On 1/14/21, 11:02 PM, "Brent Morgan via devel" <[email protected]> wrote: Hi Hui Zhou, Robert and I managed to incorporate multiple communicators- we use MPI_Gather() of Send/Receives. However the issue remains- for a small (N<50) # of threads, the calculations work and seem fine. For large (N>=50) # of threads, the issue persists. We will try compiling for ch4 tonight... But I wonder if we're doing something wrong? Should we do Gather of Gathers? Our MPI implementation is merely finding the sum of the results of the N processes, where N is large. Is MPI_Reduce going to be faster? It should *never be slower* to use MPI_Reduce, and it is likely to be faster on most machines you run it on. Ken