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Subject: Re: [OMPI devel] Open-MPI build of NAMD launched from srun over 20% slowed than with mpirun
From: Jeff Squyres (jsquyres) (jsquyres_at_[hidden])
Date: 2013-09-04 10:51:24

On Sep 4, 2013, at 4:33 AM, George Bosilca <bosilca_at_[hidden]> wrote:

> You can confirm that the slowdown happen during the MPI initialization stages by profiling the application (especially the MPI_Init call).

You can also try just launching "MPI hello world" (i.e., examples/hello_c.c). It just calls MPI_INIT / MPI_FINALIZE.

Additionally, you might want to try launching the ring program, too (examples/ring_c.c). That program sends a small message around in a ring, which forces some MPI communication to occur, and therefore does at least some level of setup in the BTLs, etc. (remember: most BTLs are lazy-connect, so they don't actually do anything until the first send. So a simple "ring" program sets up *some* BTL connections, but not nearly all of them).

Jeff Squyres
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