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

NAME

       lamssi_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.   Collec-
	   tives 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 mod-
       ule 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 associa-
	   tive, 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  met-
	   ric	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 7.1.4							    July, 2007							    lamssi_coll(7)