3.9. Progress

MPI communication operations or parallel I/O patterns typically comprise several related operations executed in one or multiple MPI processes. Examples are the point-to-point communications with one MPI process executing a send operation and another (or the same) MPI process executing a receive operation, or all MPI processes of a group executing a collective operation.

Within each MPI process parts of the communication or parallel I/O pattern are executed within the MPI procedure calls that belong to the operation in that MPI process, whereas other parts are decoupled MPI activities, i.e., they may be executed within an additional progress thread, offloaded to the network interface controller (NIC), or executed within other MPI procedure calls that are not semantically related to the given communication or parallel I/O pattern.

An MPI procedure invocation is blocked if it delays its return until some specific activity or state-change has occurred in another MPI process. An MPI procedure call that is blocked can be

• a nonlocal MPI procedure call that delays its return until a specific semantically-related MPI call on another MPI process, or
• a local MPI procedure call that delays its return until some unspecific MPI call in another MPI process causes a specific state-change in that other MPI process, or
• an MPI finalization procedure ( MPI_FINALIZE or MPI_SESSION_FINALIZE) that delays its return or exit because this MPI finalization must guarantee that all decoupled MPI activities that are related to that MPI finalization call in the calling MPI process will be executed before this MPI finalization is finished. Note that an MPI finalization procedure may execute attribute deletion callback functions prior to the finalization (see Section Allowing User Functions at MPI Finalization); these callback functions may generate additional decoupled MPI activities.

• MPI_SSEND delays its return until the matching receive operation is started at the destination MPI process (for example, by a call to MPI_RECV or to MPI_IRECV).
• MPI_RECV delays its return until the matching send operation is started at the source MPI process (for example, by a call to MPI_SEND or to MPI_ISEND).

• MPI_RSEND, if the message data cannot be entirely buffered, delays its return until the destination MPI process has received the portion of message data that cannot be buffered, which may require one or more unspecific MPI procedure call(s) at the destination MPI process.
• MPI_RECV, in case the message was buffered at the sending MPI process (e.g. with MPI_BSEND), delays its return until the message is received, which may require one or more unspecific MPI procedure calls at the sending MPI process to send the buffered data.

• blocked MPI procedures, and
• repeatedly called MPI test procedures (see below) that return flag =false.

MPI test procedures are MPI_TEST, MPI_TESTANY, MPI_TESTALL, MPI_TESTSOME, MPI_IPROBE, MPI_IMPROBE, MPI_REQUEST_GET_STATUS, MPI_WIN_TEST, and MPI_PARRIVED.

Strong progress is provided by an MPI implementation if all local procedures return independently of MPI procedure calls in other MPI processes (operation-related or not). An MPI implementation provides weak progress if it does not provide strong progress.

Advice to users.

The type of progress may influence the performance of MPI operations. A correct MPI application must be written under the assumption that only weak progress is provided. Every MPI application that is correct under weak progress will be correctly executed if strong progress is provided. In addition, the MPI standard is designed such that correctness under the assumption of strong progress should imply also correctness if only weak progress is provided by the implementation. ( End of advice to users.)

Rationale.

MPI does not guarantee progress when using synchronization methods that are not based on MPI procedures. Without guaranteed strong progress in MPI this may lead to a deadlock, see for example Section Processes and Example Progress in Section Progress. ( End of rationale.)
For further rules, see in Section MPI Procedures the definition of local MPI procedures, and all references to progress in the general index.