[Beowulf] Varying performance across identical cluster nodes.
pbisbal at pppl.gov
Fri Sep 8 11:41:14 PDT 2017
I need your assistance debugging a problem:
I have a dozen servers that are all identical hardware: SuperMicro
servers with AMD Opteron 6320 processors. Every since we upgraded to
CentOS 6, the users have been complaining of wildly inconsistent
performance across these 12 nodes. I ran LINPACK on these nodes, and was
able to duplicate the problem, with performance varying from ~14 GFLOPS
to 64 GFLOPS.
I've identified that performance on the slower nodes starts off fine,
and then slowly degrades throughout the LINPACK run. For example, on a
node with this problem, during first LINPACK test, I can see the
performance drop from 115 GFLOPS down to 11.3 GFLOPS. That constant,
downward trend continues throughout the remaining tests. At the start of
subsequent tests, performance will jump up to about 9-10 GFLOPS, but
then drop to 5-6 GLOPS at the end of the test.
Because of the nature of this problem, I suspect this might be a thermal
issue. My guess is that the processor speed is being throttled to
prevent overheating on the "bad" nodes.
But here's the thing: this wasn't a problem until we upgraded to CentOS
6. Where I work, we use a read-only NFSroot filesystem for our cluster
nodes, so all nodes are mounting and using the same exact read-only
image of the operating system. This only happens with these SuperMicro
nodes, and only with the CentOS 6 on NFSroot. RHEL5 on NFSroot worked
fine, and when I installed CentOS 6 on a local disk, the nodes worked fine.
Any ideas where to look or what to tweak to fix this? Any idea why this
is only occuring with RHEL 6 w/ NFS root OS?
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