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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link="#0563C1" vlink="#954F72"><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></body></html>