The basic concept of a parfor-loop
in MATLAB® software is the same as the standard MATLAB for-loop: MATLAB executes a series
of statements (the loop body) over a range of values. The MATLAB client
(where the parfor is issued) coordinates with MATLAB workers
comprising a parallel pool, so that the loop
iterations can be executed in parallel on the pool. The necessary
data on which parfor operates is sent from the
client to workers, where most of the computation happens, and the
results are sent back to the client and pieced together.
Because several MATLAB workers can be computing concurrently
on the same loop, a parfor-loop can provide significantly
better performance than its analogous for-loop.
Each execution of the body of a parfor-loop
is an iteration. MATLAB workers evaluate
iterations in no particular order, and independently of each other.
Because each iteration is independent, there is no guarantee that
the iterations are synchronized in any way, nor is there any need
for this. If the number of workers is equal to the number of loop
iterations, each worker performs one iteration of the loop. If there
are more iterations than workers, some workers perform more than one
loop iteration; in this case, a worker might receive multiple iterations
at once to reduce communication time.
A parfor-loop is useful
in situations where you need many loop iterations of a simple calculation,
such as a Monte Carlo simulation. parfor divides
the loop iterations into groups so that each worker executes some
portion of the total number of iterations. parfor-loops
are also useful when you have loop iterations that take a long time
to execute, because the workers can execute iterations simultaneously.
You
cannot use a parfor-loop when an iteration in your
loop depends on the results of other iterations. Each iteration must
be independent of all others. Since there is a communications cost
involved in a parfor-loop, there might be no advantage
to using one when you have only a small number of simple calculations.
The examples of this section are only to illustrate the behavior of parfor-loops,
not necessarily to show the applications best suited to them.