Rob, Thanks for your reply. Let me rephrase my question. I'm simulating what I do in a complicate code. P0 Isends data to P1, and does computations. P1 needs data to do computation, so it calls Recv. As you said, P0 is calling MPI_Test periodically. What puzzles me is why 2 calls of MPI_Test are needed to let P1 receive the data. (With Intel MPI, the number jumps to around 6.) The number of MPI_Test calls has influence on performance. Thanks. Best regards, Zhen On Mon, Apr 25, 2016 at 4:13 PM, Rob Latham <[email protected]> wrote:
On 04/25/2016 02:57 PM, Zhen Wang wrote:
Hi,
I have questions regarding MPI_Isend and MPI_Test. A sample code is attached, output is as follows.
For instance, both Isend and Recv of 0 start at 15:46:14, but 2 MPI_Test are called before Recv of 0 finishes. My understanding is first Test receives the signal that receiver is ready, and data transfer follows. Then the second Test will see the transfer is complete, and free MPI_Request. Is my understanding correct? Thanks.
MPI_Test() is non-blocking. MPI_Test() is not going to wait for anything to happen -- that's what MPI_Wait() will do.
under the hood, MPI_Test "kicks the progress engine", which means everything that can execute right now will happen. If there is anything that requires code to wait (to receive a signal, in your case), then test will return.
A data transfer might take several calls to MPI_Test to complete. Or, there's a background progress thread that's churning along and you only need to make one call to MPI_Test. It's implementation-dependent.
You are asking the wrong question, though. At this level, it doesn't matter what MPI_Test does. If your code has something it can do while the isend progresses, then go do that and call MPI_Test periodically. If you have nothing productive you can do, call MPI_Wait() (or one of the variants).
==rob
Best regards, Zhen
MPI 1: Recv of 0 started at 15:46:14. MPI 0: Isend of 0 started at 15:46:14. MPI 0: Isend of 1 started at 15:46:14. MPI 0: Isend of 2 started at 15:46:14. MPI 0: Isend of 3 started at 15:46:14. MPI 0: Isend of 4 started at 15:46:14. MPI 0: MPI_Test of 0 at 15:46:16. MPI 0: MPI_Test of 0 at 15:46:18. MPI 0: Isend of 0 finished at 15:46:18. MPI 1: Recv of 0 finished at 15:46:18. MPI 1: Recv of 1 started at 15:46:18. MPI 0: MPI_Test of 1 at 15:46:20. MPI 0: MPI_Test of 1 at 15:46:22. MPI 0: Isend of 1 finished at 15:46:22. MPI 1: Recv of 1 finished at 15:46:22. MPI 1: Recv of 2 started at 15:46:22. MPI 0: MPI_Test of 2 at 15:46:24. MPI 0: MPI_Test of 2 at 15:46:26. MPI 0: Isend of 2 finished at 15:46:26. MPI 1: Recv of 2 finished at 15:46:26. MPI 1: Recv of 3 started at 15:46:26. MPI 0: MPI_Test of 3 at 15:46:28. MPI 0: MPI_Test of 3 at 15:46:30. MPI 0: Isend of 3 finished at 15:46:30. MPI 1: Recv of 3 finished at 15:46:30. MPI 1: Recv of 4 started at 15:46:30. MPI 0: MPI_Test of 4 at 15:46:32. MPI 0: MPI_Test of 4 at 15:46:34. MPI 0: Isend of 4 finished at 15:46:34. MPI 1: Recv of 4 finished at 15:46:34.
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