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<title>Sharing a Thread with Another Main Loop</title>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="fiber.integration"></a><a name="integration"></a><a class="link" href="integration.html" title="Sharing a Thread with Another Main Loop">Sharing a
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Thread with Another Main Loop</a>
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</h2></div></div></div>
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<h4>
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<a name="fiber.integration.h0"></a>
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<span><a name="fiber.integration.overview"></a></span><a class="link" href="integration.html#fiber.integration.overview">Overview</a>
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</h4>
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<p>
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As always with cooperative concurrency, it is important not to let any one
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fiber monopolize the processor too long: that could <span class="quote">“<span class="quote">starve</span>”</span> other
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ready fibers. This section discusses a couple of solutions.
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</p>
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<h4>
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<a name="fiber.integration.h1"></a>
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<span><a name="fiber.integration.event_driven_program"></a></span><a class="link" href="integration.html#fiber.integration.event_driven_program">Event-Driven
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Program</a>
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</h4>
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<p>
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Consider a classic event-driven program, organized around a main loop that
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fetches and dispatches incoming I/O events. You are introducing <span class="bold"><strong>Boost.Fiber</strong></span>
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because certain asynchronous I/O sequences are logically sequential, and for
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those you want to write and maintain code that looks and acts sequential.
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</p>
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<p>
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You are launching fibers on the application's main thread because certain of
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their actions will affect its user interface, and the application's UI framework
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permits UI operations only on the main thread. Or perhaps those fibers need
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access to main-thread data, and it would be too expensive in runtime (or development
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time) to robustly defend every such data item with thread synchronization primitives.
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</p>
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<p>
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You must ensure that the application's main loop <span class="emphasis"><em>itself</em></span>
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doesn't monopolize the processor: that the fibers it launches will get the
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CPU cycles they need.
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</p>
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<p>
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The solution is the same as for any fiber that might claim the CPU for an extended
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time: introduce calls to <a class="link" href="fiber_mgmt/this_fiber.html#this_fiber_yield"> <code class="computeroutput">this_fiber::yield()</code></a>. The most straightforward
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approach is to call <code class="computeroutput"><span class="identifier">yield</span><span class="special">()</span></code>
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on every iteration of your existing main loop. In effect, this unifies the
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application's main loop with <span class="bold"><strong>Boost.Fiber</strong></span>'s
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internal main loop. <code class="computeroutput"><span class="identifier">yield</span><span class="special">()</span></code>
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allows the fiber manager to run any fibers that have become ready since the
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previous iteration of the application's main loop. When these fibers have had
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a turn, control passes to the thread's main fiber, which returns from <code class="computeroutput"><span class="identifier">yield</span><span class="special">()</span></code> and
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resumes the application's main loop.
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</p>
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<h4>
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<a name="fiber.integration.h2"></a>
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<span><a name="fiber.integration.integrating_with__ulink_url__http___www_boost_org_doc_libs_release_libs_asio_index_html__boost_asio__ulink_"></a></span><a class="link" href="integration.html#fiber.integration.integrating_with__ulink_url__http___www_boost_org_doc_libs_release_libs_asio_index_html__boost_asio__ulink_">Integrating
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with <a href="http://www.boost.org/doc/libs/release/libs/asio/index.html" target="_top">Boost.Asio</a></a>
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</h4>
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<p>
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More challenging is when the application's main loop is embedded in some other
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library or framework. Such an application will typically, after performing
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all necessary setup, pass control to some form of <code class="computeroutput"><span class="identifier">run</span><span class="special">()</span></code> function from which control does not return
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until application shutdown.
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</p>
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<p>
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A <a href="http://www.boost.org/doc/libs/release/libs/asio/index.html" target="_top">Boost.Asio</a>
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program might call <a href="http://www.boost.org/doc/libs/release/doc/html/boost_asio/reference/io_service/run.html" target="_top"><code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code></a>
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in this way.
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</p>
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<p>
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The trick here is to arrange to pass control to <a class="link" href="fiber_mgmt/this_fiber.html#this_fiber_yield"> <code class="computeroutput">this_fiber::yield()</code></a> frequently.
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You can use an <a href="http://www.boost.org/doc/libs/1_59_0/doc/html/boost_asio/reference/high_resolution_timer.html" target="_top">Asio
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timer</a> for this purpose. Instantiate the timer, arranging to call a
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handler function when the timer expires:
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</p>
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<p>
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[run_service]
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</p>
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<p>
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The handler function calls <code class="computeroutput"><span class="identifier">yield</span><span class="special">()</span></code>, then resets the timer and arranges to wake
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up again on expiration:
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</p>
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<p>
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[timer_handler]
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</p>
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<p>
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Then instead of directly calling <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>,
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your application would call the above <code class="computeroutput"><span class="identifier">run_service</span><span class="special">(</span><span class="identifier">io_service</span><span class="special">&)</span></code> wrapper.
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</p>
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<p>
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Since, in this example, we always pass control to the fiber manager via <code class="computeroutput"><span class="identifier">yield</span><span class="special">()</span></code>,
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the calling fiber is never blocked. Therefore there is always at least one
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ready fiber. Therefore the fiber manager never sleeps.
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</p>
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<p>
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Using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">chrono</span><span class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span class="number">0</span><span class="special">)</span></code> for <span class="emphasis"><em>every</em></span>
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keepalive timer interval would be unfriendly to other threads. When all I/O
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is pending and all fibers are blocked, the io_service and the fiber manager
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would simply spin the CPU, passing control back and forth to each other. Resetting
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the timer for <code class="computeroutput"><span class="identifier">keepalive_iterval</span></code>
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allows tuning the responsiveness of this thread relative to others in the same
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way as when <span class="bold"><strong>Boost.Fiber</strong></span> is running without
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<a href="http://www.boost.org/doc/libs/release/libs/asio/index.html" target="_top">Boost.Asio</a>.
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</p>
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<p>
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The source code above is found in <a href="../../../examples/asio/round_robin.hpp" target="_top">round_robin.hpp</a>.
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2013 Oliver Kowalke<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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</div></td>
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