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Subject: Re: [OMPI users] nonblocking send/receive question
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2010-10-12 07:52:15

On Oct 11, 2010, at 1:29 PM, Bowen Zhou wrote:

> Try MPI_Isend?

'zactly correct.

You currently have an MPI_Wait on the sender side for no reason -- the request is only filled in on the receiver. So you're waiting on an uninitialized variable on the sender.

MPI_Send is a "blocking" send. MPI_Isend is a non-blocking send.
MPI_Recv is a blocking receiver. MPI_Irecv is a non-blocking receiver.

MPI_Send is more-or-less equivalent to MPI_Isend immediately followed by an MPI_Wait on the corresponding request. Ditto with MPI_Recv / MPI_Irecv.

>> I have a glut application I am trying to add MPI to. In the display callback, for rank >= 1, I want to send data to the rank =0 process. I am not concerned at this point about sending data from the rank 0 process back to the rank >= 1 process, so my data is one direction. I would like to do this with non-blocking send/receive but I am not having much success.
>> Within my display callback I do the following:
>> if( myrank == 0 ) {
>> MPI_Irecv( receiveData, DATA_SIZE, MPI_DOUBLE, 1, 19, MPI_COMM_WORLD, &request );
>> MPI_Wait( &request, &status );
>> }
>> else if( myrank == 1 ) {
>> /* Post a receive, send a message, then wait */
>> MPI_Send( sendData, DATA_SIZE, MPI_DOUBLE, 0, 19, MPI_COMM_WORLD );
>> MPI_Wait( &request, &status );
>> }
>> But it appears that the app is still blocking after the MPI_Send.... (I have various debug prints in the actual code, this is stripped down for ease of reading). A sample app that i have that does this works... Is doing this from the glut display call back causing the problem? Any suggestions would be greatly appreciated.
>> Thanks,
>> Ed
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Jeff Squyres
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