[petsc-users] Poor speed up for KSP example 45

[Mark Adams]

I would guess that you are saturating the memory bandwidth. After you make
PETSc (make all) it will suggest that you test it (make test) and suggest
that you run streams (make streams).

I see Matt answered but let me add that when you make streams you will seed
the memory rate for 1,2,3, ... NP processes. If your machine is decent you
should see very good speed up at the beginning and then it will start to
saturate. You are seeing about 50% of perfect speedup at 16 process. I
would expect that you will see something similar with streams. Without
knowing your machine, your results look typical.

On Wed, Mar 25, 2020 at 1:05 PM Amin Sadeghi <aminthefresh at gmail.com> wrote:

> Hi,
>
> I ran KSP example 45 on a single node with 32 cores and 125GB memory using
> 1, 16 and 32 MPI processes. Here's a comparison of the time spent during
> KSP.solve:
>
> - 1 MPI process: ~98 sec, speedup: 1X
> - 16 MPI processes: ~12 sec, speedup: ~8X
> - 32 MPI processes: ~11 sec, speedup: ~9X
>
> Since the problem size is large enough (8M unknowns), I expected a speedup
> much closer to 32X, rather than 9X. Is this expected? If yes, how can it be
> improved?
>
> I've attached three log files for more details.
>
> Sincerely,
> Amin
>
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