1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Rationale</title> 5<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../index.html" title="Chapter 1. Fiber"> 8<link rel="up" href="../index.html" title="Chapter 1. Fiber"> 9<link rel="prev" href="custom.html" title="Customization"> 10<link rel="next" href="acknowledgements.html" title="Acknowledgments"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="custom.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="acknowledgements.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 27<a name="fiber.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a> 28</h2></div></div></div> 29<h4> 30<a name="fiber.rationale.h0"></a> 31 <span class="phrase"><a name="fiber.rationale.preprocessor_defines"></a></span><a class="link" href="rationale.html#fiber.rationale.preprocessor_defines">preprocessor 32 defines</a> 33 </h4> 34<div class="table"> 35<a name="fiber.rationale.preopcessor_defines"></a><p class="title"><b>Table 1.6. preopcessor defines</b></p> 36<div class="table-contents"><table class="table" summary="preopcessor defines"> 37<colgroup> 38<col> 39<col> 40</colgroup> 41<thead><tr> 42<th> 43 </th> 44<th> 45 </th> 46</tr></thead> 47<tbody> 48<tr> 49<td> 50 <p> 51 BOOST_FIBERS_NO_ATOMICS 52 </p> 53 </td> 54<td> 55 <p> 56 no <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">atomic</span><span class="special"><></span></code> 57 used, inter-thread synchronization disabled 58 </p> 59 </td> 60</tr> 61<tr> 62<td> 63 <p> 64 BOOST_FIBERS_SPINLOCK_STD_MUTEX 65 </p> 66 </td> 67<td> 68 <p> 69 use <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">mutex</span></code> as spinlock instead of default 70 <code class="computeroutput"><span class="identifier">XCHG</span></code>-sinlock with 71 backoff 72 </p> 73 </td> 74</tr> 75<tr> 76<td> 77 <p> 78 BOOST_FIBERS_SPIN_BACKOFF 79 </p> 80 </td> 81<td> 82 <p> 83 limit determines when to used <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">this_thread</span><span class="special">::</span><span class="identifier">yield</span><span class="special">()</span></code> instead of mnemonic <code class="computeroutput"><span class="identifier">pause</span><span class="special">/</span><span class="identifier">yield</span></code> during busy wait (apllies 84 on to <code class="computeroutput"><span class="identifier">XCHG</span></code>-spinlock) 85 </p> 86 </td> 87</tr> 88<tr> 89<td> 90 <p> 91 BOOST_FIBERS_SINGLE_CORE 92 </p> 93 </td> 94<td> 95 <p> 96 allways call <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">this_thread</span><span class="special">::</span><span class="identifier">yield</span><span class="special">()</span></code> without backoff during busy wait 97 (apllies on to <code class="computeroutput"><span class="identifier">XCHG</span></code>-spinlock) 98 </p> 99 </td> 100</tr> 101</tbody> 102</table></div> 103</div> 104<br class="table-break"><h4> 105<a name="fiber.rationale.h1"></a> 106 <span class="phrase"><a name="fiber.rationale.distinction_between_coroutines_and_fibers"></a></span><a class="link" href="rationale.html#fiber.rationale.distinction_between_coroutines_and_fibers">distinction 107 between coroutines and fibers</a> 108 </h4> 109<p> 110 The fiber library extends the coroutine library by adding a scheduler and synchronization 111 mechanisms. 112 </p> 113<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 114<li class="listitem"> 115 a coroutine yields 116 </li> 117<li class="listitem"> 118 a fiber blocks 119 </li> 120</ul></div> 121<p> 122 When a coroutine yields, it passes control directly to its caller (or, in the 123 case of symmetric coroutines, a designated other coroutine). When a fiber blocks, 124 it implicitly passes control to the fiber scheduler. Coroutines have no scheduler 125 because they need no scheduler.<a href="#ftn.fiber.rationale.f0" class="footnote" name="fiber.rationale.f0"><sup class="footnote">[12]</sup></a>. 126 </p> 127<h4> 128<a name="fiber.rationale.h2"></a> 129 <span class="phrase"><a name="fiber.rationale.what_about_transactional_memory"></a></span><a class="link" href="rationale.html#fiber.rationale.what_about_transactional_memory">what 130 about transactional memory</a> 131 </h4> 132<p> 133 GCC supports transactional memory since version 4.7. Unfortunately tests show 134 that transactional memory is slower (ca. 4x) than spinlocks using atomics. 135 Once transactional memory is improved (supporting hybrid tm), spinlocks will 136 be replaced by __transaction_atomic{} statements surrounding the critical sections. 137 </p> 138<h4> 139<a name="fiber.rationale.h3"></a> 140 <span class="phrase"><a name="fiber.rationale.synchronization_between_fibers_running_in_different_threads"></a></span><a class="link" href="rationale.html#fiber.rationale.synchronization_between_fibers_running_in_different_threads">synchronization 141 between fibers running in different threads</a> 142 </h4> 143<p> 144 Synchronization classes from <a href="http://www.boost.org/doc/libs/release/libs/thread/index.html" target="_top">Boost.Thread</a> 145 block the entire thread. In contrast, the synchronization classes from <span class="bold"><strong>Boost.Fiber</strong></span> block only specific fibers, so that the 146 scheduler can still keep the thread busy running other fibers in the meantime. 147 The synchronization classes from <span class="bold"><strong>Boost.Fiber</strong></span> 148 are designed to be thread-safe, i.e. it is possible to synchronize fibers running 149 in different threads as well as fibers running in the same thread. (However, 150 there is a build option to disable cross-thread fiber synchronization support; 151 see <a class="link" href="overview.html#cross_thread_sync">this description</a>.) 152 </p> 153<a name="spurious_wakeup"></a><h4> 154<a name="fiber.rationale.h4"></a> 155 <span class="phrase"><a name="fiber.rationale.spurious_wakeup"></a></span><a class="link" href="rationale.html#fiber.rationale.spurious_wakeup">spurious 156 wakeup</a> 157 </h4> 158<p> 159 Spurious wakeup can happen when using <a href="http://en.cppreference.com/w/cpp/thread/condition_variable" target="_top"><code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">condition_variable</span></code></a>: 160 the condition variable appears to be have been signaled while the awaited condition 161 may still be false. Spurious wakeup can happen repeatedly and is caused on 162 some multiprocessor systems where making <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">condition_variable</span></code> 163 wakeup completely predictable would slow down all <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">condition_variable</span></code> 164 operations.<a href="#ftn.fiber.rationale.f1" class="footnote" name="fiber.rationale.f1"><sup class="footnote">[13]</sup></a> 165 </p> 166<p> 167 <a class="link" href="synchronization/conditions.html#class_condition_variable"><code class="computeroutput">condition_variable</code></a> is not subject to spurious wakeup. 168 Nonetheless it is prudent to test the business-logic condition in a <code class="computeroutput"><span class="identifier">wait</span><span class="special">()</span></code> loop 169 — or, equivalently, use one of the <code class="computeroutput"><span class="identifier">wait</span><span class="special">(</span> <span class="identifier">lock</span><span class="special">,</span> 170 <span class="identifier">predicate</span> <span class="special">)</span></code> 171 overloads. 172 </p> 173<p> 174 See also <a class="link" href="synchronization/conditions.html#condition_variable_spurious_wakeups">No Spurious Wakeups</a>. 175 </p> 176<h4> 177<a name="fiber.rationale.h5"></a> 178 <span class="phrase"><a name="fiber.rationale.migrating_fibers_between_threads"></a></span><a class="link" href="rationale.html#fiber.rationale.migrating_fibers_between_threads">migrating 179 fibers between threads</a> 180 </h4> 181<p> 182 Support for migrating fibers between threads has been integrated. The user-defined 183 scheduler must call <a class="link" href="scheduling.html#context_detach"><code class="computeroutput">context::detach()</code></a> on a fiber-context on the 184 source thread and <a class="link" href="scheduling.html#context_attach"><code class="computeroutput">context::attach()</code></a> on the destination thread, 185 passing the fiber-context to migrate. (For more information about custom schedulers, 186 see <a class="link" href="custom.html#custom">Customization</a>.) Examples <code class="computeroutput"><span class="identifier">work_sharing</span></code> 187 and <code class="computeroutput"><span class="identifier">work_stealing</span></code> in directory 188 <code class="computeroutput"><span class="identifier">examples</span></code> might be used as a 189 blueprint. 190 </p> 191<p> 192 See also <a class="link" href="migration.html#migration">Migrating fibers between threads</a>. 193 </p> 194<h4> 195<a name="fiber.rationale.h6"></a> 196 <span class="phrase"><a name="fiber.rationale.support_for_boost_asio"></a></span><a class="link" href="rationale.html#fiber.rationale.support_for_boost_asio">support 197 for Boost.Asio</a> 198 </h4> 199<p> 200 Support for <a href="http://www.boost.org/doc/libs/release/libs/asio/index.html" target="_top">Boost.Asio</a>’s 201 <span class="emphasis"><em>async-result</em></span> is not part of the official API. However, 202 to integrate with a <a href="http://www.boost.org/doc/libs/release/doc/html/boost_asio/reference/io_service.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_service</span></code></a>, 203 see <a class="link" href="integration.html#integration">Sharing a Thread with Another Main Loop</a>. 204 To interface smoothly with an arbitrary Asio async I/O operation, see <a class="link" href="callbacks/then_there_s____boost_asio__.html#callbacks_asio">Then There’s Boost.Asio</a>. 205 </p> 206<h4> 207<a name="fiber.rationale.h7"></a> 208 <span class="phrase"><a name="fiber.rationale.tested_compilers"></a></span><a class="link" href="rationale.html#fiber.rationale.tested_compilers">tested 209 compilers</a> 210 </h4> 211<p> 212 The library was tested with GCC-5.1.1, Clang-3.6.0 and MSVC-14.0 in c++11-mode. 213 </p> 214<h4> 215<a name="fiber.rationale.h8"></a> 216 <span class="phrase"><a name="fiber.rationale.supported_architectures"></a></span><a class="link" href="rationale.html#fiber.rationale.supported_architectures">supported 217 architectures</a> 218 </h4> 219<p> 220 <span class="bold"><strong>Boost.Fiber</strong></span> depends on <a href="http://www.boost.org/doc/libs/release/libs/context/index.html" target="_top">Boost.Context</a> 221 - the list of supported architectures can be found <a href="http://www.boost.org/doc/libs/release/libs/context/doc/html/context/architectures.html" target="_top">here</a>. 222 </p> 223<div class="footnotes"> 224<br><hr style="width:100; text-align:left;margin-left: 0"> 225<div id="ftn.fiber.rationale.f0" class="footnote"><p><a href="#fiber.rationale.f0" class="para"><sup class="para">[12] </sup></a> 226 <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4024.pdf" target="_top">'N4024: 227 Distinguishing coroutines and fibers'</a> 228 </p></div> 229<div id="ftn.fiber.rationale.f1" class="footnote"><p><a href="#fiber.rationale.f1" class="para"><sup class="para">[13] </sup></a> 230 David R. Butenhof <span class="quote">“<span class="quote">Programming with POSIX Threads</span>”</span> 231 </p></div> 232</div> 233</div> 234<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 235<td align="left"></td> 236<td align="right"><div class="copyright-footer">Copyright © 2013 Oliver Kowalke<p> 237 Distributed under the Boost Software License, Version 1.0. (See accompanying 238 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>) 239 </p> 240</div></td> 241</tr></table> 242<hr> 243<div class="spirit-nav"> 244<a accesskey="p" href="custom.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="acknowledgements.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 245</div> 246</body> 247</html> 248
