247. Communication Calls

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MPI supports the following RMA communication calls: MPI_PUT and MPI_RPUT transfer data from the caller memory (origin) to the target memory; MPI_GET and MPI_RGET transfer data from the target memory to the caller memory; MPI_ACCUMULATE and MPI_RACCUMULATE update locations in the target memory, e.g., by adding to these locations values sent from the caller memory; MPI_GET_ACCUMULATE, MPI_RGET_ACCUMULATE, and MPI_FETCH_AND_OP perform atomic read-modify-write and return the data before the accumulate operation; and MPI_COMPARE_AND_SWAP performs a remote atomic compare and swap operation. These operations are nonblocking: the call initiates the transfer, but the transfer may continue after the call returns. The transfer is completed, at the origin or both the origin and the target, when a subsequent synchronization call is issued by the caller on the involved window object. These synchronization calls are described in Section Synchronization Calls . Transfers can also be completed with calls to flush routines; see Section Flush and Sync for details. For the MPI_RPUT, MPI_RGET, MPI_RACCUMULATE, and MPI_RGET_ACCUMULATE calls, the transfer can be locally completed by using the MPI test or wait operations described in Section Communication Completion .

The local communication buffer of an RMA call should not be updated, and the local communication buffer of a get call should not be accessed after the RMA call until the operation completes at the origin.

The resulting data values, or outcome, of concurrent conflicting accesses to the same memory locations is undefined; if a location is updated by a put or accumulate operation, then the outcome of loads or other RMA operations is undefined until the updating operation has completed at the target. There is one exception to this rule; namely, the same location can be updated by several concurrent accumulate calls, the outcome being as if these updates occurred in some order. In addition, the outcome of concurrent load/store and RMA updates to the same memory location is undefined. These restrictions are described in more detail in Section Semantics and Correctness .

The calls use general datatype arguments to specify communication buffers at the origin and at the target. Thus, a transfer operation may also gather data at the source and scatter it at the destination. However, all arguments specifying both communication buffers are provided by the caller.

For all RMA calls, the target process may be identical with the origin process; i.e., a process may use an RMA operation to move data in its memory.


The choice of supporting ``self-communication'' is the same as for message-passing. It simplifies some coding, and is very useful with accumulate operations, to allow atomic updates of local variables. ( End of rationale.)
MPI_PROC_NULL is a valid target rank in all MPI RMA communication calls. The effect is the same as for MPI_PROC_NULL in MPI point-to-point communication. After any RMA operation with rank MPI_PROC_NULL, it is still necessary to finish the RMA epoch with the synchronization method that started the epoch.

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