[Beowulf] Varying performance across identical cluster nodes.

[Andrew Latham]

ack, so maybe validate you can reproduce with another nfs root. Maybe a lab
setup where a single server is serving nfs root to the node. If you could
reproduce in that way then it would give some direction. Beyond that it
sounds like an interesting problem.

On Wed, Sep 13, 2017 at 12:48 PM, Prentice Bisbal <pbisbal at pppl.gov> wrote:

> Okay, based on the various responses I've gotten here and on other lists,
> I feel I need to clarify things:
>
> This problem only occurs when I'm running our NFSroot based version of the
> OS (CentOS 6). When I run the same OS installed on a local disk, I do not
> have this problem, using the same exact server(s).  For testing purposes,
> I'm using LINPACK, and running the same executable  with the same HPL.dat
> file in both instances.
>
> Because I'm testing the same hardware using different OSes, this (should)
> eliminate the problem being in the BIOS, and faulty hardware. This leads me
> to believe it's most likely a software configuration issue, like a kernel
> tuning parameter, or some other software configuration issue.
>
> These are Supermicro servers, and it seems they do not provide CPU temps.
> I do see a chassis temp, but not the temps of the individual CPUs. While I
> agree that should be the first thing I look at, it's not an option for me.
> Other tools like FLIR and Infrared thermometers aren't really an option for
> me, either.
>
> What software configuration, either a kernel a parameter, configuration of
> numad or cpuspeed, or some other setting, could affect this?
>
> Prentice
>
> On 09/08/2017 02:41 PM, Prentice Bisbal wrote:
>
>> Beowulfers,
>>
>> 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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-- 
- Andrew "lathama" Latham lathama at gmail.com http://lathama.com
<http://lathama.org> -
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