RE: NUMA or not on dual Opteron

Arjan van de Ven
2005-01-14T08:10:44Z

On Fri, 2005-01-14 at 00:04 -0800, David Schwartz wrote:
> > In article <20050113094537.GB2547@favonius> you wrote:
>
> > > I was under the impression that NUMA is useful on > 2-way systems only.
> > > Is this true, and if not, under what circumstances is NUMA useful on
> > > 2-way Opteron systems?
>
> > NUMA is good for all situations where you have more than one CPU and the
> > CPUs have different access speeds for some parts of the memory (i.e. cpu
> > local memory). This is true for SMP Opterons, not for the usual Intel
> > Boards.
>
> Not quite. It's good if and only if the NU of the MA is sufficient to
> overcome the overhead associated with the NUMA code. Whether or not this is
> true depends upon two factors:
>
> 1) How non-uniform is the memory access? On 2 CPU Opteron systems, the
> answer is generally not very at all, it's nearly uniform.
>
> 2) How much overhead does the NUMA code add? On most of the benchmarks I've
> seen, the answer is a lot, so much that the memory access would have to be
> very non-uniform (factor of 2 at least) to justify enabling the NUMA code.

btw one of the major "pain points" of numa is that there are more memory
zones, and thus making it harder on the vm to balance the memory between
them...