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From: Heywood, Todd (heywood_at_[hidden])
Date: 2007-03-22 13:30:19


Ralph,

Well, according to the FAQ, aggressive mode can be "forced" so I did try
setting OMPI_MCA_mpi_yield_when_idle=0 before running. I also tried turning
processor/memory affinity on. Efffects were minor. The MPI tasks still cycle
bewteen run and sleep states, driving up system time well over user time.

Mpstat shows SGE is indeed giving 4 or 2 slots per node as approporiate
(depending on memory) and the MPI tasks are using 4 or 2 cores, but to be
sure, I also tried running directly with a hostfile with slots=4 or slots=2.
The same behavior occurs.

This behavior is a function of the size of the job. I.e. As I scale from 200
to 800 tasks the run/sleep cycling increases, so that system time grows from
maybe half the user time to maybe 5 times user time.

This is for TCP/gigE.

Todd

On 3/22/07 12:19 PM, "Ralph Castain" <rhc_at_[hidden]> wrote:

> Just for clarification: ompi_info only shows the *default* value of the MCA
> parameter. In this case, mpi_yield_when_idle defaults to aggressive, but
> that value is reset internally if the system sees an "oversubscribed"
> condition.
>
> The issue here isn't how many cores are on the node, but rather how many
> were specifically allocated to this job. If the allocation wasn't at least 2
> (in your example), then we would automatically reset mpi_yield_when_idle to
> be non-aggressive, regardless of how many cores are actually on the node.
>
> Ralph
>
>
> On 3/22/07 7:14 AM, "Heywood, Todd" <heywood_at_[hidden]> wrote:
>
>> Yes, I'm using SGE. I also just noticed that when 2 tasks/slots run on a
>> 4-core node, the 2 tasks are still cycling between run and sleep, with
>> higher system time than user time.
>>
>> Ompi_info shows the MCA parameter mpi_yield_when_idle to be 0 (aggressive),
>> so that suggests the tasks aren't swapping out on bloccking calls.
>>
>> Still puzzled.
>>
>> Thanks,
>> Todd
>>
>>
>> On 3/22/07 7:36 AM, "Jeff Squyres" <jsquyres_at_[hidden]> wrote:
>>
>>> Are you using a scheduler on your system?
>>>
>>> More specifically, does Open MPI know that you have for process slots
>>> on each node? If you are using a hostfile and didn't specify
>>> "slots=4" for each host, Open MPI will think that it's
>>> oversubscribing and will therefore call sched_yield() in the depths
>>> of its progress engine.
>>>
>>>
>>> On Mar 21, 2007, at 5:08 PM, Heywood, Todd wrote:
>>>
>>>> P.s. I should have said this this is a pretty course-grained
>>>> application,
>>>> and netstat doesn't show much communication going on (except in
>>>> stages).
>>>>
>>>>
>>>> On 3/21/07 4:21 PM, "Heywood, Todd" <heywood_at_[hidden]> wrote:
>>>>
>>>>> I noticed that my OpenMPI processes are using larger amounts of
>>>>> system time
>>>>> than user time (via vmstat, top). I'm running on dual-core, dual-CPU
>>>>> Opterons, with 4 slots per node, where the program has the nodes to
>>>>> themselves. A closer look showed that they are constantly
>>>>> switching between
>>>>> run and sleep states with 4-8 page faults per second.
>>>>>
>>>>> Why would this be? It doesn't happen with 4 sequential jobs
>>>>> running on a
>>>>> node, where I get 99% user time, maybe 1% system time.
>>>>>
>>>>> The processes have plenty of memory. This behavior occurs whether
>>>>> I use
>>>>> processor/memory affinity or not (there is no oversubscription).
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Todd
>>>>>
>>>>> _______________________________________________
>>>>> users mailing list
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>>>>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>>>>
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