(I decided to split this into its own thread of discussion from what was developing on [1]) Recently it was mentioned on the list [1] that users would like the ability to validate multiple communicators (or windows or file handles) in a single collective call. We have discussed this a few times in the past. Defining the interface and semantics appropriately has prevented us from defining such an operation so far. So though we saw the desire for such an operation we pushed it to a separate follow-on ticket until we could get it right. [1] http://lists.mpi-forum.org/mpi3-ft/2011/09/0836.php Problem: -------------------- MPI_Comm_validate(comm, failed_grp) The current operation validates one communicator, and returns the group of globally agreed upon failed processes associated with that communicator. Users would like the ability to, in a single collective call, validate multiple communicators. This would likely result in better performance over calling MPI_Comm_validate over each individual communicator. Workaround: -------------------- Call MPI_Comm_validate over all of the communicators individually. This would involve 'num_comms' collective operations and likely impede scalability. for(i=0; i < num_comms; ++i) { MPI_Comm_validate(comm[i], failed_grps[i]); } Option A: Array of communicators -------------------- MPI_Comm_validate_many(comm[], num_comms, failed_grp) Validate 'num_comms' communicators, and return a failed group. - or - MPI_Comm_validate_many(comm[], num_comms, failed_grps[]) Validate 'num_comms' communicators, and return a failed group for each communicator. ---- In this version of the operation the user passes in an array of pointers to communicators. Since communicators are not often created in a contiguous array, pointers to communications should probably be used. The failed_grps is an array of failures in each of those communicators. Some questions: * Should all processes pass in the same set of communicators at all processes? * Should all communicators be duplicates or subsets of one another? * Does this operation run the risk of a circular dependency if the user does not pass in the same set of communicators at all participating processes? Is that something the MPI library should protect the application from? Option B: Implicit inherited validation -------------------- MPI_Comm_validate_many(comm, failed_grp) ---- The idea is to add an additional semantic (or maybe new API) to allows the validation of a communicator to automatically validates all communicators created from it (only dups and subsets of it?). The problem with this is that if an application calls MPI_Comm_validate on MPI_COMM_WORLD, it changes the semantics of communicators that libraries might be using internally without notification in those libraries. So this breaks the abstraction barrier between the two in possibly a dangerous way. Some questions: * Are there some other semantics that we can add to help protect libraries? (e.g., after implicit validation the first use of the communicator will return a special error code indicating that the communicator has been adjusted). * Are there thread safety issues involved with this? (e.g., the library operates in a concurrent thread with its own duplicate of the communicator. The application does not know about or control the concurrent thread but calls MPI_Comm_validate on its own communicator and implicitly changes the semantics of the duplicate communicator.) * It is only through the call to MPI_Comm_validate that we can provide a uniform group of failed processes globally known. For those that were implicitly validated, do we need to provide a way to access this group after the call? Does this have implications on the amount of storage required for this semantic? Option C: Explicit inherited validation -------------------- MPI_Comm_validate_link(commA, commB); MPI_Comm_validate_many(commA, failed_grp) /* Implies MPI_Comm_validate(commB, NULL) */ MPI_Comm_validate(commA, failed_grp) /* Does not imply MPI_Comm_validate(commB, NULL) */ ---- In this version the application explicitly links communicators. This prevents an application from implicitly altering derived communicators out of their scope (e.g., in use by other libraries). Some questions: * It is only through the call to MPI_Comm_validate that we can provide a uniform group of failed processes globally known. For those that were implicitly validated, do we need to provide a way to access this group after the call (e.g., for commB)? Does this have implications on the amount of storage required for this semantic? * Do we need a mechanism to 'unlink' communicators? Or determine which communicators are linked? * Can a communicator be linked to multiple other communicators? * Is the linking a unidirectional operation? (so in the example above validating commB does not validate commA unless there is a separate MPI_Comm_validate_link(commB, commA) call) Option D: Other -------------------- Something else... Thoughts? -- Josh -- Joshua Hursey Postdoctoral Research Associate Oak Ridge National Laboratory http://users.nccs.gov/~jjhursey