Josh Hursey jjhursey at open-mpi.org
Wed Feb 17 07:34:59 CST 2010

They were trying to primarily answer two questions:
  - How many nodes are in my allocation?
  - What is the maximum number of MPI processes per node that I can  
reasonably use without oversubscribing the node?

So, for example, this is useful if an application wanted to spawn 1  
process per node, then use MPI_UNIVERSE_SIZE/MPI_UNIVERSE_NUM_NODES to  
determine the maximum number of threads to start per node from each  
MPI process.

This does assume that all nodes in the system are uniform in size,  
which these applications already do.

-- Josh

On Feb 17, 2010, at 8:24 AM, Supalov, Alexander 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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