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As per my little knowledge mpirun and mpiexec both are launcher. Can anybody tell the exact difference between mpiexec and mpirun?
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Mpiexec is a replacement program for the script mpirun, which is part of the mpich package. It is used to initialize a parallel job from within a PBS batch or interactive environment. Mpiexec uses the task manager library of PBS to spawn copies of the executable on the nodes in a PBS allocation.
"mpirun" is a shell script that attempts to hide the differences in starting jobs for various devices from the user. Mpirun attempts to determine what kind of machine it is running on and start the required number of jobs on that machine.
Hostfiles my_hostfile are simple text files with hosts specified, one per line. Each host can also specify a default and maximum number of slots to be used on that host (i.e., the number of available processors on that host).
OpenMPI will default to the fastest interface for a given job layout. If a job is set to run across multiple nodes, the Infiniband interface will automatically be set. For GPU runs the default byte-transport layer is smcuda. You should not override this (e.g. by setting --mca btl openib,tcp).
mpiexec is defined in the MPI standard (well, the recent versions at least) and I refer you to those (your favourite search engine will find them for you) for details.
mpirun is a command implemented by many MPI implementations. It has never, however, been standardised and there have always been, often subtle, differences between implementations. For details see the documentation of the implementation(s) of your choice.
And yes, they are both used to launch MPI programs, these days mpiexec is generally preferable because it is standardised.
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