[Users] cactus performance

[Frank Loeffler]

Hi,

On Tue, Mar 20, 2012 at 05:14:38PM -0700, Jose Fiestas Iquira wrote:
> Is there documentation about performance of Cactus ETK in large machines. I
> have some questions regarding best performance according to initial
> conditions, calculation time required, etc.

Performance very much depends on the specific setup. One poorly scaling
function can ruin the otherwise best run.

> If there are performance plots like Flops vs. Number of nodes would help me
> as well.

Flops are very problem-dependent. There isn't such thing as flops/s for
Cactus, not even for one given machine. If we talk about the Einstein
equations and a typical production run I would expect a few percent of
the peak performance of any given CPU, as we are most of the time bound by
memory bandwidth.
Calculation time for initial data very much depends on the type of
initial data. Some initial data are setup in a few seconds, some may
need a day. Scaling of initial data computation also might be quite
different from that of the evolution, which is why sometimes it makes
sense to checkpoint right after initial data setup and restart using a
different number of cores.

> Other main question I have is if Cactus scales in large machines.

I am afraid that this question is too general. 'Cactus' itself doesn't
even deal directly with MPI, so it would be better to ask, e.g., how
well Carpet scales. And given that still quite general question it can
be said that Carpet scales to at least 100k cores - however, that again
very much depends on your setup. Using a couple of levels of mesh
refinement bring down the scaling limit to maybe 10k cores, adding
re-gridding and a few common analysis routines and we talk about 2k-4k
cores practical limit.  But again, these numbers very much depend on
your setup. You might be able to perform much better for certain
problems, and much worse for others.

> I am using McLachlan, but any other application would give me an idea of
> what I should expect for my runs in big machines.

Do the numbers I gave help?

Frank

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