Product specifications

5–Using QLogic MPI
QLogic MPI Details
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Generally, mpirun tries to distribute the specified number of processes evenly
among the nodes listed in the mpihosts file. However, if the number of
processes exceeds the number of nodes listed in the mpihosts file, then some
nodes will be assigned more than one instance of the program.
Another command line option, -ppn, instructs mpirun to assign a fixed number p
of node programs (processes) to each node, as it distributes n instances among
the nodes:
$ mpirun -np n -m mpihosts -ppn p program-name
This option overrides the :process_count specifications, if any, in the lines of
the mpihosts file. As a general rule, mpirun distributes the n node programs
among the nodes without exceeding, on any node, the maximum number of
instances specified by the :process_count option. The value of
the :process_count
option is specified by either the -ppn command line
option or in the mpihosts file.
Typically, the number of node programs should not be larger than the number of
processor cores, at least not for compute-bound programs.
This option specifies the number of processes to spawn. If this option is not set,
then environment variable MPI_NPROCS is checked. If MPI_NPROCS is not set,
the default is to determine the number of processes based on the number of hosts
in the machinefile -M or the list of hosts -H.
-ppn processes-per-node
This option creates up to the specified number of processes per node.
Each node program is started as a process on one node. While a node program
may fork child processes, the children themselves must not call MPI functions.
The -distributed=on|off option has been added to mpirun. This option
reduces overhead by enabling mpirun to start processes in parallel on multiple
nodes. Initially, mpirun spawns one mpirun child per node from the root node,
each of which in turn spawns the number of local processes for that particular
node. Control the use of distributed mpirun job spawning mechanism with this
option:
-distributed [=on|off]
The default is on. To change the default, put this option in the global
mpirun.defaults file or a user-local file. See “Environment for Node
Programs” on page 5-17 and “Environment Variables” on page 5-18 for details.
NOTE:
When the -np value is larger than the number of nodes in the mpihosts file
times the -ppn value, mpirun cycles back through the hostsfile, assigning
additional node programs per host.