[Mpi3-ft] MPI_UNIVERSE_NUM_NODES?

[Graham, Richard L.]

The number of MPI processes that can be reasonably used seems to be application specific.

I tend to agree that in the context of providing support for dynamic process creation, having the ability to query the system in some manner is a good idea.

Rich


On 2/17/10 8:24 AM, "Supalov, Alexander" <alexander.supalov at intel.com> wrote:

Thanks. Why should we care of the node number? I think users care primarily of the maximum number of the MPI processes they can reasonably use without oversubscribing the system.

-----Original Message-----
From: mpi3-ft-bounces at lists.mpi-forum.org [mailto:mpi3-ft-bounces at lists.mpi-forum.org] On Behalf Of Josh Hursey
Sent: Wednesday, February 17, 2010 2:17 PM
To: MPI 3.0 Fault Tolerance and Dynamic Process Control working Group
Subject: [Mpi3-ft] MPI_UNIVERSE_NUM_NODES?

I have been talking with a couple application groups that are
interested in using comm_spawn for dynamic resource management
scenarios. MPI_UNIVERSE_SIZE (MPI 10.5.1) is a good starting place for
them to portably find a soft limit on the number of processes that can
be started within their allocation. However, they would also really
like to know how many machines/nodes are in their allocation in an
equally portable way (So they can try to create N procs per node, for
example). Currently they do some preprocessing on the runtime
environment variables to determine how many nodes are in the
allocation. Since this is something the MPI implementation is already
doing, I suggested that it might be possible to expose this in a
portable way to the application through the MPI interface.

So I am proposing a MPI_UNIVERSE_NUM_NODES attribute to MPI_COMM_WORLD
with similar wording as MPI_UNIVERSE_SIZE. This will let the
application access the number of nodes in their allocation in a
portable manner. As for on-node topologies (i.e., sockets, cores,
etc.), there are other tools to help make that more portable (e.g.
hwloc).

I am interested in what others think of the idea of introducing a
MPI_UNIVERSE_NUM_NODES attribute to the standard.

Cheers,
Josh

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