This is not necessary stated in the standard or guaranteed by it, it's just my intuition... I expect that MPI_User_function is invoked by the thread that invokes the MPI function that uses it, and that you need a mutex to touch shared state associated with MPI_User_function when running with MPI_THREAD_MULTIPLE, but not otherwise. I would hope that MPI implementations do not serialize calls to MPI_User_function in MPI_THREAD_MULTIPLE mode, because there are many cases where MPI_User_function is trivially reentrant and I would like simultaneous progress if I call MPI_Allreduce concurrently from 2+ threads (with different communicators, of course) with the same MPI_User_function. Jeff On Thu, May 25, 2017 at 9:35 AM, Phil Ruffwind <[email protected]> wrote:

Hello all,

I have another question about MPI_User_function. Does MPI make any guarantees regarding (a) whether multiple invocations of MPI_User_function could occur simultaneously within a given process for a given MPI_Op, and regarding (b) which thread MPI_User_function gets called on?

The answer to (a) affects whether MPI_User_function would need any synchronization (like mutexes or semaphores) if it mutates global state. The answer to (b) affects whether it is possible to use thread-local state inside MPI_User_function.

I would appreciate any clarifications. Thanks. Phil _______________________________________________ mpi-forum mailing list [email protected] https://lists.mpi-forum.org/mailman/listinfo/mpi-forum

-- Jeff Hammond [email protected] http://jeffhammond.github.io/