You can have a situation where the isend/irecv pair completes at process 0 before process 1 has called irecv or waitall. Since process 0 is now busy waiting on the file, it will not make progress on MPI calls and can result in deadlock. ~Jim. On Sat, Oct 10, 2020 at 2:17 PM Skjellum, Anthony <[email protected]> wrote:

Jim, OK, my attempt at answering below.

See if you agree with my annotations.

-Tony

Anthony Skjellum, PhD

Professor of Computer Science and Chair of Excellence

Director, SimCenter

University of Tennessee at Chattanooga (UTC)

[email protected] [or [email protected]]

cell: 205-807-4968

------------------------------ *From:* mpi-forum <[email protected]> on behalf of Jim Dinan via mpi-forum <[email protected]> *Sent:* Saturday, October 10, 2020 1:31 PM *To:* Main MPI Forum mailing list <[email protected]> *Cc:* Jim Dinan <[email protected]> *Subject:* [EXT]: [Mpi-forum] Progress Question

*External Email* Hi All,

A colleague recently asked a question that I wasn't able to answer definitively. Is the following code guaranteed to make progress?

MPI_Barrier(); -- everything is uncertain to within one message, if layered on pt2pt; --- let's assume a power of 2, and recursive doubling (RD). --- At each stage, it posts an irecv and isend to its corresponding element in RD --- All stages must complete to get to the last stage. --- At the last stage, it appears like your example below for N/2 independent process pairs, which appears always to complete. Oif rank == 1 create_file("test") if rank == 0 while not_exists("test") sleep(1);

That is, can rank 1 require rank 0 to make MPI calls after its return from the barrier, in order for rank 1 to complete the barrier? If the code were written as follows:

isend(..., other_rank, &req[0]) irecv(..., other_rank, &req[1]) waitall(2, req) --- Assume both isends buffer on the send-side and return immediately--valid. --- Both irecvs are posted, but unmatched as yet. Nothing has transferred on network. --- Waitall would mark the isends done at once, and work to complete the irecvs; in that process, each would have to progress the isends across the network. On this comm and all comms, incidentally. --- When waitall returns, the data has transferred to the receiver, otherwise the irecvs aren't done. if rank == 1 create_file("test") if rank == 0 while not_exists("test") sleep(1);

I think it would clearly not guarantee progress since the send data can be buffered. Is the same true for barrier?

Cheers, ~Jim. *This message is not from a UTC.EDU <http://UTC.EDU> address. Caution should be used in clicking links and downloading attachments from unknown senders or unexpected email. *