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Subject: Re: [OMPI devel] System V Shared Memory for OpenMPI:Request forCommunity Input and Testing
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2010-05-04 10:41:12

On May 4, 2010, at 9:32 AM, Ashley Pittman wrote:

> > One thing to be careful with a run-time check is that you might not want *all* processes on a box to try to alloc a sysv segment, fork a child, try to connect, ...etc. With large count boxen, you might run out of sysv shmem segments if all procs try the test and/or run into OS serialization issues (someone here at the Forum cited a 96 core box). So you might want to have local rank 0 (or the orted? ...but that wouldn't work for srun / direct launch, etc.) do a test and communicate the results to the rest of the local procs -- maybe in the modex?
> I think as as user I'd be quite surprised if my MPI job was spawning sub-processes during MPI_Init().

Point noted. But actually -- can you give specific reasons as to why a user should care? Keep in mind that this would be a short-lived fork'ed process -- not "spawn" in the MPI sense of the word.

(I'm not advocating or refuting the idea -- I just want to understand why a user should care if the run-time system runs a quick test during startup)

Jeff Squyres
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