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

[Bakul Shah <bakul@bitblocks.com>]

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.