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Subject: Re: [OMPI users] Over committing?
From: Ralph Castain (rhc_at_[hidden])
Date: 2011-04-13 23:55:01


Have you folks used a debugger such as TotalView or padb to look at these stalls?

I ask because we discovered a long time ago that MPI collectives can "hang" in the scenario you describe. It is caused by one rank falling behind, and then never catching up due to resource allocations - i.e.., once you fall behind due to the processor being used by something else, you never catch up.

The code that causes this is generally a loop around a collective such as Allreduce. The solution was to inject a "barrier" operation in the loop periodically, thus ensuring that all ranks had an opportunity to catch up.

There is an MCA param you can set that will inject the barrier - it specifies to inject it every N collective operations (either before or after the Nth op):

-mca coll_sync_barrier_before N

or

-mca coll_sync_barrier_after N

It'll slow the job down a little, depending upon how often you inject the barrier. But it did allow us to run jobs reliably to completion when the code involved such issues.

On Apr 13, 2011, at 10:07 AM, Rushton Martin wrote:

> The 16 cores refers to x3755-m2s. We have a mix of 3550s and 3755s in
> the cluster.
>
> It could be memory, but I think not. The jobs are well within memory
> capacity, and the memory is mainly static. If out of memory then the
> jobs would be first candidate for the job. Larger jobs run on the 3755s
> which as well as more memory have local disks for paging to.
>
>
> Martin Rushton
> HPC System Manager, Weapons Technologies
> Tel: 01959 514777, Mobile: 07939 219057
> email: jmrushton_at_[hidden]
> www.QinetiQ.com
> QinetiQ - Delivering customer-focused solutions
>
> Please consider the environment before printing this email.
> -----Original Message-----
> From: users-bounces_at_[hidden] [mailto:users-bounces_at_[hidden]] On
> Behalf Of Reuti
> Sent: 13 April 2011 16:53
> To: Open MPI Users
> Subject: Re: [OMPI users] Over committing?
>
> Am 13.04.2011 um 17:09 schrieb Rushton Martin:
>
>> Version 1.3.2
>>
>> Consider a job that will run with 28 processes. The user submits it
>> with:
>>
>> $ qsub -l nodes=4:ppn=7 ...
>>
>> which reserves 7 cores on (in this case) each of x3550x014 x3550x015
>> and
>> x3550x016 x3550x020. Torque generates a file (PBS_NODEFILE) which
>> lists each node 7 times.
>>
>> The mpirun command given within the batch script is:
>>
>> $ mpirun -np 28 -machinefile $PBS_NODEFILE <executable>
>>
>> This is what I would refer to as 7+7+7+7, and it runs fine.
>>
>> The problem occurs if, for instance, a 24 core job is attempted. qsub
>
>> gets nodes=3:ppn=8 and mpirun has -np 24. The job is now running on
>> three nodes, using all eight cores on each node - 8+8+8. This sort of
>
>> job will eventually hang and has to be killed off.
>>
>> Cores Nodes Ppn Result
>> ----- ----- --- ------
>> 8 1 any works
>> 8 >1 1-7 works
>> 8 >1 8 hangs
>> 16 1 any works
>> 16 >1 1-15 works
>> 16 >1 16 hangs
>
> How many cores do you have in each system? Looks like 8 is the maximum
> IBM offers from their datasheet, and still you can request 16 per node?
>
> Can it be a memory porblem?
>
> -- Reuti
>
>
>> We have also tried test jobs on 8+7 (or 7+8) with inconclusive
> results.
>> Some of the live jobs run for a month or more and cut down versions do
>
>> not model well.
>>
>> Martin Rushton
>> HPC System Manager, Weapons Technologies
>> Tel: 01959 514777, Mobile: 07939 219057
>> email: jmrushton_at_[hidden]
>> www.QinetiQ.com
>> QinetiQ - Delivering customer-focused solutions
>>
>> Please consider the environment before printing this email.
>> -----Original Message-----
>> From: users-bounces_at_[hidden] [mailto:users-bounces_at_[hidden]]
>> On Behalf Of Ralph Castain
>> Sent: 13 April 2011 15:34
>> To: Open MPI Users
>> Subject: Re: [OMPI users] Over committing?
>>
>>
>> On Apr 13, 2011, at 8:13 AM, Rushton Martin wrote:
>>
>>> The bulk of our compute nodes are 8 cores (twin 4-core IBM x3550-m2).
>>> Jobs are submitted by Torque/MOAB. When run with up to np=8 there is
>
>>> good performance. Attempting to run with more processors brings
>>> problems, specifically if any one node of a group of nodes has all 8
>>> cores in use the job hangs. For instance running with 14 cores (7+7)
>
>>> is fine, but running with 16 (8+8) hangs.
>>>
>>>> From the FAQs I note the issues of over committing and aggressive
>>> scheduling. Is it possible for mpirun (or orted on the remote nodes)
>
>>> to be blocked from progressing by a fully committed node? We have a
>>> few
>>> x3755-m2 machines with 16 cores, and we have detected a similar issue
>
>>> with 16+16.
>>
>> I'm not entirely sure I understand your notation, but we have never
>> seen an issue when running with fully loaded nodes (i.e., where the
>> number of MPI procs on the node = the number of cores).
>>
>> What version of OMPI are you using? Are you binding the procs?
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