Re: [9fans] Barrelfish

[Bakul Shah <bakul+plan9@bitblocks.com>]

On Sun, 18 Oct 2009 01:15:45 -0000 Roman Shaposhnik <roman@shaposhnik.org>  wrote:
> On Thu, Oct 15, 2009 at 10:53 AM, Sam Watkins <sam@nipl.net> wrote:
> > On Wed, Oct 14, 2009 at 06:50:28PM -0700, Roman Shaposhnik wrote:
> >> > The mention that "... the overhead of cache coherence restricts the ab=
> ility
> >> > to scale up to even 80 cores" is also eye openeing. If we're at aprox =
> 8
> >> > cores today, thats only 5 yrs away (if we double cores every
> >> > 1.5 yrs).
> >
> > Sharing the memory between processes is a stupid approach to multi-proces=
> sing /
> > multi-threading. =A0Modern popular computer architecture and software des=
> ign is
> > fairly much uniformly stupid.

> It is. But what's your proposal on code sharing? All those PC
> registers belonging to
> different cores have to point somewhere. Is that somewhere is not shared me=
> mory
> the code has to be put there for every single core, right?

Different technoglogies/techniques make sense at different
levels of scaling and at different points in time so sharing
memory is not necessarily stupid -- unless one thinks that
any compromise (to produce usable solutions in a realistic
time frame) is stupid.

At the hardware level we do have message passing between a
processor and the memory controller -- this is exactly the
same as talking to a shared server and has the same issues of
scaling etc. If you have very few clients, a single shared
server is indeed a cost effective solution.

When you absolutely have to share state, somebody has to
mediate access to the shared state and you can't get around
the fact that it's going to cost you.  But if you know
something about the patterns of sharing, you can get away
from a single shared memory & increase concurrency.  A simple
example is a h/w fifo (to connect producer/consumer but you
also gave up some flexibility).

As the number of processors increases on a device, sharing
state between neighbors will be increasingly cheaper compared
any global sharing. Even if you use message passing, messages
between near neighbors will be far cheaper than between
processors in different neighboorhoods. So switching to
message passing is not going to fix things; you have to worry
about placement as well (just like in h/w design).