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

[Cristian Degli Esposti Boschi]

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?

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/