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Subject: Re: [OMPI users] Shared Memory Performance Problem.
From: Tim Prince (n8tm_at_[hidden])
Date: 2011-03-30 15:25:35


On 3/30/2011 10:08 AM, Eugene Loh wrote:
> Michele Marena wrote:
>> I've launched my app with mpiP both when two processes are on
>> different node and when two processes are on the same node.
>>
>> The process 0 is the manager (gathers the results only), processes 1
>> and 2 are workers (compute).
>>
>> This is the case processes 1 and 2 are on different nodes (runs in 162s).
>> @--- MPI Time (seconds)
>> ---------------------------------------------------
>> Task AppTime MPITime MPI%
>> 0 162 162 99.99
>> 1 162 30.2 18.66
>> 2 162 14.7 9.04
>> * 486 207 42.56
>>
>> The case when processes 1 and 2 are on the same node (runs in 260s).
>> @--- MPI Time (seconds)
>> ---------------------------------------------------
>> Task AppTime MPITime MPI%
>> 0 260 260 99.99
>> 1 260 39.7 15.29
>> 2 260 26.4 10.17
>> * 779 326 41.82
>>
>> I think there's a contention problem on the memory bus.
> Right. Process 0 spends all its time in MPI, presumably waiting on
> workers. The workers spend about the same amount of time on MPI
> regardless of whether they're placed together or not. The big difference
> is that the workers are much slower in non-MPI tasks when they're
> located on the same node. The issue has little to do with MPI. The
> workers are hogging local resources and work faster when placed on
> different nodes.
>> However, the message size is 4096 * sizeof(double). Maybe I are wrong
>> in this point. Is the message size too huge for shared memory?
> No. That's not very large at all.

Not even large enough to expect the non-temporal storage issue about
cache eviction to arise.

-- 
Tim Prince