[Mpi-comments] is MPI_Igather thread-safe?

[Joseph Schuchart]

Jonas,

The MPI standard states in §6.12 that

 > All processes must call collective operations (blocking and 
nonblocking) in the same order per communicator.

In your first example (the one posted on SO) this is not the case 
because there is no ordering among the threads. Two processes might post 
Igather with different root arguments in a different order. The 
operations do not match, which is erroneous. The results can a deadlock, 
an error, or the hounds of hell coming at you.

I don't think your second example is correct either: All threads start 
the same operation without inter-thread synchronization so they can be 
interleaved at the MPI level. The Igather that Thread 1 starts in 
Process 1 might be matched with the Igather started on Thread 2 on 
Process 2 (if Thread 1 starts its Igather before Thread 2 on Process 1 
and Thread 2 posts its Igather before Thread 1 on Process 2). So Thread 
1 will receive the wrong data.

HTH
Joseph



On 1/15/22 6:33 AM, Jonas Thies via mpi-comments wrote:
>
> Dear MPI forum,
>
> I posted a technical question on StackOverflow and was advised to ask 
> it in this mailing list.
>
> The original post is here: 
> https://stackoverflow.com/questions/70697604/is-mpi-igather-thread-safe
>
> In a nutshell, I am trying to call independent  MPI_Igathers in an 
> OpenMP-parallel for-loop (after testing for MPI_THREAD_MULTIPLE),
>
> and am getting a deadlock apparently caused by mismatched sends and 
> receives, even in a very much stripped down example.
>
> Any help on wether my code is not standard-conforming would be greatly 
> appreciated. I attached two versions of the code for convenience, one 
> of them is also in the StackOverflow post. The _v2 version does not 
> deadlock but produces incorrect results when run with multiple MPI 
> processes and multiple OpenMP thread per process.
>
> Thanks and best wishes,
>
> Jonas
>
> -- 
> *J. Thies*
> Assistant Professor
>
> TU Delft
> Faculty Electrical Engineering, Mathematics and Computer Science
> Institute of Applied Mathematics and High Performance Computing Center
> Mekelweg 4
> 2628 CD Delft
>
> T +31 15 27 XXXX
> *J.Thies at tudelft.nl*
>
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