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Subject: Re: [OMPI users] Some Newbie questions
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2009-06-29 07:20:10

On Jun 29, 2009, at 2:19 AM, vipin kumar wrote:

> Q. Since I am using TCP/Ethernet I guess that MPI uses SSH/RSH to
> communicate between peers.
> Ans. May be you are right. I don't know exactly how peers
> communicate in MPI environment, but I can say for sure that OpenMPI
> uses rsh/ssh as one of the available launchers.

Open MPI uses two different mechanisms for launching individual MPI
processes vs. MPI communications. rsh/ssh is one of the options that
Open MPI can use for launching MPI processes, but we don't keep those
channels open and don't use them for MPI communications. Individual,
new TCP sockets are opened for MPI_SEND / MPI_RECV (etc.) traffic.
These sockets are not encrypted (like ssh connections would be).

> Q. And for that, each peer should have the copy of the application
> right?
> Ans. Absolutely correct. But If you don't want to copy binaries
> manually you should use "--preload-binary" option. OpenMPI will copy
> the executables in remote nodes before launching processes, and will
> delete when job gets done.
> It is almost always good to use latest version. "--preload-binary"
> option may be absent in old versions.

It is new in the 1.3 series; it did not exist in the 1.2 series.

Jeff Squyres
Cisco Systems