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Subject: Re: [OMPI users] Very slow MPI_GATHER
From: Jeffrey Squyres (jsquyres_at_[hidden])
Date: 2012-02-29 12:59:19

On Feb 29, 2012, at 11:01 AM, Pinero, Pedro_jose wrote:

> I am using OMPI v.1.5.5 to communicate 200 Processes in a 2-Computers cluster connected though Ethernet, obtaining a very poor performance.

Let me making sure I'm parsing this statement properly: are you launching 200 MPI processes on 2 computers? If so, do those computers each have 100 cores?

I ask because oversubscribing MPI processes (i.e., putting more than 1 process per core) will be disastrous to performance.

> I have measured each operation time and I haver realised that the MPI_Gather operation takes about 1 second in each synchronization (only an integer is send in each case). Is this time range normal or I have a synchronization problem? Is there any way to improve this performance?

I'm afraid I can't say more without more information about your hardware and software setup. Is this a dedicated HPC cluster? Are you oversubscribing the cores? What kind of Ethernet switching gear do you have? ...etc.

Jeff Squyres
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