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parfor is a Parallel Computing Toolbox™ function similar to a for loop. Parallel statistical functions call parfor internally. parfor distributes computations to worker processors.
More caveats related to parfor appear in parfor Limitations in the Parallel Computing Toolbox documentation.
parfor does not work in parallel when called from within another parfor loop, or from an spmd block. Parallelization occurs only at the outermost level.
Suppose, for example, you want to apply jackknife to your function userfcn, which calls parfor, and you want to call jackknife in a loop. The following figure shows three cases:
The outermost loop is parfor. Only that loop runs in parallel.
The outermost parfor loop is in jackknife. Only jackknife runs in parallel.
The outermost parfor loop is in userfcn. userfcn uses parfor in parallel.
When parfor Runs in Parallel