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Subject: Re: [OMPI users] Interaction of MPI_Send and MPI_Barrier
From: Richard Treumann (treumann_at_[hidden])
Date: 2009-07-23 17:26:31


No - it is not guaranteed. (it is highly probable though)

The return from the MPI_Send only guarantees that the data is safely held
somewhere other than the send buffer so you are free to modify the send
buffer. The MPI standard does not say where the data is to be held. It only
says that once the MPI_Test is successful, the data will have been
delivered to the receive buffer.

Consider this possible scenario:

MPI_Send is for a small message:
The data is sent toward the destination
To allow the MPI_Send to complete promptly ,lib MPI makes a temporary copy
of the message
The MPI_Send returns once the copy is made
the message gets lost in the network
the MPI_Barrier does its communication without packet loss and completes
the call to MPI_Test returns false
the send side gets no ack for the lost message and eventually retransmits
it from the saved temp
This time it gets through
A later MPI_Test succeeds
An ack eventually gets back to the sender and it throws away the temp copy
of the message it was keeping in case a retransmit was needed

I am not saying any particular MPI library would work this way but it is
one kind of thing that a libmpi might do to give better performance while
maintaining the strict rules of MPI semantic. Since the MPI_Barrier does
not make any guarantee about the destination status of sends done before
it, this kind of optimization is legitimate.

If you must know that the message is received once the barrier returns, you
need to MPI_Wait the message before you call barrier.

            Dick

Dick Treumann - MPI Team
IBM Systems & Technology Group
Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846 Fax (845) 433-8363

users-bounces_at_[hidden] wrote on 07/23/2009 05:02:51 PM:

> [image removed]
>
> [OMPI users] Interaction of MPI_Send and MPI_Barrier
>
> Shaun Jackman
>
> to:
>
> Open MPI
>
> 07/23/2009 05:04 PM
>
> Sent by:
>
> users-bounces_at_[hidden]
>
> Please respond to Open MPI Users
>
> Hi,
>
> Two processes run the following program:
>
> request = MPI_Irecv
> MPI_Send (to the other process)
> MPI_Barrier
> flag = MPI_Test(request)
>
> Without the barrier, there's a race and MPI_Test may or may not return
> true, indicating whether the message has been received. With the
> barrier, is it guaranteed that the message will have been received,
> and MPI_Test will return true?
>
> Cheers,
> Shaun
> _______________________________________________
> users mailing list
> users_at_[hidden]
> http://www.open-mpi.org/mailman/listinfo.cgi/users