[Pw_forum] Quantum Espresso (4.3.2) and pure openmp runs

[Axel Kohlmeyer]

i should probably add, that you are likely
to get better performance using MPI, since
there are many ways to do rather efficient
distributed data parallelization, while OpenMP
multi-threading requires a lot of little changes
that are not always easy to add (and thus some
of which are still missing) and thus generally
parallelizes less well.

axel.

On Mon, Aug 6, 2012 at 12:20 PM, Axel Kohlmeyer <akohlmey at gmail.com> wrote:
> On Mon, Aug 6, 2012 at 12:06 PM, Cristian Degli Esposti Boschi
> <degliesposti at bo.imm.cnr.it> wrote:
>> Dear all,
>>
>> I have a (maybe basic) question on the performance of Quantum Espresso
>> with respect to openmp, even if I do not realise if the answer lies in
>> the way QE handles different levels of parallelism or lies at the system
>> level instead.
>>
>> However, I am running QE 4.3.2 on a quad-core machine with 2 CPUs 64bit.
>> SO is Debian stable, and it recognises 16 threads in the CPU table.
>>
>> I have compiled QE with pure openmp support (gcc) without any MPI
>> environment.
>>
>> The question is: even if I do NOT set the OMP_NUM_THREADS variable
>> at runtime, from the table of the "top" command my process
>> now uses 1000 % CPU time, as if it were "parallelised" over 10 threads.
>>
>> Is this real? Is there some kind of default when pw.x is invoked?
>> If so, how is openmp parallelism exploited by QE?
>
> this has nothing to do with QE but is a typical implementation default.
> if OMP_NUM_THREADS is not set, most OpenMP runtime environment
> automatically default to using all CPU cores for threads. the fact
> that you see about 1000% and not 1600% is probably due to the
> limited threading during the course of execution for your specific
> input. in any case, you are more likely to get better performance
> with hyperthreading disabled and/or OMP_NUM_THREADS set to 8.
>
> BTW: this default behavior of OpenMP is a major nuisance for
> codes that can do both MPI and OpenMP, so that any reasonable
> system admin will set up machines, so that OMP_NUM_THREADS
> is explicitly set to 1 by default in the system environment.
>
> axel.
>
>
>>
>> Thanks for your time and support.
>>
>> --
>> Cristian Degli Esposti Boschi
>> CNR-IMM, Sezione di Bologna,
>> via Gobetti, 101, 40129, Bologna, Italia
>> tel. ++39 051 6399152, fax ++39 051 6399216
>> email: degliesposti -AT- bo.imm.cnr.it
>> web:   www.bo.imm.cnr.it/site/staff/personal_pages/degliesposti/
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>
>
>
> --
> Dr. Axel Kohlmeyer  akohlmey at gmail.com  http://goo.gl/1wk0
> International Centre for Theoretical Physics, Trieste. Italy.



-- 
Dr. Axel Kohlmeyer  akohlmey at gmail.com  http://goo.gl/1wk0
International Centre for Theoretical Physics, Trieste. Italy.