[petsc-users] Advice on OpenMP/PETSc mix

[Mohammad Mirzadeh]

Thanks Aron. That would work but at the cost of wasting idle cores when
threads join and the rest of MPI-based code is running, correct?

On Fri, Apr 20, 2012 at 11:44 AM, Aron Ahmadia <aron.ahmadia at kaust.edu.sa>wrote:

> If I use, say Np = 16 processes on one node, MPI is running 16 versions of
>> the code on a single node (which has 16 cores). How does OpenMP figure out
>> how to fork? Does it fork a total of 16 threads/MPI process = 256 threads
>> or is it smart to just fork a total of 16 threads/node = 1 thread/core = 16
>> threads? I'm a bit confused here how the job is scheduled when MPI and
>> OpenMP are mixed?
>>
>
> This is one important use for OMP_NUM_THREADS.   If you're trying to
> increase the amount of memory per process, you should map one process per
> node and set OMP_NUM_THREADS to the number of OpenMP threads you'd like.
>  There are lots of tutorials and even textbooks now that discuss hybrid
> programming techniques that you should look to for more information (or you
> could try scicomp.stackexchange.com).
>
> Cheers,
> Aron
>
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