Automatic Chunking
A parallel loop construct incurs overhead cost for every chunk of work
that it schedules. Intel® oneAPI Threading Building Blocks (oneTBB)
chooses chunk sizes automatically, depending upon load balancing
needs. The heuristic attempts to limit overheads while
still providing ample opportunities for load balancing.
Typically a loop needs to take at least a million clock cycles to
make it worth using
parallel_for
. For example, a loop that takes
at least 500 microseconds on a 2 GHz processor might benefit from
parallel_for
.The default automatic chunking is recommended for most uses. As with
most heuristics, however, there are situations where controlling the
chunk size more precisely might yield better performance.