[mpich-discuss] MCS lock and MPI RMA problem

[Ask Jakobsen]

 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 at 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 at 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 at gmail.com
> http://jeffhammond.github.io/
>
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-- 
*Ask Jakobsen*
R&D

Qeye Labs
Lersø Parkallé 107
2100 Copenhagen Ø
Denmark

mobile: +45 2834 6936
email: afj at Qeye-Labs.com
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