Re: [Mpi3-ft] MPI_Recv + MPI_Comm_failure_ack

15 Mar 2013


      You're right. A blocking call shouldn't return MPI_ERR_PENDING when there
is no request to be pending. I did think we'd covered this some other way.
It's definitely the intent for both versions of receive to be able to
ignore acknowledged failures.


On Fri, Mar 15, 2013 at 1:53 PM, David Solt <[email protected]> wrote:
...
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 <[email protected]>
To:        "MPI 3.0 Fault Tolerance and Dynamic Process Control working
Group" <[email protected]>,
Date:        03/15/2013 12:45 PM
Subject:        Re: [Mpi3-ft] MPI_Recv + MPI_Comm_failure_ack
Sent by:        [email protected]
------------------------------
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 <*[email protected]*<[email protected]>>
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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