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<h1 class="topictitle1">Automatic Chunking</h1>
<div>
<p>A parallel loop construct incurs overhead cost for every chunk of work
that it schedules. Since version 2.2, Intel® Threading Building Blocks (Intel® TBB) chooses chunk sizes
automatically, depending upon load balancing needs.<a href="#ftn1"><sup><sup>[1]</sup></sup></a>
The heuristic attempts to limit overheads while still providing ample
opportunities for load balancing.
</p>
<div class="Note"><h3 class="NoteTipHead">
Caution</h3>
<p>Typically a loop needs to take at least a million clock cycles to make
it worth using
<samp class="codeph">parallel_for</samp>. For example, a loop that takes at least
500 microseconds on a 2 GHz processor might benefit from
<samp class="codeph">parallel_for</samp>.
</p>
</div>
<p>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.
</p>
</div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="../tbb_userguide/parallel_for.htm">parallel_for</a></div>
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<div class="See Also">
<h2>See Also</h2>
<div class="linklist">
<div><a href="Controlling_Chunking.htm#tutorial_Controlling_Chunking">Controlling Chunking
</a></div></div>
</div>
<p class="tfootnote"><a id="ftn1"><sup>[1]</sup></a> In Intel® TBB 2.1, the default
was
<em>not</em> automatic. Compile with
<samp class="codeph">TBB_DEPRECATED=1</samp> to get the old default behavior.</p>
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