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Subject: [OMPI users] MPI_THREAD_SINGLE vs. MPI_THREAD_FUNNELED
From: Jai Dayal (dayalsoap_at_[hidden])
Date: 2013-10-22 12:23:04


Hi All,

I, for the life of me, can't understand the difference between these two
init_thread modes.

MPI_THREAD_SINGLE states that "only one thread will execute", but
MPI_THREAD_FUNNELED states "The process may be multi-threaded, but only the
main thread will make MPI calls (all MPI calls are funneled to the main
thread)."

If I use MPI_THREAD_SINGLE, and just create a bunch of pthreads that dumbly
loop in the background, the MPI library will have no way of detecting this,
nor should this have any affects on the machine.

This is exactly the same as MPI_THREAD_FUNNELED. What exactly does it mean
with "only one thread will execute?" The openmpi library has absolutely
zero way of knowng I've spawned other pthreads, and since these pthreads
aren't actually doing MPI communication, I fail to see how this would
interfere.

I'm asking because I'm using an open_mpi build ontop of infiniband, and the
maximum thread mode is MPI_THREAD_SINGLE.

What am I misunderstanding?

Thanks,
Jai