. Even lights on the
hard drives when they were externally visible gave some signs of what was
going on. Especially when there were different lights for read and write..

It is sounding like Inferno is going to be the more practical option. I
>> believe gcc can still generate PDP-11 code, so it shouldn't be too hard to
>> try.
>>
>
> Sounds like a nifty hack. Fitting Dis into a 64k/64k split I/D space is
> the challenge.
>

 Yes indeed. I guess that is something I can try to find the time to test
in the short term. It should provide a pretty good indication of viability.

Regards,
DigbyT

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Re: [9fans] PDP11

[David Arnold]

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> On 9 Oct 2018, at 14:08, Digby R.S. Tarvin <digbyt42@gmail.com> wrote:

<…>

> So I don't think it i would be worth a substantial rewrite to get it going. It is a shame that there don't seem to have been any more powerful machines with a comparably elegant architecture and attractive front panel :)
> 
> An attractive front panel for nearly any machine is just a soldering iron, LEDs and some logic chips away. As far as elegant architectures, some are very nice: MIPS is kind of retro but elegant, RISC-V is nice, 680x0 machines can be had a reasonable prices, and POWER is kind of cool. I know I shouldn't, but I have a soft spot for ARM.
> 
> I have thought about it, but there are a couple of problems (in addition to my lack artistic talent when it comes to building physically attractive enclosures)..  One is the sheer number of LEDs required to display all of the address and data lines in a modern architecture.  Mainly an issue if I want to use the old PDP11/70 front panel that I had saved for the purpose, I suppose. The other problem is getting access to the all of the machine state that was displayable on a mini computer console. Virtual addresses, User/Kernel mode, register contents etc are all hard to get at. I have toyed with using JTAG etc, but there always seems to be something that I can't get to. So it is hard to do more than resort to a software controlled front panel. I used to have a little box of LEDs and switches that I plugged into the parallel port on PCs, and had my BSDi kernel modified to update it as part of the clock interrupt. But now the parallel ports are becoming rare and you can't update LEDs connected via USB in a single instruction... :-/

Probably not quite what you’re after, but the PiDP8 and PiDP11 kits will get you an (arguably) attractive front panel without requiring artistic talent.

   http://obsolescence.wixsite.com/obsolescence/pidp-11

I’ve not looked into how the front-panel is driven (from SIMH, I guess?), but perhaps it could be suitably massaged?



d


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Re: [9fans] PDP11

[Digby R.S. Tarvin]

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Yes, that is exactly what prompted the thinking about Plan9 on a PDP11/70.
I have already organized a PiDP11 kit to be shipped to me when I get home
in December  - so that I can experiment without running the risk of blowing
up my old original 11/70 front panel. But a (simulated) 11/70 with a nice
front panel isn't so interesting unless I have some interesting PDP11
software to run on it.

A small Plan9/Inferno implementation could be integrated into a larger
network and allow the old hardware to integrate seamlessly with other
things. Such as exporting a device that lets other hosts write to the
lights and read from the switches, for example..

Regards.
DigbyT

On Tue, 9 Oct 2018 at 14:23, David Arnold <davida@pobox.com> wrote:

> On 9 Oct 2018, at 14:08, Digby R.S. Tarvin <digbyt42@gmail.com> wrote:
>
>
> <…>
>
> So I don't think it i would be worth a substantial rewrite to get it
>>> going. It is a shame that there don't seem to have been any more powerful
>>> machines with a comparably elegant architecture and attractive front panel
>>> :)
>>>
>>
>> An attractive front panel for nearly any machine is just a soldering
>> iron, LEDs and some logic chips away. As far as elegant architectures, some
>> are very nice: MIPS is kind of retro but elegant, RISC-V is nice, 680x0
>> machines can be had a reasonable prices, and POWER is kind of cool. I know
>> I shouldn't, but I have a soft spot for ARM.
>>
>
> I have thought about it, but there are a couple of problems (in addition
> to my lack artistic talent when it comes to building physically attractive
> enclosures)..  One is the sheer number of LEDs required to display all of
> the address and data lines in a modern architecture.  Mainly an issue if I
> want to use the old PDP11/70 front panel that I had saved for the purpose,
> I suppose. The other problem is getting access to the all of the machine
> state that was displayable on a mini computer console. Virtual addresses,
> User/Kernel mode, register contents etc are all hard to get at. I have
> toyed with using JTAG etc, but there always seems to be something that I
> can't get to. So it is hard to do more than resort to a software controlled
> front panel. I used to have a little box of LEDs and switches that I
> plugged into the parallel port on PCs, and had my BSDi kernel modified to
> update it as part of the clock interrupt. But now the parallel ports are
> becoming rare and you can't update LEDs connected via USB in a single
> instruction... :-/
>
>
> Probably not quite what you’re after, but the PiDP8 and PiDP11 kits will
> get you an (arguably) attractive front panel without requiring artistic
> talent.
>
>    http://obsolescence.wixsite.com/obsolescence/pidp-11
>
> I’ve not looked into how the front-panel is driven (from SIMH, I guess?),
> but perhaps it could be suitably massaged?
>
>
>
> d
>
>

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Ethan Gardener]

On Tue, Oct 9, 2018, at 4:08 AM, Digby R.S. Tarvin wrote:
> I thought there might have been a chance of an early attempt to target the x86 because of its ubiquity and low cost - which could be useful for a networked operating system. And those were 16 bit address constrained in the early days. But its probably not an architecture you would choose to work with if you had a choice.. 68K is what I would have gone for..

Fascinating thread, but I think you're off by a decade with the 16-bit address bus comment, unless you're not actually talking about Plan 9.  The 8086 and 8088 were introduced with 20-bit addressing in 1978 and 1979 respectively.  The IBM PC, launched in 1982, had its ROM at the top of that 1MByte space, so it couldn't have been constrained in that way.  By the end of the 80s, all my schoolmates had 68k-powered computers from Commodore and Atari, showing hardware with a 24-bit address space was very much affordable and ubiquitous at the time Plan 9 development started.  Almost all of them had 512KB at the time.  A few flashy gits had 1MB machines. :)

I still wish I'd kept the better of the Atari STs which made their way down to me -- a "1040 STE" -- 1MB with a better keyboard and ROM than the earlier "STFM" models.  I remember wanting to try to run Plan 9 on it.  Let's estimate how tight it would be...

I think it would be terrible, because I got frustrated enough trying to run a 4e CPU server with graphics on a 2GB x86.  I kept running out of image memory!  The trouble was the draw device in 4th edition stores images in the same "image memory" the kernel loads programs into, and the 386 CPU kernel 'only' allocates 64MB of that. :)

1 bit per pixel would obviously improve matters by a factor of 16 compared to my setup, and 640x400 (Atari ST high resolution) would be another 5 times smaller than my screen.  Putting these numbers together with my experience, you'd have to be careful to use images sparingly on a machine with 800KB free RAM after the kernel is loaded.  That's better than I thought, probably achievable on that Atari I had, but it couldn't be used as intensively as I used Plan 9 back then.

How could it be used?  I think it would be a good idea to push the draw device back to user space and make very sure to have it check for failing malloc!  I certainly wouldn't want a terminal with a filesystem and graphics all on a single 1MByte 64000-powered computer, because a filesystem on a terminal runs in user space, and thus requires some free memory to run the programs to shut it down.  Actually, Plan 9's separation of terminal from filesystem seems quite the obvious choice when I look at it like this. :)

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[erik quanstrom]

> I think it would be terrible, because I got frustrated enough trying to run a 4e CPU server with graphics on a 2GB x86.  I kept running out of image memory!  The trouble was the draw device in 4th edition stores images in the same "image memory" the kernel loads programs into, and the 386 CPU kernel 'only' allocates 64MB of that. :)

this was changed long ago.  image memory can now be much bigger.  i never had a problem when a 4e terminal
was my daily driver.

- erik

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Digby R.S. Tarvin]

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On Tue, 9 Oct 2018 at 23:00, Ethan Gardener <eekee57@fastmail.fm> wrote:

>
> Fascinating thread, but I think you're off by a decade with the 16-bit
> address bus comment, unless you're not actually talking about Plan 9.  The
> 8086 and 8088 were introduced with 20-bit addressing in 1978 and 1979
> respectively.  The IBM PC, launched in 1982, had its ROM at the top of that
> 1MByte space, so it couldn't have been constrained in that way.  By the end
> of the 80s, all my schoolmates had 68k-powered computers from Commodore and
> Atari, showing hardware with a 24-bit address space was very much
> affordable and ubiquitous at the time Plan 9 development started.  Almost
> all of them had 512KB at the time.  A few flashy gits had 1MB machines. :)
>

Not sure I would agree with that. The 20 bit addressing of the 8086 and
8088 did not change their 16 bit nature. They were still 16 bit program
counter, with segmentation to provide access to a larger memory - similar
in principle to the PDP11 with MMU.

The first 32 bit x86 processor was the 386, which I think came out in 1985,
very close to when work on Plan9 was rumored to have  started. So it seemed
not impossible that work might have started on an older 16 bit machine,
but  at Bell Labs probably a long shot.


> I still wish I'd kept the better of the Atari STs which made their way
> down to me -- a "1040 STE" -- 1MB with a better keyboard and ROM than the
> earlier "STFM" models.  I remember wanting to try to run Plan 9 on it.
> Let's estimate how tight it would be...
>
> I think it would be terrible, because I got frustrated enough trying to
> run a 4e CPU server with graphics on a 2GB x86.  I kept running out of
> image memory!  The trouble was the draw device in 4th edition stores images
> in the same "image memory" the kernel loads programs into, and the 386 CPU
> kernel 'only' allocates 64MB of that. :)
>
> 1 bit per pixel would obviously improve matters by a factor of 16 compared
> to my setup, and 640x400 (Atari ST high resolution) would be another 5
> times smaller than my screen.  Putting these numbers together with my
> experience, you'd have to be careful to use images sparingly on a machine
> with 800KB free RAM after the kernel is loaded.  That's better than I
> thought, probably achievable on that Atari I had, but it couldn't be used
> as intensively as I used Plan 9 back then.
>
> How could it be used?  I think it would be a good idea to push the draw
> device back to user space and make very sure to have it check for failing
> malloc!  I certainly wouldn't want a terminal with a filesystem and
> graphics all on a single 1MByte 64000-powered computer, because a
> filesystem on a terminal runs in user space, and thus requires some free
> memory to run the programs to shut it down.  Actually, Plan 9's separation
> of terminal from filesystem seems quite the obvious choice when I look at
> it like this. :)
>

I went Commodore Amiga at about that time - because it at least supported
some form of multi-tasking out out the box, and I spent many happy hours
getting OS9 running on it.. An interesting architecture, capable of some
impressive graphics, but subject to quite severe limitations which made
general purpose graphics difficult. (Commodore later released SVR4 Unix for
the A3000, but limited X11 to monochrome when using the inbuilt graphics).

But being 32 bit didn't give it a huge advantage over the 16 bit x86
systems for tinkering with operating system, because the 68000 had no MMU.
It was easier to get a Unix like system going with 16 bit segmentation than
a 32 bit linear space and no hardware support for run time relocation.
(OS9 used position independent code throughout to work without an MMU, but
didn't try to implement fork() semantics).

It wasn't till the 68030 based Amiga 3000 came out in 1990 that it really
did everything I wanted. The 68020 with an optional MMU was equivalent, but
not so common in consumer machines.

Hardware progress seems to have been rather uninteresting since then. Sure,
hardware is *much* faster and *much* bigger, but fundamentally the same
architecture. Intel had a brief flirtation with a novel architecture with
the iAPX 432 in 81, but obviously found that was more profitable making the
familiar architecture bigger and faster .

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Ethan Gardener]

On Tue, Oct 9, 2018, at 11:22 PM, Digby R.S. Tarvin wrote:
> 
> 
> On Tue, 9 Oct 2018 at 23:00, Ethan Gardener <eekee57@fastmail.fm> wrote:
>> 
>> Fascinating thread, but I think you're off by a decade with the 16-bit address bus comment, unless you're not actually talking about Plan 9.  The 8086 and 8088 were introduced with 20-bit addressing in 1978 and 1979 respectively.  The IBM PC, launched in 1982, had its ROM at the top of that 1MByte space, so it couldn't have been constrained in that way.  By the end of the 80s, all my schoolmates had 68k-powered computers from Commodore and Atari, showing hardware with a 24-bit address space was very much affordable and ubiquitous at the time Plan 9 development started.  Almost all of them had 512KB at the time.  A few flashy gits had 1MB machines. :)
> 
> Not sure I would agree with that. The 20 bit addressing of the 8086 and 8088 did not change their 16 bit nature. They were still 16 bit program counter, with segmentation to provide access to a larger memory - similar in principle to the PDP11 with MMU. 

That's not at all the same as being constrained to 64KB memory.  Are we communicating at cross purposes here?  If we're not, if I haven't misunderstood you, you might want to read up on creating .exe files for MS-DOS.  

> The first 32 bit x86 processor was the 386, which I think came out in 1985, very close to when work on Plan9 was rumored to have  started. So it seemed not impossible that work might have started on an older 16 bit machine, but  at Bell Labs probably a long shot.

Mmh, rumors. I read they were starting to think about Plan 9 in 1985, but I haven't read anything about it being up and running until '89 or '90.  There's not much to go on.

>> I still wish I'd kept the better of the Atari STs which made their way down to me -- a "1040 STE" -- 1MB with a better keyboard and ROM than the earlier "STFM" models.  I remember wanting to try to run Plan 9 on it.  Let's estimate how tight it would be...
>>  
>>  I think it would be terrible, because I got frustrated enough trying to run a 4e CPU server with graphics on a 2GB x86.  I kept running out of image memory!  The trouble was the draw device in 4th edition stores images in the same "image memory" the kernel loads programs into, and the 386 CPU kernel 'only' allocates 64MB of that. :)  
>>  
>>  1 bit per pixel would obviously improve matters by a factor of 16 compared to my setup, and 640x400 (Atari ST high resolution) would be another 5 times smaller than my screen.  Putting these numbers together with my experience, you'd have to be careful to use images sparingly on a machine with 800KB free RAM after the kernel is loaded.  That's better than I thought, probably achievable on that Atari I had, but it couldn't be used as intensively as I used Plan 9 back then.  
>>  
>>  How could it be used?  I think it would be a good idea to push the draw device back to user space and make very sure to have it check for failing malloc!  I certainly wouldn't want a terminal with a filesystem and graphics all on a single 1MByte 64000-powered computer, because a filesystem on a terminal runs in user space, and thus requires some free memory to run the programs to shut it down.  Actually, Plan 9's separation of terminal from filesystem seems quite the obvious choice when I look at it like this. :)  
> 
> I went Commodore Amiga at about that time - because it at least supported some form of multi-tasking out out the box, and I spent many happy hours getting OS9 running on it.. An interesting architecture, capable of some impressive graphics, but subject to quite severe limitations which made general purpose graphics difficult. (Commodore later released SVR4 Unix for the A3000, but limited X11 to monochrome when using the inbuilt graphics).

It does sound like fun. :)  I'm not surprised by the monochrome graphics limitation after my calculations.  Still, X11 or any other window system which lacks a backing store may do better in low-memory environments than Plan 9's present draw device.  It's a shame, a backing store is a great simplification for programmers.

> But being 32 bit didn't give it a huge advantage over the 16 bit x86 systems for tinkering with operating system, because the 68000 had no MMU.  It was easier to get a Unix like system going with 16 bit segmentation than a 32 bit linear space and no hardware support for run time relocation.
> (OS9 used position independent code throughout to work without an MMU, but didn't try to implement fork() semantics).

I'm sometimes tempted to think that fork() is freakishly high-level crazy stuff. :)  Still, like backing store, it's very nice to have.

> It wasn't till the 68030 based Amiga 3000 came out in 1990 that it really did everything I wanted. The 68020 with an optional MMU was equivalent, but not so common in consumer machines.
> 
> Hardware progress seems to have been rather uninteresting since then. Sure, hardware is *much* faster and *much* bigger, but fundamentally the same architecture. Intel had a brief flirtation with a novel architecture with the iAPX 432 in 81, but obviously found that was more profitable making the familiar architecture bigger and faster .

I rather agree.  Multi-core and hyperthreading don't bring in much from an operating system designer's perspective, and I think all the interesting things about caches are means of working around their problems.

I would very much like to get my hands on a ga144 to see what sort of operating system structure would work well on 144 processors with 64KW RAM each. :)  There's 64KW ROM per processor too, a lot of stock code could go in that.  Both the RAM and ROM operate at the full speed of the processor, no caches to worry about.

A little rant about MMUs, sort-of saying "unix and C are not without complexifying nonsense":  I'm sure the MMU itself is uninteresting or even harmful to many who prefer other languages and system designs.  Just look at that other discussion about the penalties of copying versus the cache penalties of page flipping.  If that doesn't devolve into "heavy negativity," it'll only be because those who know don't write much, or those who write much don't want to provide actual figures or references to argue about.

What about all those languages which don't even give the programmer access to pointers in the first place.  Many have run directly on hardware in the past, some can now.  Do they need MMUs?  

Then there's Forth, which relies on pointers even more than C does.  I haven't read *anything* about MMUs in relation to Forth, and yet Forth is in practice as much an operating system as a language.  It runs directly on hardware.  I'm not sure of some details yet, but it looks like many operating system features either "fall out of" the language design (to use a phrase from Ken Thompson & co.), or are trivial to implement.  

There were multitasking Forth systems in the 70s.  No MMU.  The full power of pointers *at the prompt*.  Potential for stack under- and over-runs too.  And yet these were working systems, and the language hasn't been consigned to the graveyard of computing history.  My big project includes exploring how this is possible. :)  A likely possibility is the power to redefine words (functions) without affecting previous definitions.  Pointer store and fetch can trivially be redefined to check bounds.  Check your code doesn't go out of bounds, then "empty", and load it without the bounds-checking store and fetch.  

--
Progress might have been all right once, but it has gone on too long -- Ogden Nash

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Digby R.S. Tarvin]

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On Wed, 10 Oct 2018 at 21:40, Ethan Gardener <eekee57@fastmail.fm> wrote:

> >
> > Not sure I would agree with that. The 20 bit addressing of the 8086 and
> 8088 did not change their 16 bit nature. They were still 16 bit program
> counter, with segmentation to provide access to a larger memory - similar
> in principle to the PDP11 with MMU.
>
> That's not at all the same as being constrained to 64KB memory.  Are we
> communicating at cross purposes here?  If we're not, if I haven't
> misunderstood you, you might want to read up on creating .exe files for
> MS-DOS.


Agreed, but the PDP11/70 was not constrained to 64KB memory either.

I do recall the MS-DOS small/large/medium etc models that used the
segmentation in various ways to mitigate the limitations of being a 16 bit
computer. Similar techniques were possible on the PDP11, for example
Modula-2/VRS under RT-11 used the MMU to transparently support 4MB programs
back in 1984 (it used trap instructions to implement subroutine calls).

It wasn't possible under Unix, of course, because there were no system
calls for manipulating the mmu. Understandable, as it would have
complicated the security model in a multi-tasking system. Something neither
MS-DOS or RT-11 had to deal with.

Address space manipulation was more convenient with Intel segmentation
because the instruction set included procedure call/return instructions
that manipulated the segmentation registers, but the situation was not
fundamentally different.  They were both 16 bit machines with hacks to give
access to a larger than 64K physical memory.

The OS9 operating system allowed some control of application memory maps in
a unix like environement by supporting dynamic (but explicit) link and
unlink of subroutine and data modules - which would be added and removed
from your 64K address space as required.So more analogous to memory based
overlays.


> > I went Commodore Amiga at about that time - because it at least
> supported some form of multi-tasking out out the box, and I spent many
> happy hours getting OS9 running on it.. An interesting architecture,
> capable of some impressive graphics, but subject to quite severe
> limitations which made general purpose graphics difficult. (Commodore later
> released SVR4 Unix for the A3000, but limited X11 to monochrome when using
> the inbuilt graphics).
>
> It does sound like fun. :)  I'm not surprised by the monochrome graphics
> limitation after my calculations.  Still, X11 or any other window system
> which lacks a backing store may do better in low-memory environments than
> Plan 9's present draw device.  It's a shame, a backing store is a great
> simplification for programmers.
>

X11 does, of course, support the concept of a backing store. It just
doesn't mandate it. It was an expensive thing to provide back when X11 was
young, so pretty rare. I remember finding the need to be able to re-create
windows on demand rather annoying when I first learned to program in Xlib,
but once you get used to it I find it can lead to benefits when you have to
retain a knowledge of how an image is created, not just the end result.


> > But being 32 bit didn't give it a huge advantage over the 16 bit x86
> systems for tinkering with operating system, because the 68000 had no MMU.
> It was easier to get a Unix like system going with 16 bit segmentation than
> a 32 bit linear space and no hardware support for run time relocation.
> > (OS9 used position independent code throughout to work without an MMU,
> but didn't try to implement fork() semantics).
>
> I'm sometimes tempted to think that fork() is freakishly high-level crazy
> stuff. :)  Still, like backing store, it's very nice to have.
>

I agree. Very elegant when you compare it to the hoops you have to jump
through to initialize the child process environment in systems with the
more common combined 'forkexec' semantics, but a real sticking point for
low end hardware.


> > It wasn't till the 68030 based Amiga 3000 came out in 1990 that it
> really did everything I wanted. The 68020 with an optional MMU was
> equivalent, but not so common in consumer machines.
> >
> > Hardware progress seems to have been rather uninteresting since then.
> Sure, hardware is *much* faster and *much* bigger, but fundamentally the
> same architecture. Intel had a brief flirtation with a novel architecture
> with the iAPX 432 in 81, but obviously found that was more profitable
> making the familiar architecture bigger and faster .
>
> I rather agree.  Multi-core and hyperthreading don't bring in much from an
> operating system designer's perspective, and I think all the interesting
> things about caches are means of working around their problems.


I don't think anyone would bother with multiple cores or caches if that
same performance could be achieved without them.  They just buy a bit more
performance at the cost of additional software complexity.

I would very much like to get my hands on a ga144 to see what sort of
> operating system structure would work well on 144 processors with 64KW RAM
> each. :)  There's 64KW ROM per processor too, a lot of stock code could go
> in that.  Both the RAM and ROM operate at the full speed of the processor,
> no caches to worry about.
>

Interesting. I hadn't come across those before....


> A little rant about MMUs, sort-of saying "unix and C are not without
> complexifying nonsense":  I'm sure the MMU itself is uninteresting or even
> harmful to many who prefer other languages and system designs.  Just look
> at that other discussion about the penalties of copying versus the cache
> penalties of page flipping.  If that doesn't devolve into "heavy
> negativity," it'll only be because those who know don't write much, or
> those who write much don't want to provide actual figures or references to
> argue about.
>

I think if you are going to postulate about this, we need to refine our
terms a bit. The term MMU encompasses too many quite different concepts.
For example:
1. run time relocation
2. virtual address space expansion (use more memory than can be directly
addressed)
3. virtual memory expansion (appear to use more memory than you physically
have)
4. process interference protection
5. process privacy protection

I'm sure there are more. For example, on the 68K processors, OS-9/68K had
no support for the first three - virtual and physical addresses were always
the same. For Unix style timesharing it required all compilers to generate
position independent code. There was no swapping or virtual memory. It did
use an MMU via a module called SPU - the 'System Protection Unit', which
mapped all of your program code as read only, your data as read/write, and
made everything else inaccessible. That sort of functionality is invaluable
while developing, because you don't want faulty programs to change the
kernel or other programs, and trapping attempts to do so makes it easier to
identify faults.
However with suitable privileges, any process can request that arbitrary
memory addresses be made readable or writable, and if desired, the SPU
could be omitted from the system, either removing an MMU  performance
penalty or allowing the application to run on cheaper, non-mmu equipped
hardware.

The executables were re-entrant and position independent, but one an
instance started executing it could not be moved (calculated pointers
stored in data etc). So this software solution would not have been enough
to support efficient swapping or paging. It was ok in this case, because it
was intended as a real-time system and you don't do swapping in that
situation.

What about all those languages which don't even give the programmer access
> to pointers in the first place.  Many have run directly on hardware in the
> past, some can now.  Do they need MMUs?
>


> Then there's Forth, which relies on pointers even more than C does.  I
> haven't read *anything* about MMUs in relation to Forth, and yet Forth is
> in practice as much an operating system as a language.  It runs directly on
> hardware.  I'm not sure of some details yet, but it looks like many
> operating system features either "fall out of" the language design (to use
> a phrase from Ken Thompson & co.), or are trivial to implement.
>
> There were multitasking Forth systems in the 70s.  No MMU.  The full power
> of pointers *at the prompt*.  Potential for stack under- and over-runs
> too.  And yet these were working systems, and the language hasn't been
> consigned to the graveyard of computing history.  My big project includes
> exploring how this is possible. :)  A likely possibility is the power to
> redefine words (functions) without affecting previous definitions.  Pointer
> store and fetch can trivially be redefined to check bounds.  Check your
> code doesn't go out of bounds, then "empty", and load it without the
> bounds-checking store and fetch.
>

MMU's are probably more important in multi-user than just multi-tasking
systems. The Amiga's, as I mentioned, were multi-tasking without the need
for an MMU. The result was a very fast system, resulting in some of the
impressive graphics and games. But also the need for frequent reboots when
developing.

But you are right, if you try hard enough, you can replace hardware memory
management with software - a sandboxed environment with program development
tools with strong typing and no low level access. Look, for example, and
the old Burroughs B6700 which had a security paradigm based on making it
impossible for unprivileged users to generate machine code. Compilers had
to blessed with trusted status, and only code generated by trusted
compilers would be executed.. I don't recall many details, other than it
had an interesting tagged architecture.

An extreme example would be an emulator - sandboxing users without any
actual hardware protection, albeit at significant performance cost.

Forth, Basic and all the other common development environments common on
person computers before the MMU availability were fine because there was
generally only one user, the language provided a lot or
protection/restriction (and was often interpreted), and if you managed to
crash it, it was generally quick to restart.

So I still tend to feel that MMUs were a valuable advance in computer
architecture that I would hate to have to live without..

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Lyndon Nerenberg]

Digby R.S. Tarvin writes:

> Agreed, but the PDP11/70 was not constrained to 64KB memory either.

> I do recall the MS-DOS small/large/medium etc models that used the
> segmentation in various ways to mitigate the limitations of being a 16 bit
> computer. Similar techniques were possible on the PDP11, for example

Coincidental to this conversation, I'm currently reading "The Apollo
Guidance Computer: Architecture and Operation" by _Framk O'Brien_.
(ISBN 978-1-4419-0876-6)  Very interesting to see what you can do with
a 15 bit architecture when sufficiently motivated.

--lyndon

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Digby R.S. Tarvin]

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Oh yes, I read Eldon Halls book on that quite a few years ago. Meetings
held to discuss competing potential uses for a word of memory that had
become free.

That one would be a challenging Plan9 port..

On Fri, 12 Oct 2018 at 05:13, Lyndon Nerenberg <lyndon@orthanc.ca> wrote:

> Digby R.S. Tarvin writes:
>
> > Agreed, but the PDP11/70 was not constrained to 64KB memory either.
>
> > I do recall the MS-DOS small/large/medium etc models that used the
> > segmentation in various ways to mitigate the limitations of being a 16
> bit
> > computer. Similar techniques were possible on the PDP11, for example
>
> Coincidental to this conversation, I'm currently reading "The Apollo
> Guidance Computer: Architecture and Operation" by _Framk O'Brien_.
> (ISBN 978-1-4419-0876-6)  Very interesting to see what you can do with
> a 15 bit architecture when sufficiently motivated.
>
> --lyndon
>
>

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Lyndon Nerenberg]

Digby R.S. Tarvin writes:

> Oh yes, I read Eldon Halls book on that quite a few years ago. Meetings
> held to discuss competing potential uses for a word of memory that had
> become free.

> That one would be a challenging Plan9 port..

And yet Plan9 was not there to save the day.  Such a pity.

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[erik quanstrom]

>  From what I recall, PDP11 hardware memory management was based on
> segmentation rather than paging (64K divided into 16 variable sized
> segments), and Unix did swapping rather than paging (a process is either
> completely in memory or completely on disk). It does relocation and

completely in memory /and running/. or swapped out.

- erik

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Nils M Holm]

On 2018-10-08T15:29:02+1100, Digby R.S. Tarvin wrote:
> A native Inferno port would certainly be a lot easier, but I think you
> might be a bit pessimistic about would can fit into a 64K address space
> machine. The 11/70 certainly managed to run a very respectable V7 Unix
> supporting 20-30 simultaneous active users in its day, [...]

The 11/70 was a completely different beast than, say, an 11/03.
The 70 had a backplane with 22 address lines, a MMU, and up to
4M bytes of memory. So while its processes were limited to
64K+64K bytes, I would not consider it to be a typical 16-bit
machine.

--
Nils M Holm  < n m h @ t 3 x . o r g >  www.t3x.org

Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Digby R.S. Tarvin]

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I quite agree - the PDP 11/70 was quite a high end 16 bit machine, but it
was the machine that I was talking about and the one I would most like to
revisit (although I wouldn't turn down an 11/40 if somebody offered me a
working one). I don't think I would contemplate putting Plan9 on a machine
with no MMU or a 64K physical memory limit.

My first reasonable multi-user, multi-tasking computer system (back in the
early 80s)  was home made 6809 machine with 6829 MMU and eventually 1MB of
ram, running OS-9/6809. It initially ran with 64K for programs and and the
rest of memory was a big ram disk - because what else could you do with
such a ridiculous amount of memory. It did pretty well at providing a
personal Unix like environment, although counldn't reproduce the fork()
semantics and there was no memory protection, and the memory contraints
meant always running the C compiler one pass at a time.. But we eventually
ported 'Level 2' OS-9 which could use a mapping ram/MMU, and with that I
had a quite robust multi-user system, with up to 64K available per process,
and 64K available for the kernel. I was able to get most Unix programs
running on it (except for a few with big tables that compiled to larger
than 64K) and no longer had to worry about exiting the editor before doing
a compile. Most of the core system utilities were written in assembly
language - so the equivalent of 'ls' for example, required no more than a
256 byte memory allocation. And all executables were loaded read-only and
re-entrant (shared text) which helped. The only real Achilles heal was the
6809 had no illegal instruction trapping, so executing data could
occasionally  result in an unrecoverable freeze..

I never liked the 68K version os OS-9 quite as much. Because of the larger
address space it used the MMU for protection only, with no address
translation - so the kernel was mapped into the same address space as the
user programs but just not accessible in user mode. It just didn't seem as
elegant.

Anyway, thats why I don't see 64K per process as necessarily being
inadequate for a lean operating system, although it would be easy enough to
write extravagant code that would not run in 64K, or a design that relied
on a large virtual address space - especially if you were used to relying
on virtual memory. I just don't know if how small Plan9 can go, and unless
someone has already explored those limits, I suppose rather than
speculating i'll just have to plan on a little experimentation when I get a
bit of spare time.

Regards,
Digby



On Mon, 8 Oct 2018 at 19:13, Nils M Holm <nmh@t3x.org> wrote:

> On 2018-10-08T15:29:02+1100, Digby R.S. Tarvin wrote:
> > A native Inferno port would certainly be a lot easier, but I think you
> > might be a bit pessimistic about would can fit into a 64K address space
> > machine. The 11/70 certainly managed to run a very respectable V7 Unix
> > supporting 20-30 simultaneous active users in its day, [...]
>
> The 11/70 was a completely different beast than, say, an 11/03.
> The 70 had a backplane with 22 address lines, a MMU, and up to
> 4M bytes of memory. So while its processes were limited to
> 64K+64K bytes, I would not consider it to be a typical 16-bit
> machine.
>
> --
> Nils M Holm  < n m h @ t 3 x . o r g >  www.t3x.org
>
>

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Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[Nils M Holm]

On 2018-10-08T05:38:07+0200, Lucio De Re wrote:
> You really must be thinking of Inferno, native, running in a host with
> 1MiB of memory. 64KiB isn't enough for anything other than maybe CPM.
> Even MPM won't cut it, I don't think.

There were serveral UNIX 6th Edition-based "Mini Unix" variants
for the PDP-11/03 and other 16-bit systems. Then there is UZI,
the Unix Z80 Implementation, which can run multiple processes
(with swapping) in 64K bytes of RAM. CP/M ran in much less than
64KB.

--
Nils M Holm  < n m h @ t 3 x . o r g >  www.t3x.org