The main reason to run statistical computations in parallel is to gain speed, meaning to reduce the execution time of your program or functions. Factors Affecting Speed discusses the main items affecting the speed of programs or functions. Factors Affecting Results discusses details that can cause a parallel run to give different results than a serial run.
Some factors that can affect the speed of execution of parallel processing are:
Parallel environment setup. It takes time to run
begin computing in parallel. If your computation is fast, the setup
time can exceed any time saved by computing in parallel.
Parallel overhead. There is overhead in communication and coordination when running in parallel. If function evaluations are fast, this overhead could be an appreciable part of the total computation time. Thus, solving a problem in parallel can be slower than solving the problem serially. For an example, see Improving Optimization Performance with Parallel Computing in MATLAB® Digest, March 2009.
parfor loops. This
is described in Working with parfor.
not work in parallel when called from within another
If you have programmed your custom functions to take advantage of
parallel processing, the limitation of no nested
can cause a parallel function to run slower than expected.
When executing serially,
run slightly slower than
Passing parameters. Parameters are automatically passed to worker sessions during the execution of parallel computations. If there are many parameters, or they take a large amount of memory, passing parameters can slow the execution of your computation.
Contention for resources: network and computing. If the pool of workers has low bandwidth or high latency, parallel computation can be slow.
Some factors can affect results when using parallel processing.
There are several caveats related to
in parfor Limitations in
the Parallel Computing Toolbox™ documentation. Some important factors
Persistent or global variables. If any functions use persistent or global variables, these variables can take different values on different worker processors. Furthermore, they might not be cleared properly on the worker processors.
Accessing external files. External files can be accessed unpredictably during a parallel computation. The order of computations is not guaranteed during parallel processing, so external files can be accessed in unpredictable order, leading to unpredictable results. Furthermore, if multiple processors try to read an external file simultaneously, the file can become locked, leading to a read error, and halting function execution.
Noncomputational functions, such as
keyboard, can behave badly when used in your
custom functions. When called in a
these functions are executed on worker machines. This can cause a
worker to become nonresponsive, since it is waiting for input.
parfor does not allow
The random numbers you use can affect the results of your computations. See Reproducibility in Parallel Statistical Computations.