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 <robl@mcs.anl.gov> 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.



_______________________________________________
discuss mailing list     discuss@mpich.org
To manage subscription options or unsubscribe:
https://lists.mpich.org/mailman/listinfo/discuss

_______________________________________________
discuss mailing list     discuss@mpich.org
To manage subscription options or unsubscribe:
https://lists.mpich.org/mailman/listinfo/discuss