6.4 Synchronization Calls
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:
- 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.
- 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.
- 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 6.1 illustrates the general synchronization
pattern for active target
communication.
Figure 6.1:
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 6.1 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 6.2.
Figure 6.2:
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 6.3 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 6.3:
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.
Subsections
MPI-Standard for MARMOT