In this code, all processes post a barrier, sleep a while for good measure, then first Test and then Wait for the barrier. The Test says no and the wait succeeds. A test should be like a non-blocking wait, so it should succeed. What am I missing?

  printf("[%d] posting barrier, then sleeping\n",procid);
  MPI_Request final_barrier;

  int all_done_flag=0;
  printf("[%d] all done: %d\n",procid,all_done_flag);

  printf("[%d] concluded\n",procid);
  • $\begingroup$ If I just do two MPI_Test's in a row instead of one, the second one succeeds. Some googling led me to Why does MPI_Iprobe return false when message has definitely been sent (4 years old now), could the answer there be it? $\endgroup$
    – Kirill
    Jun 5, 2018 at 17:48
  • $\begingroup$ Do you do the two tests both before the wait? I tried two tests followed by a wait, and then the second test is still not true. A test after the wait will succeed (that's what your link is doing: an Iprobe after a blocking Probe) but that's not interesting to me. $\endgroup$ Jun 5, 2018 at 18:12
  • $\begingroup$ I used gist.github.com/ikirill/30118fc521144e0e7d71a066261cbdde. In that answer (not the question) it seems to me they used a busy-loop with only an Iprobe inside. $\endgroup$
    – Kirill
    Jun 5, 2018 at 18:22
  • $\begingroup$ So part of the answer is that Test is a local operation: does only local testing without any interaction with other processes, while Wait is a non-local operation. Another aspect of the problem is "progress": MPI needs to be activated every once in a while for everything to progress as it should. $\endgroup$ Jun 13, 2018 at 13:34


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