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<h1 class="topictitle1">Task Scheduler</h1>
<div>
<div class="section">
<p>Intel® Threading Building Blocks (Intel® TBB)
provides a task scheduler, which is the engine that drives the algorithm
templates and task groups. You may also call it directly. Using tasks is often
simpler and more efficient than using threads, because the task scheduler takes
care of a lot of details.
</p>
<p>The tasks are quanta of computation. The scheduler
maps these onto physical threads. The mapping is non-preemptive. Each thread
has a method
<samp class="codeph">execute(</samp>). Once a thread starts running
<samp class="codeph">execute()</samp>, the task is bound to that thread until
<samp class="codeph">execute()</samp> returns. During that time, the thread
services other tasks only when it waits on its predecessor tasks, at which time
it may run the predecessor tasks, or if there are no pending predecessor tasks,
the thread may service tasks created by other threads.
</p>
<p>The task scheduler is intended for parallelizing
computationally intensive work. Because task objects are not scheduled
preemptively, they should generally avoid making calls that might block for
long periods, because meanwhile that thread is precluded from servicing other
tasks.
</p>
<div class="Note"><h3 class="NoteTipHead">
Caution</h3>
<p>There is no guarantee that
<em>potentially</em> parallel tasks
<em>actually</em> execute in parallel, because the scheduler adjusts
actual parallelism to fit available worker threads. For example, given a single
worker thread, the scheduler creates no actual parallelism. For example, it is
generally unsafe to use tasks in a producer consumer relationship, because
there is no guarantee that the consumer runs at all while the producer is
running.
</p>
</div>
<p>Potential parallelism is typically generated by a
<em>split/join</em> pattern. Two basic patterns of split/join are
supported. The most efficient is continuation-passing form, in which the
programmer constructs an explicit "continuation" task. The parent task creates
child tasks and specifies a continuation task to be executed when the children
complete. The continuation inherits the parent's ancestor. The parent task then
exits; it does not block on its children. The children subsequently run, and
after they (or their continuations) finish, the continuation task starts
running. The figure, "Continuation-passing Style," shows the steps. The running
tasks at each step are shaded.
</p>
<div class="fignone" id="fig4"><a name="fig4"><!-- --></a><span class="figcap">Continuation-passing Style</span>
<br><img src="Resources/06000005.png"><br>
</div>
<p>Explicit continuation passing is efficient, because
it decouples the thread's stack from the tasks. However, it is more difficult
to program. A second pattern is "blocking style", which uses implicit
continuations. It is sometimes less efficient in performance, but more
convenient to program. In this pattern, the parent task blocks until its
children complete, as shown in the figure below.
</p>
<div class="fignone" id="fig5"><a name="fig5"><!-- --></a><span class="figcap">Blocking Style</span>
<br><img src="Resources/08000006.png"><br>
</div>
<p>The convenience comes with a price. Because the
parent blocks, its thread's stack cannot be popped yet. The thread must be
careful about what work it takes on, because continually stealing and blocking
could cause the stack to grow without bound. To solve this problem, the
scheduler constrains a blocked thread such that it never executes a task that
is less deep than its deepest blocked task. This constraint may impact
performance because it limits available parallelism, and tends to cause threads
to select smaller (deeper) subtrees than they would otherwise choose.
</p>
</div>
</div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="../reference/reference.htm">Intel® Threading Building Blocks Reference Manual</a></div>
</div>
<div class="See Also">
<ul class="ullinks">
<li class="ulchildlink"><a href="../reference/task_scheduler/scheduling_algorithm.htm">Scheduling Algorithm</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/task_scheduler_init_cls.htm">task_scheduler_init Class</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/task_cls.htm">task Class</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/task_allocation.htm">task Allocation</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/explicit_task_destruction.htm">Explicit task Destruction</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/recycling_tasks.htm">Recycling Tasks</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/synchronization.htm">Synchronization</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/task_context.htm">task Context</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/cancellation.htm">Cancellation</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/priorities.htm">Priorities</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/affinity.htm">Affinity</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/task_debugging.htm">task Debugging</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/empty_task_cls.htm">empty_task Class</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/task_list_cls.htm">task_list Class</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/task_group_context.htm">task_group_context</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/task_scheduler_observer.htm">task_scheduler_observer</a><br>
</li>
<li class="ulchildlink"><a href="../reference/task_scheduler/catalog_of_recommended_task_patterns.htm">Catalog of Recommended task Patterns</a><br>
</li>
</ul>
<h2>See Also</h2>
<div class="linklist">
<div><a href="algorithms.htm">Algorithms
</a></div>
<div><a href="task_groups.htm">Task Groups
</a></div></div>
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