[Mpi3-subsetting] Subsetting slides

[Richard Treumann]

One argument for subsetting is that MPI is somehow not well suited to
massive master-worker applications. It does seem reasonable to me that pure
master-worker applications have little use for many of MPI's features.
Most communication may be point to point because workers are not expected
to cooperate or communicate among themselves.  Groups, topologies, new
communicators, MPI-IO etc are not very useful.

In the simple case, the master hands each worker a piece of work to do and
the worker runs on its own until the work is done. As a worker finishes, it
sends an answer to the master and waits to be handed another piece of work.
Some pieces of work are brief and some are not so workers are not synched
with each other and the master does not know who will have an answer next.

My impression is that the biggest problem with MPI for this model is that
with one master and many workers, it is not hard to make an application
that can survive the loss of one or more workers.  The master just hands
out the lost work assignment again.  MPI is not an ideal fit because the
loss of one member of MPI_COMM_WORLD will probably bring down the job.  A
simple subset will not resolve this.  An Fault Tolerant subset could be
considered if this is an important rationale for a subset.

Also, lots of people seem to like the idea of getting rid of MPI_ANY_SOURCE
in a subset.  I am one of these people.  MPI_ANY_SOURCE is a problem.  The
difficulty is that the master in a master worker algorithm is likely to
find MPI_ANY_SOURCE and MPI_PROBE useful.

               Dick

Dick Treumann  -  MPI Team/TCEM
IBM Systems & Technology Group
Dept 0lva / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846         Fax (845) 433-8363





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