ceph/src/boost/libs/fiber/doc/html/fiber/migration.html

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  25 <div class="section">
  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="fiber.migration"></a><a name="migration"></a><a class="link" href="migration.html" title="Migrating fibers between threads">Migrating fibers
  28     between threads</a>
  29 </h2></div></div></div>
  30 <h4>
  31 <a name="fiber.migration.h0"></a>
  32       <span><a name="fiber.migration.overview"></a></span><a class="link" href="migration.html#fiber.migration.overview">Overview</a>
  33     </h4>
  34 <p>
  35       Each fiber owns a stack and manages its execution state, including all registers
  36       and CPU flags, the instruction pointer and the stack pointer. That means, in
  37       general, a fiber is not bound to a specific thread.<sup>[<a name="fiber.migration.f0" href="#ftn.fiber.migration.f0" class="footnote">2</a>]</sup><sup>,</sup><sup>[<a name="fiber.migration.f1" href="#ftn.fiber.migration.f1" class="footnote">3</a>]</sup>
  38     </p>
  39 <p>
  40       Migrating a fiber from a logical CPU with heavy workload to another logical
  41       CPU with a lighter workload might speed up the overall execution. Note that
  42       in the case of NUMA-architectures, it is not always advisable to migrate data
  43       between threads. Suppose fiber <span class="emphasis"><em>f</em></span> is running on logical
  44       CPU <span class="emphasis"><em>cpu0</em></span> which belongs to NUMA node <span class="emphasis"><em>node0</em></span>.
  45       The data of <span class="emphasis"><em>f</em></span> are allocated on the physical memory located
  46       at <span class="emphasis"><em>node0</em></span>. Migrating the fiber from <span class="emphasis"><em>cpu0</em></span>
  47       to another logical CPU <span class="emphasis"><em>cpuX</em></span> which is part of a different
  48       NUMA node <span class="emphasis"><em>nodeX</em></span> might reduce the performance of the application
  49       due to increased latency of memory access.
  50     </p>
  51 <p>
  52       Only fibers that are contained in <a class="link" href="scheduling.html#class_algorithm"><code class="computeroutput">algorithm</code></a>&#8217;s ready queue can
  53       migrate between threads. You cannot migrate a running fiber, nor one that is
  54       <a class="link" href="overview.html#blocking"><span class="emphasis"><em>blocked</em></span></a>. You cannot migrate
  55       a fiber if its <a class="link" href="scheduling.html#context_is_context"><code class="computeroutput">context::is_context()</code></a> method returns <code class="computeroutput"><span class="keyword">true</span></code> for <code class="computeroutput"><span class="identifier">pinned_context</span></code>.
  56     </p>
  57 <p>
  58       In <span class="bold"><strong>Boost.Fiber</strong></span> a fiber is migrated by invoking
  59       <a class="link" href="scheduling.html#context_detach"><code class="computeroutput">context::detach()</code></a> on the thread from which the fiber migrates
  60       and <a class="link" href="scheduling.html#context_attach"><code class="computeroutput">context::attach()</code></a> on the thread to which the fiber migrates.
  61     </p>
  62 <p>
  63       Thus, fiber migration is accomplished by sharing state between instances of
  64       a user-coded <a class="link" href="scheduling.html#class_algorithm"><code class="computeroutput">algorithm</code></a> implementation running on different threads.
  65       The fiber&#8217;s original thread calls <a class="link" href="scheduling.html#algorithm_awakened"><code class="computeroutput">algorithm::awakened()</code></a>, passing
  66       the fiber&#8217;s <a class="link" href="scheduling.html#class_context"><code class="computeroutput">context</code></a><code class="literal">*</code>. The <code class="computeroutput"><span class="identifier">awakened</span><span class="special">()</span></code> implementation calls <a class="link" href="scheduling.html#context_detach"><code class="computeroutput">context::detach()</code></a>.
  67     </p>
  68 <p>
  69       At some later point, when the same or a different thread calls <a class="link" href="scheduling.html#algorithm_pick_next"><code class="computeroutput">algorithm::pick_next()</code></a>,
  70       the <code class="computeroutput"><span class="identifier">pick_next</span><span class="special">()</span></code>
  71       implementation selects a ready fiber and calls <a class="link" href="scheduling.html#context_attach"><code class="computeroutput">context::attach()</code></a> on
  72       it before returning it.
  73     </p>
  74 <p>
  75       As stated above, a <code class="computeroutput"><span class="identifier">context</span></code>
  76       for which <code class="computeroutput"><span class="identifier">is_context</span><span class="special">(</span><span class="identifier">pinned_context</span><span class="special">)</span>
  77       <span class="special">==</span> <span class="keyword">true</span></code>
  78       must never be passed to either <a class="link" href="scheduling.html#context_detach"><code class="computeroutput">context::detach()</code></a> or <a class="link" href="scheduling.html#context_attach"><code class="computeroutput">context::attach()</code></a>.
  79       It may only be returned from <code class="computeroutput"><span class="identifier">pick_next</span><span class="special">()</span></code> called by the <span class="emphasis"><em>same</em></span> thread
  80       that passed that context to <code class="computeroutput"><span class="identifier">awakened</span><span class="special">()</span></code>.
  81     </p>
  82 <h4>
  83 <a name="fiber.migration.h1"></a>
  84       <span><a name="fiber.migration.example_of_work_sharing"></a></span><a class="link" href="migration.html#fiber.migration.example_of_work_sharing">Example
  85       of work sharing</a>
  86     </h4>
  87 <p>
  88       In the example <a href="../../../examples/work_sharing.cpp" target="_top">work_sharing.cpp</a>
  89       multiple worker fibers are created on the main thread. Each fiber gets a character
  90       as parameter at construction. This character is printed out ten times. Between
  91       each iteration the fiber calls <a class="link" href="fiber_mgmt/this_fiber.html#this_fiber_yield"><code class="computeroutput">this_fiber::yield()</code></a>. That puts
  92       the fiber in the ready queue of the fiber-scheduler <span class="emphasis"><em>shared_ready_queue</em></span>,
  93       running in the current thread. The next fiber ready to be executed is dequeued
  94       from the shared ready queue and resumed by <span class="emphasis"><em>shared_ready_queue</em></span>
  95       running on <span class="emphasis"><em>any participating thread</em></span>.
  96     </p>
  97 <p>
  98       All instances of <span class="emphasis"><em>shared_ready_queue</em></span> share one global concurrent
  99       queue, used as ready queue. This mechanism shares all worker fibers between
 100       all instances of <span class="emphasis"><em>shared_ready_queue</em></span>, thus between all
 101       participating threads.
 102     </p>
 103 <h4>
 104 <a name="fiber.migration.h2"></a>
 105       <span><a name="fiber.migration.setup_of_threads_and_fibers"></a></span><a class="link" href="migration.html#fiber.migration.setup_of_threads_and_fibers">Setup
 106       of threads and fibers</a>
 107     </h4>
 108 <p>
 109       In <code class="computeroutput"><span class="identifier">main</span><span class="special">()</span></code>
 110       the fiber-scheduler is installed and the worker fibers and the threads are
 111       launched.
 112     </p>
 113 <p>
 114 </p>
 115 <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">use_scheduling_algorithm</span><span class="special">&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">algo</span><span class="special">::</span><span class="identifier">shared_work</span> <span class="special">&gt;();</span> <a class="co" name="fiber.migration.c0" href="migration.html#fiber.migration.c1"><img src="../../../../../doc/src/images/callouts/1.png" alt="1" border="0"></a>
 116
 117 <span class="keyword">for</span> <span class="special">(</span> <span class="keyword">char</span> <span class="identifier">c</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">(</span><span class="string">"abcdefghijklmnopqrstuvwxyz"</span><span class="special">))</span> <span class="special">{</span> <a class="co" name="fiber.migration.c2" href="migration.html#fiber.migration.c3"><img src="../../../../../doc/src/images/callouts/2.png" alt="2" border="0"></a>
 118     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">fiber</span><span class="special">([</span><span class="identifier">c</span><span class="special">](){</span> <span class="identifier">whatevah</span><span class="special">(</span> <span class="identifier">c</span><span class="special">);</span> <span class="special">}).</span><span class="identifier">detach</span><span class="special">();</span>
 119     <span class="special">++</span><span class="identifier">fiber_count</span><span class="special">;</span> <a class="co" name="fiber.migration.c4" href="migration.html#fiber.migration.c5"><img src="../../../../../doc/src/images/callouts/3.png" alt="3" border="0"></a>
 120 <span class="special">}</span>
 121 <span class="identifier">barrier</span> <span class="identifier">b</span><span class="special">(</span> <span class="number">4</span><span class="special">);</span>
 122 <span class="identifier">std</span><span class="special">::</span><span class="identifier">thread</span> <span class="identifier">threads</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span> <a class="co" name="fiber.migration.c6" href="migration.html#fiber.migration.c7"><img src="../../../../../doc/src/images/callouts/4.png" alt="4" border="0"></a>
 123     <span class="identifier">std</span><span class="special">::</span><span class="identifier">thread</span><span class="special">(</span> <span class="identifier">thread</span><span class="special">,</span> <span class="special">&amp;</span> <span class="identifier">b</span><span class="special">),</span>
 124     <span class="identifier">std</span><span class="special">::</span><span class="identifier">thread</span><span class="special">(</span> <span class="identifier">thread</span><span class="special">,</span> <span class="special">&amp;</span> <span class="identifier">b</span><span class="special">),</span>
 125     <span class="identifier">std</span><span class="special">::</span><span class="identifier">thread</span><span class="special">(</span> <span class="identifier">thread</span><span class="special">,</span> <span class="special">&amp;</span> <span class="identifier">b</span><span class="special">)</span>
 126 <span class="special">};</span>
 127 <span class="identifier">b</span><span class="special">.</span><span class="identifier">wait</span><span class="special">();</span> <a class="co" name="fiber.migration.c8" href="migration.html#fiber.migration.c9"><img src="../../../../../doc/src/images/callouts/5.png" alt="5" border="0"></a>
 128 <span class="special">{</span>
 129     <span class="identifier">lock_t</span><a class="co" name="fiber.migration.c10" href="migration.html#fiber.migration.c11"><img src="../../../../../doc/src/images/callouts/6.png" alt="6" border="0"></a> <span class="identifier">lk</span><span class="special">(</span> <span class="identifier">mtx_count</span><span class="special">);</span>
 130     <span class="identifier">cnd_count</span><span class="special">.</span><span class="identifier">wait</span><span class="special">(</span> <span class="identifier">lk</span><span class="special">,</span> <span class="special">[](){</span> <span class="keyword">return</span> <span class="number">0</span> <span class="special">==</span> <span class="identifier">fiber_count</span><span class="special">;</span> <span class="special">}</span> <span class="special">);</span> <a class="co" name="fiber.migration.c12" href="migration.html#fiber.migration.c13"><img src="../../../../../doc/src/images/callouts/7.png" alt="7" border="0"></a>
 131 <span class="special">}</span> <a class="co" name="fiber.migration.c14" href="migration.html#fiber.migration.c15"><img src="../../../../../doc/src/images/callouts/8.png" alt="8" border="0"></a>
 132 <span class="identifier">BOOST_ASSERT</span><span class="special">(</span> <span class="number">0</span> <span class="special">==</span> <span class="identifier">fiber_count</span><span class="special">);</span>
 133 <span class="keyword">for</span> <span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">thread</span> <span class="special">&amp;</span> <span class="identifier">t</span> <span class="special">:</span> <span class="identifier">threads</span><span class="special">)</span> <span class="special">{</span> <a class="co" name="fiber.migration.c16" href="migration.html#fiber.migration.c17"><img src="../../../../../doc/src/images/callouts/9.png" alt="9" border="0"></a>
 134     <span class="identifier">t</span><span class="special">.</span><span class="identifier">join</span><span class="special">();</span>
 135 <span class="special">}</span>
 136 </pre>
 137 <p>
 138     </p>
 139 <div class="calloutlist"><table border="0" summary="Callout list">
 140 <tr>
 141 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c1"></a><a href="#fiber.migration.c0"><img src="../../../../../doc/src/images/callouts/1.png" alt="1" border="0"></a> </p></td>
 142 <td valign="top" align="left"><p>
 143           Install the scheduling algorithm <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">algo</span><span class="special">::</span><span class="identifier">shared_work</span></code>
 144           in the main thread too, so each new fiber gets launched into the shared
 145           pool.
 146         </p></td>
 147 </tr>
 148 <tr>
 149 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c3"></a><a href="#fiber.migration.c2"><img src="../../../../../doc/src/images/callouts/2.png" alt="2" border="0"></a> </p></td>
 150 <td valign="top" align="left"><p>
 151           Launch a number of worker fibers; each worker fiber picks up a character
 152           that is passed as parameter to fiber-function <code class="computeroutput"><span class="identifier">whatevah</span></code>.
 153           Each worker fiber gets detached.
 154         </p></td>
 155 </tr>
 156 <tr>
 157 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c5"></a><a href="#fiber.migration.c4"><img src="../../../../../doc/src/images/callouts/3.png" alt="3" border="0"></a> </p></td>
 158 <td valign="top" align="left"><p>
 159           Increment fiber counter for each new fiber.
 160         </p></td>
 161 </tr>
 162 <tr>
 163 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c7"></a><a href="#fiber.migration.c6"><img src="../../../../../doc/src/images/callouts/4.png" alt="4" border="0"></a> </p></td>
 164 <td valign="top" align="left"><p>
 165           Launch a couple of threads that join the work sharing.
 166         </p></td>
 167 </tr>
 168 <tr>
 169 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c9"></a><a href="#fiber.migration.c8"><img src="../../../../../doc/src/images/callouts/5.png" alt="5" border="0"></a> </p></td>
 170 <td valign="top" align="left"><p>
 171           sync with other threads: allow them to start processing
 172         </p></td>
 173 </tr>
 174 <tr>
 175 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c11"></a><a href="#fiber.migration.c10"><img src="../../../../../doc/src/images/callouts/6.png" alt="6" border="0"></a> </p></td>
 176 <td valign="top" align="left"><p>
 177           <code class="computeroutput"><span class="identifier">lock_t</span></code> is typedef'ed as
 178           <a href="http://en.cppreference.com/w/cpp/thread/unique_lock" target="_top"><code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_lock</span></code></a>&lt; <a href="http://en.cppreference.com/w/cpp/thread/mutex" target="_top"><code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">mutex</span></code></a> &gt;
 179         </p></td>
 180 </tr>
 181 <tr>
 182 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c13"></a><a href="#fiber.migration.c12"><img src="../../../../../doc/src/images/callouts/7.png" alt="7" border="0"></a> </p></td>
 183 <td valign="top" align="left"><p>
 184           Suspend main fiber and resume worker fibers in the meanwhile. Main fiber
 185           gets resumed (e.g returns from <code class="computeroutput"><span class="identifier">condition_variable_any</span><span class="special">::</span><span class="identifier">wait</span><span class="special">()</span></code>) if all worker fibers are complete.
 186         </p></td>
 187 </tr>
 188 <tr>
 189 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c15"></a><a href="#fiber.migration.c14"><img src="../../../../../doc/src/images/callouts/8.png" alt="8" border="0"></a> </p></td>
 190 <td valign="top" align="left"><p>
 191           Releasing lock of mtx_count is required before joining the threads, otherwise
 192           the other threads would be blocked inside condition_variable::wait() and
 193           would never return (deadlock).
 194         </p></td>
 195 </tr>
 196 <tr>
 197 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c17"></a><a href="#fiber.migration.c16"><img src="../../../../../doc/src/images/callouts/9.png" alt="9" border="0"></a> </p></td>
 198 <td valign="top" align="left"><p>
 199           wait for threads to terminate
 200         </p></td>
 201 </tr>
 202 </table></div>
 203 <p>
 204       The start of the threads is synchronized with a barrier. The main fiber of
 205       each thread (including main thread) is suspended until all worker fibers are
 206       complete. When the main fiber returns from <a class="link" href="synchronization/conditions.html#condition_variable_wait"><code class="computeroutput">condition_variable::wait()</code></a>,
 207       the thread terminates: the main thread joins all other threads.
 208     </p>
 209 <p>
 210 </p>
 211 <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">thread</span><span class="special">(</span> <span class="identifier">barrier</span> <span class="special">*</span> <span class="identifier">b</span><span class="special">)</span> <span class="special">{</span>
 212     <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostringstream</span> <span class="identifier">buffer</span><span class="special">;</span>
 213     <span class="identifier">buffer</span> <span class="special">&lt;&lt;</span> <span class="string">"thread started "</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">this_thread</span><span class="special">::</span><span class="identifier">get_id</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 214     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">buffer</span><span class="special">.</span><span class="identifier">str</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">flush</span><span class="special">;</span>
 215     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">use_scheduling_algorithm</span><span class="special">&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">algo</span><span class="special">::</span><span class="identifier">shared_work</span> <span class="special">&gt;();</span> <a class="co" name="fiber.migration.c18" href="migration.html#fiber.migration.c19"><img src="../../../../../doc/src/images/callouts/1.png" alt="1" border="0"></a>
 216     <span class="identifier">b</span><span class="special">-&gt;</span><span class="identifier">wait</span><span class="special">();</span> <a class="co" name="fiber.migration.c20" href="migration.html#fiber.migration.c21"><img src="../../../../../doc/src/images/callouts/2.png" alt="2" border="0"></a>
 217     <span class="identifier">lock_t</span> <span class="identifier">lk</span><span class="special">(</span> <span class="identifier">mtx_count</span><span class="special">);</span>
 218     <span class="identifier">cnd_count</span><span class="special">.</span><span class="identifier">wait</span><span class="special">(</span> <span class="identifier">lk</span><span class="special">,</span> <span class="special">[](){</span> <span class="keyword">return</span> <span class="number">0</span> <span class="special">==</span> <span class="identifier">fiber_count</span><span class="special">;</span> <span class="special">}</span> <span class="special">);</span> <a class="co" name="fiber.migration.c22" href="migration.html#fiber.migration.c23"><img src="../../../../../doc/src/images/callouts/3.png" alt="3" border="0"></a>
 219     <span class="identifier">BOOST_ASSERT</span><span class="special">(</span> <span class="number">0</span> <span class="special">==</span> <span class="identifier">fiber_count</span><span class="special">);</span>
 220 <span class="special">}</span>
 221 </pre>
 222 <p>
 223     </p>
 224 <div class="calloutlist"><table border="0" summary="Callout list">
 225 <tr>
 226 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c19"></a><a href="#fiber.migration.c18"><img src="../../../../../doc/src/images/callouts/1.png" alt="1" border="0"></a> </p></td>
 227 <td valign="top" align="left"><p>
 228           Install the scheduling algorithm <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">fibers</span><span class="special">::</span><span class="identifier">algo</span><span class="special">::</span><span class="identifier">shared_work</span></code>
 229           in order to join the work sharing.
 230         </p></td>
 231 </tr>
 232 <tr>
 233 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c21"></a><a href="#fiber.migration.c20"><img src="../../../../../doc/src/images/callouts/2.png" alt="2" border="0"></a> </p></td>
 234 <td valign="top" align="left"><p>
 235           sync with other threads: allow them to start processing
 236         </p></td>
 237 </tr>
 238 <tr>
 239 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c23"></a><a href="#fiber.migration.c22"><img src="../../../../../doc/src/images/callouts/3.png" alt="3" border="0"></a> </p></td>
 240 <td valign="top" align="left"><p>
 241           Suspend main fiber and resume worker fibers in the meanwhile. Main fiber
 242           gets resumed (e.g returns from <code class="computeroutput"><span class="identifier">condition_variable_any</span><span class="special">::</span><span class="identifier">wait</span><span class="special">()</span></code>) if all worker fibers are complete.
 243         </p></td>
 244 </tr>
 245 </table></div>
 246 <p>
 247       Each worker fiber executes function <code class="computeroutput"><span class="identifier">whatevah</span><span class="special">()</span></code> with character <code class="computeroutput"><span class="identifier">me</span></code>
 248       as parameter. The fiber yields in a loop and prints out a message if it was
 249       migrated to another thread.
 250     </p>
 251 <p>
 252 </p>
 253 <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">whatevah</span><span class="special">(</span> <span class="keyword">char</span> <span class="identifier">me</span><span class="special">)</span> <span class="special">{</span>
 254     <span class="keyword">try</span> <span class="special">{</span>
 255         <span class="identifier">std</span><span class="special">::</span><span class="identifier">thread</span><span class="special">::</span><span class="identifier">id</span> <span class="identifier">my_thread</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">this_thread</span><span class="special">::</span><span class="identifier">get_id</span><span class="special">();</span> <a class="co" name="fiber.migration.c24" href="migration.html#fiber.migration.c25"><img src="../../../../../doc/src/images/callouts/1.png" alt="1" border="0"></a>
 256         <span class="special">{</span>
 257             <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostringstream</span> <span class="identifier">buffer</span><span class="special">;</span>
 258             <span class="identifier">buffer</span> <span class="special">&lt;&lt;</span> <span class="string">"fiber "</span> <span class="special">&lt;&lt;</span> <span class="identifier">me</span> <span class="special">&lt;&lt;</span> <span class="string">" started on thread "</span> <span class="special">&lt;&lt;</span> <span class="identifier">my_thread</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span><span class="special">;</span>
 259             <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">buffer</span><span class="special">.</span><span class="identifier">str</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">flush</span><span class="special">;</span>
 260         <span class="special">}</span>
 261         <span class="keyword">for</span> <span class="special">(</span> <span class="keyword">unsigned</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="number">10</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span> <a class="co" name="fiber.migration.c26" href="migration.html#fiber.migration.c27"><img src="../../../../../doc/src/images/callouts/2.png" alt="2" border="0"></a>
 262             <span class="identifier">boost</span><span class="special">::</span><span class="identifier">this_fiber</span><span class="special">::</span><span class="identifier">yield</span><span class="special">();</span> <a class="co" name="fiber.migration.c28" href="migration.html#fiber.migration.c29"><img src="../../../../../doc/src/images/callouts/3.png" alt="3" border="0"></a>
 263             <span class="identifier">std</span><span class="special">::</span><span class="identifier">thread</span><span class="special">::</span><span class="identifier">id</span> <span class="identifier">new_thread</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">this_thread</span><span class="special">::</span><span class="identifier">get_id</span><span class="special">();</span> <a class="co" name="fiber.migration.c30" href="migration.html#fiber.migration.c31"><img src="../../../../../doc/src/images/callouts/4.png" alt="4" border="0"></a>
 264             <span class="keyword">if</span> <span class="special">(</span> <span class="identifier">new_thread</span> <span class="special">!=</span> <span class="identifier">my_thread</span><span class="special">)</span> <span class="special">{</span> <a class="co" name="fiber.migration.c32" href="migration.html#fiber.migration.c33"><img src="../../../../../doc/src/images/callouts/5.png" alt="5" border="0"></a>
 265                 <span class="identifier">my_thread</span> <span class="special">=</span> <span class="identifier">new_thread</span><span class="special">;</span>
 266                 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostringstream</span> <span class="identifier">buffer</span><span class="special">;</span>
 267                 <span class="identifier">buffer</span> <span class="special">&lt;&lt;</span> <span class="string">"fiber "</span> <span class="special">&lt;&lt;</span> <span class="identifier">me</span> <span class="special">&lt;&lt;</span> <span class="string">" switched to thread "</span> <span class="special">&lt;&lt;</span> <span class="identifier">my_thread</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span><span class="special">;</span>
 268                 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">buffer</span><span class="special">.</span><span class="identifier">str</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">flush</span><span class="special">;</span>
 269             <span class="special">}</span>
 270         <span class="special">}</span>
 271     <span class="special">}</span> <span class="keyword">catch</span> <span class="special">(</span> <span class="special">...</span> <span class="special">)</span> <span class="special">{</span>
 272     <span class="special">}</span>
 273     <span class="identifier">lock_t</span> <span class="identifier">lk</span><span class="special">(</span> <span class="identifier">mtx_count</span><span class="special">);</span>
 274     <span class="keyword">if</span> <span class="special">(</span> <span class="number">0</span> <span class="special">==</span> <span class="special">--</span><span class="identifier">fiber_count</span><span class="special">)</span> <span class="special">{</span> <a class="co" name="fiber.migration.c34" href="migration.html#fiber.migration.c35"><img src="../../../../../doc/src/images/callouts/6.png" alt="6" border="0"></a>
 275         <span class="identifier">lk</span><span class="special">.</span><span class="identifier">unlock</span><span class="special">();</span>
 276         <span class="identifier">cnd_count</span><span class="special">.</span><span class="identifier">notify_all</span><span class="special">();</span> <a class="co" name="fiber.migration.c36" href="migration.html#fiber.migration.c37"><img src="../../../../../doc/src/images/callouts/7.png" alt="7" border="0"></a>
 277     <span class="special">}</span>
 278 <span class="special">}</span>
 279 </pre>
 280 <p>
 281     </p>
 282 <div class="calloutlist"><table border="0" summary="Callout list">
 283 <tr>
 284 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c25"></a><a href="#fiber.migration.c24"><img src="../../../../../doc/src/images/callouts/1.png" alt="1" border="0"></a> </p></td>
 285 <td valign="top" align="left"><p>
 286           get ID of initial thread
 287         </p></td>
 288 </tr>
 289 <tr>
 290 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c27"></a><a href="#fiber.migration.c26"><img src="../../../../../doc/src/images/callouts/2.png" alt="2" border="0"></a> </p></td>
 291 <td valign="top" align="left"><p>
 292           loop ten times
 293         </p></td>
 294 </tr>
 295 <tr>
 296 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c29"></a><a href="#fiber.migration.c28"><img src="../../../../../doc/src/images/callouts/3.png" alt="3" border="0"></a> </p></td>
 297 <td valign="top" align="left"><p>
 298           yield to other fibers
 299         </p></td>
 300 </tr>
 301 <tr>
 302 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c31"></a><a href="#fiber.migration.c30"><img src="../../../../../doc/src/images/callouts/4.png" alt="4" border="0"></a> </p></td>
 303 <td valign="top" align="left"><p>
 304           get ID of current thread
 305         </p></td>
 306 </tr>
 307 <tr>
 308 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c33"></a><a href="#fiber.migration.c32"><img src="../../../../../doc/src/images/callouts/5.png" alt="5" border="0"></a> </p></td>
 309 <td valign="top" align="left"><p>
 310           test if fiber was migrated to another thread
 311         </p></td>
 312 </tr>
 313 <tr>
 314 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c35"></a><a href="#fiber.migration.c34"><img src="../../../../../doc/src/images/callouts/6.png" alt="6" border="0"></a> </p></td>
 315 <td valign="top" align="left"><p>
 316           Decrement fiber counter for each completed fiber.
 317         </p></td>
 318 </tr>
 319 <tr>
 320 <td width="5%" valign="top" align="left"><p><a name="fiber.migration.c37"></a><a href="#fiber.migration.c36"><img src="../../../../../doc/src/images/callouts/7.png" alt="7" border="0"></a> </p></td>
 321 <td valign="top" align="left"><p>
 322           Notify all fibers waiting on <code class="computeroutput"><span class="identifier">cnd_count</span></code>.
 323         </p></td>
 324 </tr>
 325 </table></div>
 326 <h4>
 327 <a name="fiber.migration.h3"></a>
 328       <span><a name="fiber.migration.scheduling_fibers"></a></span><a class="link" href="migration.html#fiber.migration.scheduling_fibers">Scheduling
 329       fibers</a>
 330     </h4>
 331 <p>
 332       The fiber scheduler <code class="computeroutput"><span class="identifier">shared_ready_queue</span></code>
 333       is like <code class="computeroutput"><span class="identifier">round_robin</span></code>, except
 334       that it shares a common ready queue among all participating threads. A thread
 335       participates in this pool by executing <a class="link" href="fiber_mgmt/fiber.html#use_scheduling_algorithm"><code class="computeroutput">use_scheduling_algorithm()</code></a>
 336 before
 337       any other <span class="bold"><strong>Boost.Fiber</strong></span> operation.
 338     </p>
 339 <p>
 340       The important point about the ready queue is that it&#8217;s a class static, common
 341       to all instances of shared_ready_queue. Fibers that are enqueued via <a class="link" href="scheduling.html#algorithm_awakened"><code class="computeroutput">algorithm::awakened()</code></a> (fibers
 342       that are ready to be resumed) are thus available to all threads. It is required
 343       to reserve a separate, scheduler-specific queue for the thread&#8217;s main fiber
 344       and dispatcher fibers: these may <span class="emphasis"><em>not</em></span> be shared between
 345       threads! When we&#8217;re passed either of these fibers, push it there instead of
 346       in the shared queue: it would be Bad News for thread B to retrieve and attempt
 347       to execute thread A&#8217;s main fiber.
 348     </p>
 349 <p>
 350       [awakened_ws]
 351     </p>
 352 <p>
 353       When <a class="link" href="scheduling.html#algorithm_pick_next"><code class="computeroutput">algorithm::pick_next()</code></a> gets called inside one thread,
 354       a fiber is dequeued from <span class="emphasis"><em>rqueue_</em></span> and will be resumed in
 355       that thread.
 356     </p>
 357 <p>
 358       [pick_next_ws]
 359     </p>
 360 <p>
 361       The source code above is found in <a href="../../../examples/work_sharing.cpp" target="_top">work_sharing.cpp</a>.
 362     </p>
 363 <div class="footnotes">
 364 <br><hr width="100" align="left">
 365 <div class="footnote"><p><sup>[<a name="ftn.fiber.migration.f0" href="#fiber.migration.f0" class="para">2</a>] </sup>
 366         The <span class="quote">&#8220;<span class="quote">main</span>&#8221;</span> fiber on each thread, that is, the fiber on which
 367         the thread is launched, cannot migrate to any other thread. Also <span class="bold"><strong>Boost.Fiber</strong></span> implicitly creates a dispatcher fiber
 368         for each thread &#8212; this cannot migrate either.
 369       </p></div>
 370 <div class="footnote"><p><sup>[<a name="ftn.fiber.migration.f1" href="#fiber.migration.f1" class="para">3</a>] </sup>
 371         Of course it would be problematic to migrate a fiber that relies on <a class="link" href="overview.html#thread_local_storage">thread-local storage</a>.
 372       </p></div>
 373 </div>
 374 </div>
 375 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 376 <td align="left"></td>
 377 <td align="right"><div class="copyright-footer">Copyright &#169; 2013 Oliver Kowalke<p>
 378         Distributed under the Boost Software License, Version 1.0. (See accompanying
 379         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>)
 380       </p>
 381 </div></td>
 382 </tr></table>
 383 <hr>
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