[Mpi3-ft] MPI_Recv + MPI_Comm_failure_ack

[David Solt]

I'm pretty sure the intent was that MPI_Recv should NOT return 
MPI_ERR_PENDING as there is no request on which the error can be pending, 
but I don't know if much thought was given to allowing MPI_Recv to ignore 
acknowledge ranks.
Dave



From:   Wesley Bland <wbland at icl.utk.edu>
To:     "MPI 3.0 Fault Tolerance and Dynamic Process Control working 
Group" <mpi3-ft at lists.mpi-forum.org>, 
Date:   03/15/2013 12:45 PM
Subject:        Re: [Mpi3-ft] MPI_Recv + MPI_Comm_failure_ack
Sent by:        mpi3-ft-bounces at lists.mpi-forum.org



I think you are correct in your evaluation, though I also think that 
wasn't our intent. I think the intent (unless I'm forgetting a discussion) 
was to allow MPI_ERR_PENDING to be returned by MPI_RECV and let 
MPI_COMM_FAILURE_ACK cover both cases. Can anyone else confirm that this 
was the goal.

If that's the case, it's something we'll need to fix in the text.

Thanks,
Wesley


On Fri, Mar 15, 2013 at 12:32 PM, David Solt <dsolt at us.ibm.com> wrote:
Based on the proposal: 

MPI_Comm_failure_ack(blah, blah) 

This local operation gives the users a way to acknowledge all locally 
noticed failures on 
comm. After the call, unmatched MPI_ANY_SOURCE receptions that would have 
raised an 
error code MPI_ERR_PENDING due to process failure (see Section 17.2.2) 
proceed without 
further reporting of errors due to those acknowledged failures. 

I think this clearly indicates that MPI_Recv is uninfluenced by calls to 
MPI_Comm_failure_ack.  Therefore, there is no way to call 
MPI_Recv(MPI_ANY_SOURCE) and ignore failures reported by 
MPI_Comm_failure_ack.   

I believe the following code will NOT work (i.e. after the first failure, 
the MPI_Recv will continuously fail): 


MPI_Comm_size(intercomm, &size); 
while (failures < size) { 
        err = MPI_Recv(blah, blah, MPI_ANY_SOURCE, intercomm, &status); 
        if (err == MPI_PROC_FAILED) { 
                MPI_Comm_failure_ack(intercomm); 
                MPI_Comm_failure_get_acked(intercomm, &group); 
                MPI_Group_size(group, &failures); 
        } else { 
                /* process received data */ 
        } 
} 

and has to be written as: 

MPI_Comm_size(intercomm, &size); 
while (failures < size) { 

        if (request == MPI_REQUEST_NULL) { 
                err = MPI_Irecv(blah, blah, MPI_ANY_SOURCE, intercomm, 
&request); 
        } 
        err = MPI_Wait(&request, &status); 

        if (err == MPI_ERR_PENDING) { 
                MPI_Comm_failure_ack(intercomm); 
                MPI_Comm_failure_get_acked(intercomm, &group); 
                MPI_Group_size(group, &failures); 
        } else { 
                /* process received data */ 
        } 
} 

Am I correct in my thinking? 
If so, was there a reason why MPI_Recv could not also "obey" 
MPI_Comm_failure_ack calls? 

Thanks, 
Dave
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