1.. _When_Not_to_Use_Queues:
2
3When Not to Use Queues
4======================
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6
7Queues are widely used in parallel programs to buffer consumers from
8producers. Before using an explicit queue, however, consider using
9``parallel_for_each`` ``parallel_pipeline`` instead. These is often more
10efficient than queues for the following reasons:
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12
13-  A queue is inherently a bottle neck, because it must maintain
14   first-in first-out order.
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17-  A thread that is popping a value may have to wait idly until the
18   value is pushed.
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21-  A queue is a passive data structure. If a thread pushes a value, it
22   could take time until it pops the value, and in the meantime the
23   value (and whatever it references) becomes "cold" in cache. Or worse
24   yet, another thread pops the value, and the value (and whatever it
25   references) must be moved to the other processor.
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28In contrast, ``parallel_pipeline`` avoids these bottlenecks. Because its
29threading is implicit, it optimizes use of worker threads so that they
30do other work until a value shows up. It also tries to keep items hot in
31cache.
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