[petsc-dev] Proper matrix size to choose when evaluating MatMult?

Mark Adams mfadams at lbl.gov
Fri Feb 21 16:38:39 CST 2020

On Fri, Feb 21, 2020 at 4:51 PM Junchao Zhang via petsc-dev <
petsc-dev at mcs.anl.gov> wrote:

> Hello,
> I want to evaluate MatMult on GPU.  I took a 2M x 2M matrix and ran with 6
> mpi ranks and 6 GPUs.  It took about 0.9 seconds.  A kernel launch or a
> stream synchronization took about 10us.

Your call, but you should run the code once and then run it in a new timer.
I've seen some big "warm up costs" GPUs today.

> Compared with MatMult, they are tiny. Does it mean we can ignore them?
> What is a proper size to evaluate MatMult?

It depends on the purpose/audience for the study. There is no right size
other than being much larger than the launch cost, perhaps.

> I heard it is a few thousand rows per MPI rank.  Why?
> Thanks.
> --Junchao Zhang
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