[Beowulf] Weird blade performs worse as more cpus are used?

Joe Landman joe.landman at gmail.com
Thu Sep 14 08:42:44 PDT 2017

On 09/14/2017 11:34 AM, Faraz Hussain wrote:
> Earlier I had posted about one of our blades running 30-50% slower 
> than other  ones despite having identical hardware and OS. I followed 
> the suggestions and compared cpu temperature, memory, dmesg and 
> sysctl. Everything looks the same.
> I then used "perf stat" to compare speed of pigz ( parralel gzip ). 
> The results are quite interesting. Using one cpu, the slow blade is as 
> fast as the rest! But as I use more cpus, the speed decreases linearly 
> from 3.1Ghz to 0.4 Ghz. See snippets from "perf stat" command below. 
> All tests were on /tmp to eliminate any nfs issue. And same behavior 
> is observed with any multi-threaded program.

What does numastat report?  /tmp is a ramdisk or tmpfs?  Are the 
nodes/cpus otherwise idle?  What does lscpu on a good/bad node report?

If it decreases on a 1/Ncpu curve, then you have a fixed sized resource 
bandwidth contention issue you are fighting.   The question is what.


Joe Landman
e: joe.landman at gmail.com
t: @hpcjoe
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