Hmm, it seems I've stumbled upon a variant of Listing 1.1 in your paper :-) I'm a big fan of buffer pools, so I am unfazed by the potential performance impact of sbrk(). If you need to malloc() so much memory that sbrk() is called, I suspect that the message transfer time is going to take longer than a system call. But I confess to being somewhat ignorant of the performance of slow, general purpose operating systems like Linux. Jeff On Fri, Jul 13, 2012 at 2:19 PM, Torsten Hoefler <[email protected]> wrote:
Jeff,
I was reading through www.unixer.de/publications/img/mprobe-proposal-rev4.pdf and - just for fun - wondered if there is a workaround, not because MPI_Mprobe shouldn't be in the MPI-3 standard, but because some folks might want a workaround for backwards compatibility if they are forced to use MPI-2 somewhere.
I apologize in advance if something equivalent to what I say below was discussed at the Forum and I was not present.
Torsten and coworkers say the following. ============================ For example, the following code can not be executed concurrently by two threads in an MPI process, because a message could be found by the MPI Probe in both threads, while only one of the threads could successfully receive the message (the other will block): MPI_Status status; int value; MPI_Probe(MPI_ANY_SOURCE, /*tag=*/0, MPI_COMM_WORLD, &status); MPI_Recv(&value, 1, MPI_INT, status.MPI_SOURCE, /*tag=*/0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
<snip>
There is no known workaround that addresses all of the problems with MPI Probe and MPI Iprobe in multi-threaded MPI applications. ============================
Obviously, a fat mutex around this block solves the problem, but the time spent in the mutex will scale with the message size. I was curious the following workaround is reasonable when MPI-2 must be used. ============================ MPI_Status status; MPI_Request request; AppropriateMutex mutex; int value;
ACQUIRE_MUTEX(&mutex); MPI_Probe(MPI_ANY_SOURCE, /*tag=*/0, MPI_COMM_WORLD, &status); /* ? */ MPI_Irecv(&value, 1, MPI_INT, status.MPI_SOURCE, /*tag=*/0, MPI_COMM_WORLD, &request); RELEASE_MUTEX(&mutex); MPI_Wait(&request, MPI_STATUS_IGNORE); ============================ This is correct. But most use-cases for probe require a malloc and malloc is not always fast (especially if it needs to execute sbrk). This will still have the contention problem (assume 1000's of cores!). Mprobe allows to use wait-free algorithms for the queue management.
Are there nuances regarding the use of MPI that I have missed? Nope.
Do the real-world use cases have too much to do in the "/* ? */" to make this viable? I think so (at least all use-cases I know). Do you have ones that only require trivial "/* ? */"?
This was brought up at a point when proposals were much harsher reviewed than today and we had many discussions and even a full EuroMPI paper on this http://www.unixer.de/publications/index.php?pub=103 .
All the Best, Torsten
-- ### qreharg rug ebs fv crryF ------------- http://www.unixer.de/ ----- Torsten Hoefler | Performance Modeling and Simulation Lead Blue Waters Directorate | University of Illinois (UIUC) 1205 W Clark Street | Urbana, IL, 61801 NCSA Building | +01 (217) 244-7736 _______________________________________________ mpi-forum mailing list [email protected] http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
-- Jeff Hammond Argonne Leadership Computing Facility University of Chicago Computation Institute [email protected] / (630) 252-5381 http://www.linkedin.com/in/jeffhammond https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond