[IPOL discuss] hyperthreading and openmp

[Nicolas Limare]

Hi,

> I send this mail to ask for advice or opinion on the default number of
> threads that an OMP demo should use.  Either as a constant, or as a
> way to compute this number from the system.
> 
> I suspect that the best choice is one less than the real number of
> processors (thus, 15 for the green server).

There are 32 real CPUs on the green server. Hyperthreadong has been
disabled on the advise of Jerôme, because it is only useful for idle
processes (such as web servers waiting for a connexion), not for busy
ones. This server has 4 CPUs with 8 cores each.

> Above this point, there are diminishing returns in increasing the
> number of threads while the total CPU time, which could be used for
> other demos, is wasted.

I think the "good number" of processes depends a lot on the algorithm
and on the size of your data. Moreover, the parallel gain is limited
by the part of your code which is effectively parallelisable; if 95%
of your program is perfectly parallelisable, you will never get better
than x20 performance gain, even with an infinity of
processors[1]. This, plus the overhead cost of every multithread
branch, could explain your results. Did you try with some data
substancially larger? Do you get the same graph?

[1] http://en.wikipedia.org/wiki/Amdahl%27s_law

I think any tentative of fine tuning will eventually be inadapted when
running on another machine or larger data. So in the absence of a
better wisdom, I would take no decision and keep the default settings,
ie use as many threads as CPUs. According to your graph, 32 threads is
not worse than other values. If you want to save on the total CPU time
(and power use), you can decide to only use half of the resources. You
can get the number of available CPUs from the openmp library[2], with:

    #ifdef _OPENMP
    #include <omp.h>
    omp_set_num_threads(omp_get_max_threads() / 2);
    #endif

[2] https://computing.llnl.gov/tutorials/openMP/#OMP_GET_MAX_THREADS
    https://computing.llnl.gov/tutorials/openMP/#OMP_SET_NUM_THREADS


-- 
Nicolas LIMARE - CMLA - ENS Cachan    http://www.cmla.ens-cachan.fr/~limare/
IPOL - image processing on line                          http://www.ipol.im/
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