lamssi_collectives - overview of LAM's MPI collective SSI modules
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:
- 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.
- Collective functions for IMPI communicators. These are
mostly un-implemented; only the basics exist: MPI_BARRIER and
- Shared memory collectives.
- 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
More collective modules are likely to be implemented in the future.
In the discussion below, the parameters are discussed in terms of kind
. Unlike other SSI module kinds, since coll modules are
selected on a per-communicator basis, the kind
specified as attributes to a parent communicator.
Need to write much more here.
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
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.
The coll modules accept several parameters:
- 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.
- 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.
- 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
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
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
algorithms will be used. The
attribute is checked at every invocation of the
lamssi(7), mpirun(1), LAM User's Guide