[petsc-users] Parallel processes run significantly slower

[Stefano Zampini]

You are creating the matrix on the wrong communicator if you want it
parallel. You are using PETSc.COMM_SELF

On Thu, Jan 11, 2024, 19:28 Steffen Wilksen | Universitaet Bremen <
swilksen at itp.uni-bremen.de> wrote:

> Hi all,
>
> I'm trying to do repeated matrix-vector-multiplication of large sparse
> matrices in python using petsc4py. Even the most simple method of
> parallelization, dividing up the calculation to run on multiple processes
> indenpendtly, does not seem to give a singnificant speed up for large
> matrices. I constructed a minimal working example, which I run using
>
> mpiexec -n N python parallel_example.py,
>
> where N is the number of processes. Instead of taking approximately the
> same time irrespective of the number of processes used, the calculation is
> much slower when starting more MPI processes. This translates to little to
> no speed up when splitting up a fixed number of calculations over N
> processes. As an example, running with N=1 takes 9s, while running with N=4
> takes 34s. When running with smaller matrices, the problem is not as severe
> (only slower by a factor of 1.5 when setting MATSIZE=1e+5 instead of
> MATSIZE=1e+6). I get the same problems when just starting the script four
> times manually without using MPI.
> I attached both the script and the log file for running the script with
> N=4. Any help would be greatly appreciated. Calculations are done on my
> laptop, arch linux version 6.6.8 and PETSc version 3.20.2.
>
> Kind Regards
> Steffen
>
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