Parallelization on muli-CPU hardware?

[Alex Martelli]

Nicolas Lehuen <nicolas.lehuen at thecrmcompany.com> wrote:
   ...
> > > > Sorry, I don't get your point.  Sure, Windows makes process creation
> > > > hideously expensive and has no forking.  But all kinds of BSD, including
> > > > MacOSX, are just great at forking.  Why is a preference for multiple
> > > > processes over threads "forgetting about BSD, MacOSX", or any other
> > > > flavour of Unix for that matter?
   ...
> mod_python's market (pure guess). So the multithreading issues have not
> many chances of being fixed soon. That is why I said that a
> "Linux-centered mindset forgetting [other OSes, none in particular]" is
> hindering the bugfix process.
   ...
> I do hope the point above is cleared.

It's basically "retracted", as I see thing, except that whatever's left
still doesn't make any sense to me.  Why should a Linux user need to
"forget" another system, that's just as perfect for multiprocessing as
Linux, before deciding he's got better things to do with his or her time
than work on a problem which multiprocessing finesses>

> Now, you propose to share objects between Python VMs using shared memory.

Not necessarily -- as you say, if you need synchronization the problem
is just about as hard (maybe even worse).  I was just trying to
understand the focus on multithreading vs multiprocessing, it now
appears it's more of a focus on the shared-memory paradigm of
multiprocessing in general.


> > > Obviously they won't be any worse. Well, to be precise, it still depends
> > on the OS, because the scheduler must know the difference between 2
> > processors and a 2-core processor to efficiently balance the work, but
   ...
> I am way beyond my competences here, but I've read some articles about
> hyperthreading-aware schedulers (in WinXP, Win2003, and patches for
> Linux). The idea is that on multi-core CPUs, threads from the same
> process should be ran on the same core for maximum cache efficiency,
> whereas different processes can freely run on different cores. I've read

And how is this a difference between two processors and two cores within
the same processor, which is what I quoted you above as saying?  If two
CPUs do not share caches, the CPU-affinity issues (of processing units
that share address spaces vs ones that don't) would appear to be the
same.  If two CPUs share some level of cache (as some multi-CPU designs
do), that's different from the case where the CPUs share no cache but to
share RAM.

> But apart from this caveat, yes, multi-threads and multi-processes
> application equally benefit from multi-core CPUs.

So it would seem to me, yes -- except that if CPUs share caches this may
help (perhaps) used of shared memory (if the cache design is optimized
for that and the scheduler actively helps), even though even in that
case it doesn't seem to me you can reach the same bandwidth that
hypertransport promises for a more streaming/message passing approach.


Alex