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Subject: Re: [OMPI users] Process Migration
From: Eugene Loh (eugene.loh_at_[hidden])
Date: 2011-11-10 10:44:33

On 11/10/2011 5:19 AM, Jeff Squyres wrote:
> On Nov 10, 2011, at 8:11 AM, Mudassar Majeed wrote:
>> Thank you for your reply. I am implementing a load balancing function for MPI, that will balance the computation load and the communication both at a time. So my algorithm assumes that all the cores may at the end get different number of processes to run.
> Are you talking about over-subscribing cores? I.e., putting more than 1 MPI process on each core?
> In general, that's not a good idea.
>> In the beginning (before that function will be called), each core will have equal number of processes. So I am thinking either to start more processes on each core (than needed) and run my function for load balancing and then block the remaining processes (on each core). In this way I will be able to achieve different number of processes per core.
> Open MPI spins aggressively looking for network progress. For example, if you block in an MPI_RECV waiting for a message, Open MPI is actively banging on the CPU looking for network progress. Because of this (and other reasons), you probably do not want to over-subscribe your processors (meaning: you probably don't want to put more than 1 process per core).
Or, introduce your own MPI_Test/sleep loop if you really feel that you
otherwise want to oversubscribe. Watch out for pitfalls.