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<h1 class="topictitle1">Task-Based Programming</h1>
<div>
<p>When striving for performance, programming in terms of threads can be a
poor way to do multithreaded programming. It is much better to formulate your
program in terms of
<em>logical tasks</em>, not threads, for several reasons.
</p>
<ul type="disc">
<li>
<p>Matching parallelism to available resources
</p>
</li>
<li>
<p>Faster task startup and shutdown
</p>
</li>
<li>
<p>More efficient evaluation order
</p>
</li>
<li>
<p>Improved load balancing
</p>
</li>
<li>
<p>Higher–level thinking
</p>
</li>
</ul>
<p>The following paragraphs explain these points in detail.
</p>
<p>The threads you create with a threading package are
<em>logical</em> threads, which map onto the
<em>physical threads</em> of the hardware. For computations that do not
wait on external devices, highest efficiency usually occurs when there is
exactly one running logical thread per physical thread. Otherwise, there can be
inefficiencies from the mismatch<em>. Undersubscription</em> occurs when there
are not enough running logical threads to keep the physical threads working.
<em>Oversubscription</em> occurs when there are more running logical
threads than physical threads. Oversubscription usually leads to
<em>time sliced</em> execution of logical threads, which incurs overheads
as discussed in Appendix A,
<em>Costs of Time Slicing</em>. The scheduler tries to avoid
oversubscription, by having one logical thread per physical thread, and mapping
tasks to logical threads, in a way that tolerates interference by other threads
from the same or other processes.
</p>
<p>The key advantage of tasks versus logical threads is that tasks are much
<em>lighter weight</em> than logical threads. On Linux systems, starting
and terminating a task is about 18 times faster than starting and terminating a
thread. On Windows systems, the ratio is more than 100. This is because a
thread has its own copy of a lot of resources, such as register state and a
stack. On Linux, a thread even has its own process id. A task in Intel®
Threading Building Blocks, in contrast, is typically a small routine, and also,
cannot be preempted at the task level (though its logical thread can be
preempted).
</p>
<p>Tasks in Intel® Threading Building Blocks are efficient too because
<em>the scheduler is unfair.</em> Thread schedulers typically distribute
time slices in a round-robin fashion. This distribution is called "fair",
because each logical thread gets its fair share of time. Thread schedulers are
typically fair because it is the safest strategy to undertake without
understanding the higher-level organization of a program. In task-based
programming, the task scheduler does have some higher-level information, and so
can sacrifice fairness for efficiency. Indeed, it often delays starting a task
until it can make useful progress.
</p>
<p>The scheduler does
<em>load balancing.</em> In addition to using the right number of threads,
it is important to distribute work evenly across those threads. As long as you
break your program into enough small tasks, the scheduler usually does a good
job of assigning tasks to threads to balance load. With thread-based
programming, you are often stuck dealing with load-balancing
yourself, which can be tricky to get right.
</p>
<div class="Note"><h3 class="NoteTipHead">
Tip</h3>
<p>Design your programs to try to create many more tasks than there are
threads, and let the task scheduler choose the mapping from tasks to threads.
</p>
</div>
<p>Finally, the main advantage of using tasks instead of threads is that
they let you think at a higher, task-based, level. With thread-based
programming, you are forced to think at the low level of physical threads to
get good efficiency, because you have one logical thread per physical thread to
avoid undersubscription or oversubscription. You also have to deal with the
relatively coarse grain of threads. With tasks, you can concentrate on the
logical dependences between tasks, and leave the efficient scheduling to the
scheduler.
</p>
</div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="../tbb_userguide/The_Task_Scheduler.htm">The Task Scheduler</a></div>
</div>
<div class="See Also">
<h2>See Also</h2>
<div class="linklist">
<div><a href="How_Task_Scheduling_Works.htm#tutorial_How_Task_Scheduling_Works">How Task Scheduling Works
</a></div></div>
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