I haven't worked on rma before but was working on a problem and ran into this **comment in MPI_Win_post: "Starts an RMA exposure epoch for the local window associated with win. **Only the processes belonging to group should access the window with RMA calls on win during this epoch. Each process in group must issue a matching call to MPI_Win_start. MPI_Win_post does not block." Would overlapping epochs be violating the ** line? I decided I probably need to support this but I wondered if it's bending or breaking the 'rules'? The problem (code at the bottom of this email) is using a cartesian communicator and alternating "left/right' accumulates with 'up/down' accumulates on a single win. So: - Ranks 0,1,2,3 are doing a left/right accumulate. - Ranks 4,5,6,7 are doing a left/right accumulate. - ... and then sometimes... - Ranks 0,1,2,3 complete and enter the 'up/down' accumulate epoch -- Rank 0 does MPI_Win_post to ranks 4,12 -- Rank 1 doesn MPI_Win_post to ranks 5,13 ... So is Rank 0 posting to Rank 4 while 4 is still in the epoch with 5/6/7 a violation of "Only the processes belonging to group should access the window with RMA calls on win during this epoch"? From Rank 4's point of view, rank 0 isn't in the group for the current win/epoch. Putting a barrier (or something) in between or using two different win's fixes it. I like using two win's since it separates the epochs and clearly doesn't use the wrong group/rank on the win. /* RMA transfers in left-right direction */ MPI_Win_post(grp_lr, 0, win); MPI_Win_start(grp_lr, 0, win); MPI_Accumulate(&i, 1, MPI_INT, ranks_lr[LEFT] , 0, 1, MPI_INT, MPI_SUM, win); MPI_Accumulate(&i, 1, MPI_INT, ranks_lr[RIGHT], 0, 1, MPI_INT, MPI_SUM, win); MPI_Win_complete(win); MPI_Win_wait(win); /* RMA transfers in up-down direction */ MPI_Win_post(grp_ud, 0, win); MPI_Win_start(grp_ud, 0, win); MPI_Accumulate(&i, 1, MPI_INT, ranks_ud[UP] , 0, 1, MPI_INT, MPI_SUM, win); MPI_Accumulate(&i, 1, MPI_INT, ranks_ud[DOWN], 0, 1, MPI_INT, MPI_SUM, win); MPI_Win_complete(win); MPI_Win_wait(win);