Hi Jeff,

Thanks a lot for your answer, It really makes sense.

Do you know where can I find more information about the *process launch on 
remote nodes* of MPI and all the alternatives I have?

Thanks and regards.

From: users <users-boun...@lists.open-mpi.org> on behalf of Jeff Squyres 
(jsquyres) via users <users@lists.open-mpi.org>
Sent: Wednesday, December 19, 2018 7:18:23 PM
To: Open MPI User's List
Cc: Jeff Squyres (jsquyres)
Subject: Re: [OMPI users] It's possible to get mpi working without ssh?

On Dec 19, 2018, at 11:42 AM, Daniel Edreira <dedre...@plainconcepts.com> wrote:
> Does anyone know if there's a possibility to configure a cluster of nodes to 
> communicate with each other with mpirun without using SSH?
> Someone is asking me about making a cluster with Infiniband that does not use 
> SSH to communicate using OpenMPI.

I'm not entirely clear what you're asking.  You mention both "ssh" and 
"communication" -- they're kinda different things.

Communication: Over an InfiniBand cluster, we'd recommend that you use the UCX 
library for Open MPI to communicate over IB.  I.e., when your MPI application 
invokes API calls like MPI_Send() and MPI_Recv(), they'll use the UCX library 
underneath and utilize native IB-style communication (i.e., they're not using 
the POSIX sockets API -- they're using the native UCX/verbs APIs for RDMA OS 
bypass and offload, ...etc.).

Ssh: ssh is not used for *communication*, per se; it's used for *starting Linux 
processes on remote nodes.  Open MPI can use SSH to start processes on remote 
nodes, but it can also use other mechanisms (e.g., if you have a resource 
manager such as SLURM, Open MPI can use SLURM's native remote process launching 
mechanism instead of SSH).

These two things are orthogonal to each other: you can use whatever 
*communication* mechanism you want for MPI APIs (e.g., the POSIX sockets API or 
the UCX library or ...several other APIs...), and use whatever *process launch* 
mechanism you want (e.g., SSH or SLURM's native remote process launch or 
...several others...).  Put simply: the choice of MPI communication layer does 
not imply anything about the remote process launch mechanism, and vice versa.

Make sense?

Jeff Squyres

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