Re: performance decreased

Re: performance decreased

[Francisco Llaryora]

Hi, to all.

First, thanks Tobias Burnus for the answers.
I read  https://computing.llnl.gov/tutorials/openMP/,but i didn't see it.
Just i can't access to the node, i.e. i no have jobs running.
For the next time i will read the gcc documentation.

Thanks Tim Prince, for redirecting my mail.
>how did it do with taskset or GOMP_OMP_AFFINITY settings, perhaps with a more up to date libgomp?
i don't use GOMP_OMP_AFFINITY settings. i can't update libgomp.
i don't try use taskset sorry, be couse the multithreading was off.
>Do you have another job running?
no, i have more results, with other size problem (i publish the median
of the results).

Regards!

Re: performance decreased

[Tim Prince]

On 11/7/2011 1:32 PM, Francisco Llaryora wrote:

> I want to tell you about a bizarre behavior in executables compiled
> with gcc 4.2.1 compiler.
> A few weeks ago i did must to paralelize a lattice boltzmann algorithm
> using OMP directives  (with adding own optimizations) to pass my
> High-performance computing course.
>
> I compile my c programm like this:
> $gcc -O3 -fopenmp -lm lattice-boltzmann.c -o output
>
> In lattice-boltzmann.c code i get the time with 2 calls to :omp_get_wtime().
>
> And then run the output after set OMP_NUM_THREADS value:
> $export OMP_NUM_THREADS=XXX
> $./output #without taskset programm
>
> With the purpose of measuring the SpedUp by changing the number of threads.
> I did run fourty times by changing the value OMP_NUM_THREAD   from 1 to 40.
> I run it in a node with 40 cores Xenon.4 Processors with 10 cores each one.
> The next is time in sec, no speedUP:
> 3.972002
> 2.052636
> 1.430107
> 1.111162
> 0.938760
> 0.811720
> 0.719483
> 0.666361
> 0.621093
> 0.563764
> 0.546574
> 0.510918
> 0.497733
> 0.481643
> 0.476792
> 0.454967
> 0.476283
> 0.445797
> 0.433787
> 0.426813
> 0.432890
> 0.436993
> 0.401258
> 0.424988
> 0.409322
> 0.416022
> 0.475070
> 0.425502
> 0.414787
> 0.434697
> 0.450460
> 0.428303
> 0.452609
> 0.453830
> 0.450324
> 0.461843
> 0.466831
> 0.464153
> 0.761500//39 threads
> 29.927848//40 threads
>
> Why performance decreased when the number of threads approaches the
> number of cores?
> What version of gcc resolve this behavior? Or How i resolve this behavior?
> The problem is not in the decreased of performance is in the magnitude
> with which it does.
>
> I compile my c programm like this too (intel64 compiler ver 11.1):
> $icc -fast -openmp lattice-boltzmann.c -o output
> The performance also decreases but not in the same magnitude.
> 0.336723//39 threads
> 0.756676//40 threads, and not: 14.22 sec for example.
>
redirecting to gcc-help.
And how did it do with taskset or GOMP_OMP_AFFINITY settings, perhaps 
with a more up to date libgomp?
Do you have another job running?


-- 
Tim Prince