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