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lamssi_coll(7)               LAM SSI COLL OVERVIEW              lamssi_coll(7)

NAME
       LAM SSI collectives - overview of LAM's MPI collective SSI modules

DESCRIPTION
       The "kind" for collectives SSI modules is "coll".  Specifically, the
       string "coll" (without the quotes) is the prefix that should be used
       with the mpirun command line with the -ssi switch.  For example:

       mpirun -ssi coll_base_crossover 4 C my_mpi_program

       LAM currently has three coll modules:

       lam_basic
           A full implementation of MPI collectives on intracommunicators.
           The algorithms are the same as were in the LAM 6.5 series.
           Collectives on intercommunicators are undefined, and will result in
           run-time errors.

       impi
           Collective functions for IMPI communicators.  These are mostly un-
           implemented; only the basics exist: MPI_BARRIER and MPI_REDUCE.

       shmem
           Shared memory collectives.

       smp SMP-aware collectives (based on the MagPIe algorithms).  The
           following algorithms provide SMP-aware performance on
           multiprocessors: MPI_ALLREDUCE, MPI_ALLTOALL, MPI_ALLTOALLV,
           MPI_BARRIER, MPI_BCAST, MPI_GATHER, MPI_GATHERV, MPI_REDUCE,
           MPI_SCATTER, and MPI_SCATTERV.  Note that the reduction algorithms
           must be specifically enabled by marking the operations as
           associative before they will be used.  All other MPI collectives
           will fall back to their lam_basic equivalents.

       More collective modules are likely to be implemented in the future.

COLL MODULE PARAMETERS
       In the discussion below, the parameters are discussed in terms of kind
       and value.  Unlike other SSI module kinds, since coll modules are
       selected on a per-communicator basis, the kind and value may be
       specified as attributes to a parent communicator.

       Need to write much more here.

   Selecting a coll module
       coll modules are selected on a per-communicator basis.  They are
       selected when the communicator is created, and remain the active coll
       module for the life of that communicator.  For example, different coll
       modules may be assigned to MPI_COMM_WORLD and MPI_COMM_SELF.  In most
       cases LAM/MPI will select the best coll module automatically.  For
       example, when a communicator spans multiple nodes and at least one node
       has multiple MPI processes, the smp module will automatically be
       selected.

       However, the LAM_MPI_SSI_COLL keyval can be used to set an attribute on
       a communicator that is used to create a new communicator.  The
       attribute should have the value of the string name of the coll module
       to use.  If that module cannot be used, an MPI exception will occur.
       This attribute is only examined on the parent communicator when a new
       communicator is created.

   coll SSI Parameters
       The coll modules accept several parameters:

       coll_associative
           Because of specific wording in the MPI standard, LAM/MPI can
           effectively not assume that any reduction operator is associative
           (at least, not without additional overhead).  Hence, LAM/MPI relies
           on the user to indicate that certain operations are associative.
           If the user sets the coll_associative SSI parameter to 1, LAM/MPI
           may assume that the reduction operator is assocative, and may be
           able to optimize the overall reduction operation.  If it is 0 or
           undefined, LAM/MPI will assume that the reduction operation is not
           associative, and will use strict linear ordering of reduction
           operations (regardless of data locality).  This attribute is
           checked every time a reduction operator is invoked.  The User's
           Guide contains more information on this topic.

       coll_crossover
           This parameter determines the maximum number of processes in a
           communicator that will use linear algorithms.  This SSI parameter
           is only checked during MPI_INIT.

       coll_reduce_crossover
           During reduction operations, it makes sense to use the number of
           bytes to be transferred rather than the number of processes as a
           metric whether to use linear or logrithmic algorithms.  This
           parameter indicates the maxmimum number of bytes to be transferred
           by each process by a linear algorithm.  This SSI parameter is only
           checked during MPI_INIT.

   Notes on the smp coll Module
       The smp coll module is based on the algorithms from the MagPIe project.
       It is not yet complete; there are still more algorithms that can be
       optmized for SMP-aware execution -- by the time that LAM/MPI was frozen
       in preparation for release, only some of the algorithms had been
       completed.  It is expected that future versions of LAM/MPI will have
       more SMP-optimized algorithms.

       The User's Guide contains much more detail about the smp module.  In
       particular, the coll_associative SSI parameter must be 1 for the SMP-
       aware reduction algorithms to be used.  If it is 0 or undefined, the
       corresponding lam_basic algorithms will be used.  The coll_associative
       attribute is checked at every invocation of the reduction algorithms.

SEE ALSO
       lamssi(7), mpirun(1), LAM User's Guide

LAM 7.1.5b2                       June, 2008                    lamssi_coll(7)