# 6.4. Synchronization Calls

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RMA communications fall in two categories:

- active target communication, where data is moved from the memory of one process to the memory of another, and both are explicitly involved in the communication. This communication pattern is similar to message passing, except that all the data transfer arguments are provided by one process, and the second process only participates in the synchronization.
- passive target communication, where data is moved from the memory of one process to the memory of another, and only the origin process is explicitly involved in the transfer. Thus, two origin processes may communicate by accessing the same location in a target window. The process that owns the target window may be distinct from the two communicating processes, in which case it does not participate explicitly in the communication. This communication paradigm is closest to a shared memory model, where shared data can be accessed by all processes, irrespective of location.

RMA communication calls with argument win must occur at a process only within an access epoch for win. Such an epoch starts with an RMA synchronization call on win; it proceeds with zero or more RMA communication calls ( MPI_PUT, MPI_GET or MPI_ACCUMULATE) on win; it completes with another synchronization call on win. This allows users to amortize one synchronization with multiple data transfers and provide implementors more flexibility in the implementation of RMA operations.

Distinct access epochs for win at the same process must be disjoint. On the other hand, epochs pertaining to different win arguments may overlap. Local operations or other MPI calls may also occur during an epoch.

In active target communication, a target window can be accessed by RMA operations only within an exposure epoch. Such an epoch is started and completed by RMA synchronization calls executed by the target process. Distinct exposure epochs at a process on the same window must be disjoint, but such an exposure epoch may overlap with exposure epochs on other windows or with access epochs for the same or other win arguments. There is a one-to-one matching between access epochs at origin processes and exposure epochs on target processes: RMA operations issued by an origin process for a target window will access that target window during the same exposure epoch if and only if they were issued during the same access epoch.

In passive target communication the target process does not execute RMA synchronization calls, and there is no concept of an exposure epoch.

MPI provides three synchronization mechanisms:

1. The MPI_WIN_FENCE collective synchronization call supports a simple synchronization pattern that is often used in parallel computations: namely a loosely-synchronous model, where global computation phases alternate with global communication phases. This mechanism is most useful for loosely synchronous algorithms where the graph of communicating processes changes very frequently, or where each process communicates with many others.

This call is used for active target communication. An access epoch at an origin process or an exposure epoch at a target process are started and completed by calls to MPI_WIN_FENCE. A process can access windows at all processes in the group of win during such an access epoch, and the local window can be accessed by all processes in the group of win during such an exposure epoch.
2. The four functions MPI_WIN_START, MPI_WIN_COMPLETE, MPI_WIN_POST and MPI_WIN_WAIT can be used to restrict synchronization to the minimum: only pairs of communicating processes synchronize, and they do so only when a synchronization is needed to order correctly RMA accesses to a window with respect to local accesses to that same window. This mechanism may be more efficient when each process communicates with few (logical) neighbors, and the communication graph is fixed or changes infrequently.

These calls are used for active target communication. An access epoch is started at the origin process by a call to MPI_WIN_START and is terminated by a call to MPI_WIN_COMPLETE. The start call has a group argument that specifies the group of target processes for that epoch. An exposure epoch is started at the target process by a call to MPI_WIN_POST and is completed by a call to MPI_WIN_WAIT. The post call has a group argument that specifies the set of origin processes for that epoch.
3. Finally, shared and exclusive locks are provided by the two functions MPI_WIN_LOCK and MPI_WIN_UNLOCK. Lock synchronization is useful for MPI applications that emulate a shared memory model via MPI calls; e.g., in a ``billboard'' model, where processes can, at random times, access or update different parts of the billboard.

These two calls provide passive target communication. An access epoch is started by a call to MPI_WIN_LOCK and terminated by a call to MPI_WIN_UNLOCK. Only one target window can be accessed during that epoch with win.

Figure 1 illustrates the general synchronization pattern for active target communication.

Figure 1: active target communication. Dashed arrows represent synchronizations (ordering of events).

The synchronization between `post` and `start` ensures that the put call of the origin process does not start until the target process exposes the window (with the `post` call); the target process will expose the window only after preceding local accesses to the window have completed. The synchronization between `complete` and `wait` ensures that the put call of the origin process completes before the window is unexposed (with the `wait` call). The target process will execute following local accesses to the target window only after the `wait` returned.

Figure 1 shows operations occurring in the natural temporal order implied by the synchronizations: the `post` occurs before the matching `start`, and `complete` occurs before the matching `wait`. However, such strong synchronization is more than needed for correct ordering of window accesses. The semantics of MPI calls allow weak synchronization, as illustrated in Figure 2 .

Figure 2: active target communication, with weak synchronization. Dashed arrows represent synchronizations (ordering of events)

The access to the target window is delayed until the window is exposed, after the `post`. However the `start` may complete earlier; the `put` and `complete` may also terminate earlier, if put data is buffered by the implementation. The synchronization calls order correctly window accesses, but do not necessarily synchronize other operations. This weaker synchronization semantic allows for more efficient implementations.

Figure 3 illustrates the general synchronization pattern for passive target communication. The first origin process communicates data to the second origin process, through the memory of the target process; the target process is not explicitly involved in the communication.

Figure 3: passive target communication. Dashed arrows represent synchronizations (ordering of events).

The `lock` and `unlock` calls ensure that the two RMA accesses do not occur concurrently. However, they do not ensure that the `put` by origin 1 will precede the `get` by origin 2.

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