[petsc-users] Advice on OpenMP/PETSc mix

[Aron Ahmadia]

Yes, that's right.  Though if you're memory-bound you may not notice as
great of an effect.

A

On Fri, Apr 20, 2012 at 9:49 PM, Mohammad Mirzadeh <mirzadeh at gmail.com>wrote:

> 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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