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<p><font size="-1">Hello<br>
</font></p>
<p><font size="-1">it is a strange behavior which does not depend on
the program, there may be many reasons, it's very hard to guess:
<br>
</font></p>
<p><font size="-1">starting from the most trivial things:<br>
</font></p>
<p><font size="-1">* it could be that some other application is
using the same processors as you at the same time ?<br>
</font></p>
<p><font size="-1">* you are using a file system that is not very
efficient, e.g. you are using the home filesystem instead of the
scratch disk or something of the kind. <br>
</font></p>
<p><font size="-1">* you are using multithreading but you don't have
enough processors to do that ? Try to set the envinroment
variable OMP_NUM_THREADS to 1 before running. <br>
</font></p>
<p><font size="-1">I hope it helps <br>
</font></p>
<p><font size="-1">Pietro <br>
</font></p>
<p><font size="-1"><br>
</font></p>
<div class="moz-cite-prefix">On 06/06/19 13:21, Julien Barbaud
wrote:<br>
</div>
<blockquote type="cite"
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<div class="WordSection1">
<p class="MsoNormal"><span lang="FR">Dear users,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="FR"><o:p> </o:p></span></p>
<p class="MsoNormal">I am still struggling to understand the
parallel performances of QE on the cluster of my university. I
have to say right off the bat that this problem might have
more to do with the parallel scheduling in our cluster.
However, after many discussions with the people responsible
for the cluster, they don’t seem to see where the problem
would be on their side. So I want to check if that could be a
more common problem and if you would have some suggestions
about it.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The problem in a nutshell: the performance
of a pw.x run seems completely random on our cluster.
Launching the same job on the same number of procs can result
in calculation times differing by a factor of 5 of more. This
is of course a huge issue in planning how many cores I want to
use, or just trying to have a clue of what’s going on.<o:p></o:p></p>
<p class="MsoNormal">When the speed is particularly low, it
seems to be materialized by a WALL time much higher than the
CPU time.<o:p></o:p></p>
<p class="MsoNormal"> To exemplify, here is the same code ran on
3, 6 and 9 cores, with the corresponding CPU and WALL time:<o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">Procs CPU
time WALL time<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">-------
------------ -------------<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">3
6m56.69s 28m33.48s </span><span
style="font-family:Wingdings;color:#1F497D">à</span><span
style="color:#1F497D"> big difference: bad parallelization<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">6
4m 9.56s 4m20.65s </span><span
style="font-family:Wingdings;color:#1F497D">à</span><span
style="color:#1F497D"> good parallelization<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">9
5min42s 21m13.10s </span><span
style="font-family:Wingdings;color:#1F497D">à</span><span
style="color:#1F497D"> bad parallelization<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black">The huge
difference between CPU time and WALL time is an issue. But
even looking at the CPU time alone, it doesn’t seem to scale
well, as I would not expect the 9 cores to be slower than
the 6 (but I lack experience on this). <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">If I launch the
job again right after on 6 cores, I get something much
slower. This pattern shows up for different inputs, so I
does not seem to be related to that directly. The example is
from a vc-relax run stopped after 4 iterations<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black">This all feels
very random, but do you have an idea why this would happen ?
Am I doing something wrong ?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black">Another example
with a run on 3 iterations, for 3,6,9 procs, repeated twice
to show the “random” variations between 2 runs:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Procs CPU
time WALL time<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">-------
------------ -------------<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">3
6m25.61s 16m17.82s <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">6
3m18.12s 7m16.88s<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">9
2m31.85s 6m32.46s 10s <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Procs CPU
time WALL time<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">-------
------------ -------------<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">3
7m17.83s 22m53.90s<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">6
3m42.18s 3m50.74s<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">9
5m38.31 9m21.52s<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks in advance, <o:p></o:p></p>
<p class="MsoNormal">Julien<o:p></o:p></p>
</div>
<br>
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