[petsc-users] KSPSolve doesn't seem to scale. (Must be doing something wrong...)

[Karl Rupp]

Hi William,

I couldn't find something really suspicious in the logs, so the lack of 
scalability may be due to hardware limitations. Did you run all MPI 
processes on the same machine? How many CPU sockets? If it is a 
single-socket machine, chances are good that you saturate the memory 
channels pretty well with one process already. With higher process 
counts the cache per process is reduced, thus reducing cache reuse. This 
is the only reasonable explanation why the execution time for VecMDot 
goes up from e.g. 7 seconds for one and two processes to about 24 for 
four and eight processes.

I suggest you try to run the same code across multiple machines if 
possible, you should see better scalability there. Also, for 
benchmarking purposes try to replace the ILU preconditioner with e.g. 
Jacobi, this should give you better scalability (provided that the 
solver still converges, of course...)

Best regards,
Karli


On 03/14/2014 10:45 PM, William Coirier wrote:
> I've written a parallel, finite-volume, transient thermal conduction solver using PETSc primitives, and so far things have been going great. Comparisons to theory for a simple problem (transient conduction in a semi-infinite slab) looks good, but I'm not getting very good parallel scaling behavior with the KSP solver. Whether I use the default KSP/PC or other sensible combinations, the time spent in KSPSolve seems to not scale well at all.
>
> I seem to have loaded up the problem well enough. The PETSc logging/profiling has been really useful for reworking various code segments, and right now, the bottleneck is KSPSolve, and I can't seem to figure out how to get it to scale properly.
>
> I'm attaching output produced with -log_summary, -info, -ksp_view and -pc_view all specified on the command line for 1, 2, 4 and 8 processes.
>
> If you guys have any suggestions, I'd definitely like to hear them! And I apologize in advance if I've done something stupid. All the documentation has been really helpful.
>
> Thanks in advance...
>
> Bill Coirier
>
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