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