Why does MPICH need to dupe the comm for every window? I guess performing a collective in MPI_WIN_FENCE is one reason. Does MPI_WIN_FREE need an internal comm? Are there others? If I were to provide an info saying that I only use passive target, do you still need to dupe the comm? If there is any way to avoid one comm per window, I would like to do that. And yes, I am indeed asserting that 2048 windows is not enough. Thanks, Jeff jrhammon-mac01:ticket459 jrhammon$ mpiexec -n 2 ./test_win_sync.x 10000 Fatal error in MPI_Win_allocate_shared: Other MPI error, error stack: MPI_Win_allocate_shared(162).........: MPI_Win_allocate_shared(size=4, MPI_INFO_NULL, comm=0x84000004, baseptr=0x7fff53c163c0, win=0x7fff53c0a780) failed MPID_Win_allocate_shared(248)........: win_init(294)........................: MPIR_Comm_dup_impl(57)...............: MPIR_Comm_copy(1773).................: MPIR_Get_contextid(829)..............: MPIR_Get_contextid_sparse_group(1203): Too many communicators (0/2048 free on this process; ignore_id=0) Fatal error in MPI_Win_allocate_shared: Other MPI error, error stack: MPI_Win_allocate_shared(162).........: MPI_Win_allocate_shared(size=4, MPI_INFO_NULL, comm=0x84000002, baseptr=0x7fff572853c0, win=0x7fff57279780) failed MPID_Win_allocate_shared(248)........: win_init(294)........................: MPIR_Comm_dup_impl(57)...............: MPIR_Comm_copy(1773).................: MPIR_Get_contextid(829)..............: MPIR_Get_contextid_sparse_group(1203): Too many communicators (0/2048 free on this process; ignore_id=0) -- Jeff Hammond [email protected] http://jeffhammond.github.io/