[mpich-discuss] Hang in MPI_Win_Flush during scenario in one-sided communication

[Rajeev Thakur]

For this to work you need to enable asynchronous progress in MPICH at least on the target side. You can do that by setting the environment variable MPICH_ASYNC_PROGRESS to 1 before running mpiexec.

Rajeev

On Sep 22, 2014, at 4:53 PM, Neil Spruit <nrspruit at gmail.com>
 wrote:

> Hello,
> 
>  
> I have been experimenting with mpich’s implementation of one-sided communication with no remote synchronization that was introduced in the mpi3.0 spec and I have run into a hang issue during connect/accept MPI scenarios.
> 
> 
>  
> Here is the situation,  I have two applications that connect to each other after they have been launched separately with mpiexec on two different hosts.
> 
>  
> After connecting, both hosts allocate memory and run MPI_Win_Create as the following:
> 
> Host1_program:
> 
>    void* buffer = mmap(0, buf_length, PROT_READ | PROT_WRITE, MAP_SHARED|MAP_POPULATE|MAP_ANON, -1, 0);
> 
>     MPI_Intercomm_merge(connected_comm, 1, &comm);
> 
>     MPI_Comm_rank(comm, &myrank);
> 
>     MPI_Win_create(NULL, 0, 1, MPI_INFO_NULL, comm, &win);
> 
> Host2_program:
> 
>    void* buffer = mmap(0, buf_length, PROT_READ | PROT_WRITE, MAP_SHARED|MAP_POPULATE|MAP_ANON, -1, 0);
> 
>     MPI_Intercomm_merge(connected_comm, 1, &comm);
> 
>     MPI_Comm_rank(comm, &myrank);
> 
>     MPI_Win_create(buffer, buf_length, 1, MPI_INFO_NULL, comm, &win);
> 
>  
>  
> Once the MPI_Window has been created I execute the following on host1:
> 
>  Host1_program(continued):
> 
>        err = MPI_Win_lock(MPI_LOCK_SHARED, target_rank, 0, win);
> 
>         if(err != MPI_SUCCESS)
> 
>         {
> 
>             return 1;
> 
>         }
> 
>         err = MPI_Put(buffer, buf_length, MPI_BYTE, target_rank, 0, buf_length, MPI_BYTE, win);
> 
>         if(err != MPI_SUCCESS)
> 
>         {
> 
>             return 1;
> 
>         }
> 
>         err = MPI_Win_flush(target_rank, win);
> 
>         if(err != MPI_SUCCESS)
> 
>         {
> 
>             return 1;
> 
>         }
> 
>         err = MPI_Win_unlock(target_rank, win);
> 
>         if(err != MPI_SUCCESS)
> 
>         {
> 
>             return 1;
> 
>         }
> 
> While this is running, host2’s program is simply sleeping in a getchar().
> 
>  
> This should run and complete, however, if the other host has not called another mpi function such as MPI_Recv and instead is in a sleep, getchar(), etc. the MPI_Win_flush above will simply hang.
> 
>  
> Why is the one-sided operation hanging until another MPI operation is called on the target host? My understanding is that utilizing MPI_Win_Flush does not require any action by the target host to complete the operation.
> 
>  
> This is a stack trace of where the flush gets stuck:
> 
> #0  0x0000003b456dea28 in poll () from /lib64/libc.so.6
> 
> #1  0x00007fe1c08004ee in MPID_nem_tcp_connpoll ()
> 
>    from /usr/local/lib/libmpich.so.12
> 
> #2  0x00007fe1c07efab7 in MPIDI_CH3I_Progress ()
> 
>    from /usr/local/lib/libmpich.so.12
> 
> #3  0x00007fe1c07d35ff in MPIDI_CH3I_Wait_for_lock_granted ()
> 
>    from /usr/local/lib/libmpich.so.12
> 
> #4  0x00007fe1c07d88bf in MPIDI_Win_flush () from /usr/local/lib/libmpich.so.12
> 
> #5  0x00007fe1c08c1b46 in PMPI_Win_flush () from /usr/local/lib/libmpich.so.12
> 
> #6  0x000000000040115c in main ()
> 
>  
> According to this it seems that somehow the remote host is holding something after MPI_Win_Create that in my scenario gets unlocked if I call MPI_Recv on the connected_comm from above after the MPI_Win_Create.
> 
>  
> Thank you for your time and I look forward to your reply.
> 
>  
> Respectfully,
> 
> Neil Spruit
> 
> 
> 
> P.S. MPICH version info
> 
>     Version:                                 3.1
> 
>     Release Date:                        Thu Feb 20 11:41:13 CST 2014
> 
>  
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