My understanding is that this code assumes asynchronous progress.
An example of when the processes hang is as follows:

1) P0 Finishes MCSLockAcquire()
2) P1 is busy waiting in MCSLockAcquire() at
do {
      MPI_Win_sync(win);
   } while (lmem[blocked] == 1);
3) P0 executes MCSLockRelease()
4) P0 waits on MPI_Win_lock_all() inside MCSLockRlease()

Hang!

For P1 to get out of the loop, P0 has to get out of MPI_Win_lock_all() and executes its Compare_and_swap().

For P0 to get out MPI_Win_lock_all(), it needs an ACK from P1 that it got the lock.

P1 does not make communication progress because MPI_Win_sync is not required to do so. It only synchronizes private and public copies.

For this hang to disappear, one can either trigger progress manually by using heavy-duty synchronization calls instead of Win_sync (e.g., Win_unlock_all + Win_lock_all), or enable asynchronous progress.

To enable asynchronous progress in MPICH, set the MPIR_CVAR_ASYNC_PROGRESS env var to 1.

Halim
www.mcs.anl.gov/~aamer

On 3/6/17 1:11 PM, Ask Jakobsen wrote:

 I am testing on x86_64 platform.

I have tried to built both the mpich and the mcs lock code with -O0 to
avoid agressive optimization. After your suggestion I have also tried to
make volatile int *pblocked pointing to lmem[blocked] in the MCSLockAcquire
function and volatile int *pnextrank pointing to lmem[nextRank] in
MCSLockRelease, but it does not appear to make a difference.

On suggestion from Richard Warren I have also tried building the code using
openmpi-2.0.2 without any luck (however it appears to acquire the lock a
couple of extra times before failing) which I find troubling.

I think I will give up using local load/stores and will see if I can figure
out if rewrite using MPI calls like MPI_Fetch_and_op  as you suggest.
Thanks for your help.

On Mon, Mar 6, 2017 at 7:20 PM, Jeff Hammond <jeff.science@gmail.com> wrote:

What processor architecture are you testing?

Maybe set lmem to volatile or read it with MPI_Fetch_and_op rather than a
load.  MPI_Win_sync cannot prevent the compiler from caching *lmem in a
register.

Jeff

On Sat, Mar 4, 2017 at 12:30 AM, Ask Jakobsen <afj@qeye-labs.com> wrote:

Hi,

I have downloaded the source code for the MCS lock from the excellent
book "Using Advanced MPI" from http://www.mcs.anl.gov/researc
h/projects/mpi/usingmpi/examples-advmpi/rma2/mcs-lock.c

I have made a very simple piece of test code for testing the MCS lock but
it works at random and often never escapes the busy loops in the acquire
and release functions (see attached source code). The code appears
semantically correct to my eyes.

#include <stdio.h>
#include <mpi.h>
#include "mcs-lock.h"

int main(int argc, char *argv[])
{
  MPI_Win win;
  MPI_Init( &argc, &argv );

  MCSLockInit(MPI_COMM_WORLD, &win);

  int rank, size;
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  MPI_Comm_size(MPI_COMM_WORLD, &size);

  printf("rank: %d, size: %d\n", rank, size);


  MCSLockAcquire(win);
  printf("rank %d aquired lock\n", rank);   fflush(stdout);
  MCSLockRelease(win);


  MPI_Win_free(&win);
  MPI_Finalize();
  return 0;
}


I have tested on several hardware platforms and mpich-3.2 and mpich-3.3a2
but with no luck.

It appears that the MPI_Win_Sync are not "refreshing" the local data or I
have a bug I can't spot.

A simple unfair lock like http://www.mcs.anl.gov/researc
h/projects/mpi/usingmpi/examples-advmpi/rma2/ga_mutex1.c works perfectly.

Best regards, Ask Jakobsen


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Jeff Hammond
jeff.science@gmail.com
http://jeffhammond.github.io/

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