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<title>Migrating fibers between threads</title>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="fiber.migration"></a><a name="migration"></a><a class="link" href="migration.html" title="Migrating fibers between threads">Migrating fibers
between threads</a>
</h2></div></div></div>
<h4>
<a name="fiber.migration.h0"></a>
<span><a name="fiber.migration.overview"></a></span><a class="link" href="migration.html#fiber.migration.overview">Overview</a>
</h4>
<p>
Each fiber owns a stack and manages its execution state, including all registers
and CPU flags, the instruction pointer and the stack pointer. That means, in
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>
</p>
<p>
Migrating a fiber from a logical CPU with heavy workload to another logical
CPU with a lighter workload might speed up the overall execution. Note that
in the case of NUMA-architectures, it is not always advisable to migrate data
between threads. Suppose fiber <span class="emphasis"><em>f</em></span> is running on logical
CPU <span class="emphasis"><em>cpu0</em></span> which belongs to NUMA node <span class="emphasis"><em>node0</em></span>.
The data of <span class="emphasis"><em>f</em></span> are allocated on the physical memory located
at <span class="emphasis"><em>node0</em></span>. Migrating the fiber from <span class="emphasis"><em>cpu0</em></span>
to another logical CPU <span class="emphasis"><em>cpuX</em></span> which is part of a different
NUMA node <span class="emphasis"><em>nodeX</em></span> might reduce the performance of the application
due to increased latency of memory access.
</p>
<p>
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
migrate between threads. You cannot migrate a running fiber, nor one that is
<a class="link" href="overview.html#blocking"><span class="emphasis"><em>blocked</em></span></a>. You cannot migrate
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>.
</p>
<p>
In <span class="bold"><strong>Boost.Fiber</strong></span> a fiber is migrated by invoking
<a class="link" href="scheduling.html#context_detach"><code class="computeroutput">context::detach()</code></a> on the thread from which the fiber migrates
and <a class="link" href="scheduling.html#context_attach"><code class="computeroutput">context::attach()</code></a> on the thread to which the fiber migrates.
</p>
<p>
Thus, fiber migration is accomplished by sharing state between instances of
a user-coded <a class="link" href="scheduling.html#class_algorithm"><code class="computeroutput">algorithm</code></a> implementation running on different threads.
The fiber&#8217;s original thread calls <a class="link" href="scheduling.html#algorithm_awakened"><code class="computeroutput">algorithm::awakened()</code></a>, passing
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>.
</p>
<p>
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>,
the <code class="computeroutput"><span class="identifier">pick_next</span><span class="special">()</span></code>
implementation selects a ready fiber and calls <a class="link" href="scheduling.html#context_attach"><code class="computeroutput">context::attach()</code></a> on
it before returning it.
</p>
<p>
As stated above, a <code class="computeroutput"><span class="identifier">context</span></code>
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>
<span class="special">==</span> <span class="keyword">true</span></code>
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>.
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
that passed that context to <code class="computeroutput"><span class="identifier">awakened</span><span class="special">()</span></code>.
</p>
<h4>
<a name="fiber.migration.h1"></a>
<span><a name="fiber.migration.example_of_work_sharing"></a></span><a class="link" href="migration.html#fiber.migration.example_of_work_sharing">Example
of work sharing</a>
</h4>
<p>
In the example <a href="../../../examples/work_sharing.cpp" target="_top">work_sharing.cpp</a>
multiple worker fibers are created on the main thread. Each fiber gets a character
as parameter at construction. This character is printed out ten times. Between
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
the fiber in the ready queue of the fiber-scheduler <span class="emphasis"><em>shared_ready_queue</em></span>,
running in the current thread. The next fiber ready to be executed is dequeued
from the shared ready queue and resumed by <span class="emphasis"><em>shared_ready_queue</em></span>
running on <span class="emphasis"><em>any participating thread</em></span>.
</p>
<p>
All instances of <span class="emphasis"><em>shared_ready_queue</em></span> share one global concurrent
queue, used as ready queue. This mechanism shares all worker fibers between
all instances of <span class="emphasis"><em>shared_ready_queue</em></span>, thus between all
participating threads.
</p>
<h4>
<a name="fiber.migration.h2"></a>
<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
of threads and fibers</a>
</h4>
<p>
In <code class="computeroutput"><span class="identifier">main</span><span class="special">()</span></code>
the fiber-scheduler is installed and the worker fibers and the threads are
launched.
</p>
<p>
</p>
<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>
<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>
<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>
<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>
<span class="special">}</span>
<span class="identifier">barrier</span> <span class="identifier">b</span><span class="special">(</span> <span class="number">4</span><span class="special">);</span>
<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>
<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>
<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>
<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>
<span class="special">};</span>
<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>
<span class="special">{</span>
<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>
<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>
<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>
<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>
<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>
<span class="identifier">t</span><span class="special">.</span><span class="identifier">join</span><span class="special">();</span>
<span class="special">}</span>
</pre>
<p>
</p>
<div class="calloutlist"><table border="0" summary="Callout list">
<tr>
<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>
<td valign="top" align="left"><p>
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>
in the main thread too, so each new fiber gets launched into the shared
pool.
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
Launch a number of worker fibers; each worker fiber picks up a character
that is passed as parameter to fiber-function <code class="computeroutput"><span class="identifier">whatevah</span></code>.
Each worker fiber gets detached.
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
Increment fiber counter for each new fiber.
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
Launch a couple of threads that join the work sharing.
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
sync with other threads: allow them to start processing
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
<code class="computeroutput"><span class="identifier">lock_t</span></code> is typedef'ed as
<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;
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
Suspend main fiber and resume worker fibers in the meanwhile. Main fiber
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.
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
Releasing lock of mtx_count is required before joining the threads, otherwise
the other threads would be blocked inside condition_variable::wait() and
would never return (deadlock).
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
wait for threads to terminate
</p></td>
</tr>
</table></div>
<p>
The start of the threads is synchronized with a barrier. The main fiber of
each thread (including main thread) is suspended until all worker fibers are
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>,
the thread terminates: the main thread joins all other threads.
</p>
<p>
</p>
<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>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">ostringstream</span> <span class="identifier">buffer</span><span class="special">;</span>
<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>
<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>
<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>
<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>
<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>
<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>
<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>
<span class="special">}</span>
</pre>
<p>
</p>
<div class="calloutlist"><table border="0" summary="Callout list">
<tr>
<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>
<td valign="top" align="left"><p>
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>
in order to join the work sharing.
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
sync with other threads: allow them to start processing
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
Suspend main fiber and resume worker fibers in the meanwhile. Main fiber
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.
</p></td>
</tr>
</table></div>
<p>
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>
as parameter. The fiber yields in a loop and prints out a message if it was
migrated to another thread.
</p>
<p>
</p>
<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>
<span class="keyword">try</span> <span class="special">{</span>
<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>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">ostringstream</span> <span class="identifier">buffer</span><span class="special">;</span>
<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>
<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>
<span class="special">}</span>
<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>
<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>
<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>
<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>
<span class="identifier">my_thread</span> <span class="special">=</span> <span class="identifier">new_thread</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">ostringstream</span> <span class="identifier">buffer</span><span class="special">;</span>
<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>
<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>
<span class="special">}</span>
<span class="special">}</span>
<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>
<span class="special">}</span>
<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>
<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>
<span class="identifier">lk</span><span class="special">.</span><span class="identifier">unlock</span><span class="special">();</span>
<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>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
</p>
<div class="calloutlist"><table border="0" summary="Callout list">
<tr>
<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>
<td valign="top" align="left"><p>
get ID of initial thread
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
loop ten times
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
yield to other fibers
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
get ID of current thread
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
test if fiber was migrated to another thread
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
Decrement fiber counter for each completed fiber.
</p></td>
</tr>
<tr>
<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>
<td valign="top" align="left"><p>
Notify all fibers waiting on <code class="computeroutput"><span class="identifier">cnd_count</span></code>.
</p></td>
</tr>
</table></div>
<h4>
<a name="fiber.migration.h3"></a>
<span><a name="fiber.migration.scheduling_fibers"></a></span><a class="link" href="migration.html#fiber.migration.scheduling_fibers">Scheduling
fibers</a>
</h4>
<p>
The fiber scheduler <code class="computeroutput"><span class="identifier">shared_ready_queue</span></code>
is like <code class="computeroutput"><span class="identifier">round_robin</span></code>, except
that it shares a common ready queue among all participating threads. A thread
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>
before
any other <span class="bold"><strong>Boost.Fiber</strong></span> operation.
</p>
<p>
The important point about the ready queue is that it&#8217;s a class static, common
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
that are ready to be resumed) are thus available to all threads. It is required
to reserve a separate, scheduler-specific queue for the thread&#8217;s main fiber
and dispatcher fibers: these may <span class="emphasis"><em>not</em></span> be shared between
threads! When we&#8217;re passed either of these fibers, push it there instead of
in the shared queue: it would be Bad News for thread B to retrieve and attempt
to execute thread A&#8217;s main fiber.
</p>
<p>
[awakened_ws]
</p>
<p>
When <a class="link" href="scheduling.html#algorithm_pick_next"><code class="computeroutput">algorithm::pick_next()</code></a> gets called inside one thread,
a fiber is dequeued from <span class="emphasis"><em>rqueue_</em></span> and will be resumed in
that thread.
</p>
<p>
[pick_next_ws]
</p>
<p>
The source code above is found in <a href="../../../examples/work_sharing.cpp" target="_top">work_sharing.cpp</a>.
</p>
<div class="footnotes">
<br><hr width="100" align="left">
<div class="footnote"><p><sup>[<a name="ftn.fiber.migration.f0" href="#fiber.migration.f0" class="para">2</a>] </sup>
The <span class="quote">&#8220;<span class="quote">main</span>&#8221;</span> fiber on each thread, that is, the fiber on which
the thread is launched, cannot migrate to any other thread. Also <span class="bold"><strong>Boost.Fiber</strong></span> implicitly creates a dispatcher fiber
for each thread &#8212; this cannot migrate either.
</p></div>
<div class="footnote"><p><sup>[<a name="ftn.fiber.migration.f1" href="#fiber.migration.f1" class="para">3</a>] </sup>
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>.
</p></div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2013 Oliver Kowalke<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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>)
</p>
</div></td>
</tr></table>
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