[Mpi3-ft] MPI_ANY_SOURCE

[Darius Buntinas]

I suspect that we'd get thread issues with the warning approach:

    Two threads post AS receives.
    Thread A does a Wait and gets the warning, then does a reenable_AS
    Thread B does a Wait and hangs because all of its senders are dead

We didn't have this problem before because thread B's receives would have completed with an error and so it would have gotten an error when it tried to wait on it.

We handled this case by using reader-writer locks to prevent another thread from reenabling anysources between when the user checks that potential senders are alive and when it posts the receive.  Now we'd need to hold the lock between when we check for senders and when the request actually completes.

Does that mean we need to explicitly reenable each AS receive?

-d


On Oct 5, 2011, at 1:34 PM, Josh Hursey wrote:

> Currently we state:
> ----------------------
> 17.6 Point-to-Point Communication
> ...
> When a process detects a new process failure, the ability to perform
> wildcard receives (i.e., receives where MPI_ANY_SOURCE has been
> specified for the source parameter) will be disabled on all
> communicators that contain the failed process. When wildcard receives
> are disabled on a communicator, all pending wildcard receive
> operations on that communicator are completed and an error with class
> MPI_ERR_PROC_FAIL_STOP will be returned for those operations. Any new
> wildcard receive operations posted to a communicator with disabled
> wildcard receives will be immediately completed and return an error
> code of the class MPI_ERR_PROC_FAIL_STOP.
> Wildcard receives can be re-enabled with the
> MPI_COMM_REENABLE_ANY_SOURCE function described below.
> ----------------------
> 
> The problem is that by completing the pending wildcard receives with
> an error, we may cause unintended matching of concurrent receives. For
> example:
> -------------------
> Proc 0                Proc 1
> Irecv(ANY,TAGX); [A]
> Irecv(1, TAGX); [B]
> Waitall()
> /******* Proc 2 fails *********/
>                      Send(0, TAGX) [C]
>                      Send(0, TAGX) [D]
> -------------------
> The intention is that Irecv[A] will match Send[C] and Irecv[B] will
> match Send[D]. But if another process fails (proc 2 in this example),
> then Irecv[A] will complete in error. Then Irecv[B] will match
> Send[C], and Send[D] will remain unmatched.
> 
> 
> We need a mechanism that provides notification to the application
> waiting on an ANY_SOURCE receive that some process failed in this
> communicator while still providing the necessary matching guarantees.
> The user can then choose if they want to cancel the operation or
> continue waiting.
> 
> 
> On the teleconference today, we were thinking through a warning based
> concept. Instead of completing the pending nonblocking ANY_SOURCE
> receives, we would return a special warning code (say
> MPI_WARN_PROC_FAIL_STOP). The message would -not- be completed, but
> return to the user to decide if they wish to continue waiting or
> cancel the offending receive operation.
> 
> For blocking ANY_SOURCE receives, the user would be returned an error
> (MPI_ERR_PROC_FAIL_STOP), and the operation would be completed in
> error. Since we do not have a request handle, there is no way to keep
> this receive active while the situation is resolved. So we complete
> the receive with an error, in a sense canceling the receive operation.
> 
> For nonblocking ANY_SOURCE receives, the user would be returned a
> warning (MPI_WARN_PROC_FAIL_STOP), and the request would remain
> active. The application would then be able to either re-enable
> ANY_SOURCE (via the API call) or cancel the offending receive
> operation. The user is in control over the matching guarantees since
> the request remains active, but cannot complete without action from
> the user.
> 
> 
> In the example above, Proc 0 would return MPI_ERR_IN_STATUS from
> MPI_WAITALL. The status of the active requests would be
> MPI_ERR_PENDING (per section 3.7.3), except for the offending
> ANY_SOURCE operations which will have the error field set to
> MPI_WARN_PROC_FAIL_STOP. So Irecv[A] would be MPI_ERR_PENDING, and
> Irecv[B] would be MPI_WARN_PROC_FAIL_STOP. The user can then associate
> the error with a specific request. Once a request is identified the
> user can either cancel it, or re-enable ANY_SOURCE on the communicator
> and continue waiting.
> 
> 
> So I think this fixes the matching issue with the interface, but I'm
> not sure if it addresses the concerns of the hardware matching folks.
> Additionally, this proposal adds a new state to the request concept
> 'active but not completable without action by the user' and we need to
> think if this requires semantic qualification elsewhere in the
> standard.
> 
> 
> What to folks think about this solution?
> 
> -- Josh
> 
> -- 
> Joshua Hursey
> Postdoctoral Research Associate
> Oak Ridge National Laboratory
> http://users.nccs.gov/~jjhursey
> _______________________________________________
> mpi3-ft mailing list
> mpi3-ft at lists.mpi-forum.org
> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft