[mpich-discuss] Isend and Test

[Rob Latham]

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