Re: dual-core amd question
Thank you. Then, one additional question that I forgot
before. Planning to replace the 1GB modules with 2GB
modules, is that correct to replace couples of modules
(one left, one right) at different times, and not all
eigth modules at the same time? I plan to use the
machine with a mixture of 1G-1GB and 2GB-2GB couples
of memory modules until I get money to replace all.
francesco pietra
--- Jo Shields <jms@osc.ox.ac.uk> wrote:
> On Thu, 2007-01-18 at 11:12 -0800, Francesco Pietra
> wrote:
> > Why 4 processors? Moreover, with such a system is
> that
> > command optimized or should MPI used instead? In
> other
> > words, is my arrangement "shared memory" or should
> be
> > considered a cluster?
>
> 4 processors because 2 x 2 = 4
>
> > That because someone has recently warned me that
> > "dual-opteron is not shared memory [I knew that of
> > course]. Each cpu has its own memory, and they
> can
> > access each other's. If your program uses MPI then
> > that's the best way of having it."
>
> Dual Opteron is definitely shared memory in the
> modern sense of the term
> - it's actually NUMA, which means each processor has
> its own supply of
> memory but can request memory from other processors
> (whereas with
> 'traditional' SMP, there's one big pool of memory
> and one pool of
> processors). Massive 'SMP' systems such as SGI's
> Itanium-powered Altix
> are NUMA.
>
> Performance, however, is a bigger question. Is there
> a hit when
> accessing a different processor's memory? Yes. Would
> MPI be faster?
> Possibly, but probably not on a 2x2 machine. Opteron
> suffers from
> diminishing returns (especially with a 2.6 kernel),
> so adding more cores
> takes you further & further from optimal performance
> - meaning MPI
> becomes a more interesting prospect at that point.
>
> --Jo Shields
>
>
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