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