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  27 <div><h2 class="title">
  28 <a name="lockfree"></a>Chapter&#160;18.&#160;Boost.Lockfree</h2></div>
  29 <div><div class="author"><h3 class="author">
  30 <span class="firstname">Tim</span> <span class="surname">Blechmann</span>
  31 </h3></div></div>
  32 <div><p class="copyright">Copyright &#169; 2008-2011 Tim
  33       Blechmann</p></div>
  34 <div><div class="legalnotice">
  35 <a name="lockfree.legal"></a><p>
  36         Distributed under the Boost Software License, Version 1.0. (See accompanying
  37         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>)
  38       </p>
  39 </div></div>
  40 </div></div>
  41 <div class="toc">
  42 <p><b>Table of Contents</b></p>
  43 <dl class="toc">
  44 <dt><span class="section"><a href="lockfree.html#lockfree.introduction___motivation">Introduction &amp;
  45     Motivation</a></span></dt>
  46 <dt><span class="section"><a href="lockfree/examples.html">Examples</a></span></dt>
  47 <dt><span class="section"><a href="lockfree/rationale.html">Rationale</a></span></dt>
  48 <dd><dl>
  49 <dt><span class="section"><a href="lockfree/rationale.html#lockfree.rationale.data_structures">Data Structures</a></span></dt>
  50 <dt><span class="section"><a href="lockfree/rationale.html#lockfree.rationale.memory_management">Memory Management</a></span></dt>
  51 <dt><span class="section"><a href="lockfree/rationale.html#lockfree.rationale.aba_prevention">ABA Prevention</a></span></dt>
  52 <dt><span class="section"><a href="lockfree/rationale.html#lockfree.rationale.interprocess_support">Interprocess
  53       Support</a></span></dt>
  54 </dl></dd>
  55 <dt><span class="section"><a href="lockfree/reference.html">Reference</a></span></dt>
  56 <dd><dl>
  57 <dt><span class="section"><a href="lockfree/reference.html#header.boost.lockfree.policies_hpp">Header &lt;boost/lockfree/policies.hpp&gt;</a></span></dt>
  58 <dt><span class="section"><a href="lockfree/reference.html#header.boost.lockfree.queue_hpp">Header &lt;boost/lockfree/queue.hpp&gt;</a></span></dt>
  59 <dt><span class="section"><a href="lockfree/reference.html#header.boost.lockfree.spsc_queue_hpp">Header &lt;boost/lockfree/spsc_queue.hpp&gt;</a></span></dt>
  60 <dt><span class="section"><a href="lockfree/reference.html#header.boost.lockfree.stack_hpp">Header &lt;boost/lockfree/stack.hpp&gt;</a></span></dt>
  61 </dl></dd>
  62 <dt><span class="section"><a href="lockfree/appendices.html">Appendices</a></span></dt>
  63 <dd><dl>
  64 <dt><span class="section"><a href="lockfree/appendices.html#lockfree.appendices.supported_platforms___compilers">Supported
  65       Platforms &amp; Compilers</a></span></dt>
  66 <dt><span class="section"><a href="lockfree/appendices.html#lockfree.appendices.future_developments">Future Developments</a></span></dt>
  67 <dt><span class="section"><a href="lockfree/appendices.html#lockfree.appendices.references">References</a></span></dt>
  68 </dl></dd>
  69 </dl>
  70 </div>
  71 <div class="section">
  72 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  73 <a name="lockfree.introduction___motivation"></a><a class="link" href="lockfree.html#lockfree.introduction___motivation" title="Introduction &amp; Motivation">Introduction &amp;
  74     Motivation</a>
  75 </h2></div></div></div>
  76 <h3>
  77 <a name="lockfree.introduction___motivation.h0"></a>
  78       <span class="phrase"><a name="lockfree.introduction___motivation.introduction__amp__terminology"></a></span><a class="link" href="lockfree.html#lockfree.introduction___motivation.introduction__amp__terminology">Introduction
  79       &amp; Terminology</a>
  80     </h3>
  81 <p>
  82       The term <span class="bold"><strong>non-blocking</strong></span> denotes concurrent data
  83       structures, which do not use traditional synchronization primitives like guards
  84       to ensure thread-safety. Maurice Herlihy and Nir Shavit (compare <a href="http://books.google.com/books?id=pFSwuqtJgxYC" target="_top">"The
  85       Art of Multiprocessor Programming"</a>) distinguish between 3 types
  86       of non-blocking data structures, each having different properties:
  87     </p>
  88 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  89 <li class="listitem">
  90           data structures are <span class="bold"><strong>wait-free</strong></span>, if every
  91           concurrent operation is guaranteed to be finished in a finite number of
  92           steps. It is therefore possible to give worst-case guarantees for the number
  93           of operations.
  94         </li>
  95 <li class="listitem">
  96           data structures are <span class="bold"><strong>lock-free</strong></span>, if some
  97           concurrent operations are guaranteed to be finished in a finite number
  98           of steps. While it is in theory possible that some operations never make
  99           any progress, it is very unlikely to happen in practical applications.
 100         </li>
 101 <li class="listitem">
 102           data structures are <span class="bold"><strong>obstruction-free</strong></span>,
 103           if a concurrent operation is guaranteed to be finished in a finite number
 104           of steps, unless another concurrent operation interferes.
 105         </li>
 106 </ul></div>
 107 <p>
 108       Some data structures can only be implemented in a lock-free manner, if they
 109       are used under certain restrictions. The relevant aspects for the implementation
 110       of <code class="literal">boost.lockfree</code> are the number of producer and consumer
 111       threads. <span class="bold"><strong>Single-producer</strong></span> (<span class="bold"><strong>sp</strong></span>)
 112       or <span class="bold"><strong>multiple producer</strong></span> (<span class="bold"><strong>mp</strong></span>)
 113       means that only a single thread or multiple concurrent threads are allowed
 114       to add data to a data structure. <span class="bold"><strong>Single-consumer</strong></span>
 115       (<span class="bold"><strong>sc</strong></span>) or <span class="bold"><strong>Multiple-consumer</strong></span>
 116       (<span class="bold"><strong>mc</strong></span>) denote the equivalent for the removal
 117       of data from the data structure.
 118     </p>
 119 <h3>
 120 <a name="lockfree.introduction___motivation.h1"></a>
 121       <span class="phrase"><a name="lockfree.introduction___motivation.properties_of_non_blocking_data_structures"></a></span><a class="link" href="lockfree.html#lockfree.introduction___motivation.properties_of_non_blocking_data_structures">Properties
 122       of Non-Blocking Data Structures</a>
 123     </h3>
 124 <p>
 125       Non-blocking data structures do not rely on locks and mutexes to ensure thread-safety.
 126       The synchronization is done completely in user-space without any direct interaction
 127       with the operating system <a href="#ftn.lockfree.introduction___motivation.f0" class="footnote" name="lockfree.introduction___motivation.f0"><sup class="footnote">[4]</sup></a>. This implies that they are not prone to issues like priority inversion
 128       (a low-priority thread needs to wait for a high-priority thread).
 129     </p>
 130 <p>
 131       Instead of relying on guards, non-blocking data structures require <span class="bold"><strong>atomic operations</strong></span> (specific CPU instructions executed
 132       without interruption). This means that any thread either sees the state before
 133       or after the operation, but no intermediate state can be observed. Not all
 134       hardware supports the same set of atomic instructions. If it is not available
 135       in hardware, it can be emulated in software using guards. However this has
 136       the obvious drawback of losing the lock-free property.
 137     </p>
 138 <h3>
 139 <a name="lockfree.introduction___motivation.h2"></a>
 140       <span class="phrase"><a name="lockfree.introduction___motivation.performance_of_non_blocking_data_structures"></a></span><a class="link" href="lockfree.html#lockfree.introduction___motivation.performance_of_non_blocking_data_structures">Performance
 141       of Non-Blocking Data Structures</a>
 142     </h3>
 143 <p>
 144       When discussing the performance of non-blocking data structures, one has to
 145       distinguish between <span class="bold"><strong>amortized</strong></span> and <span class="bold"><strong>worst-case</strong></span> costs. The definition of 'lock-free' and
 146       'wait-free' only mention the upper bound of an operation. Therefore lock-free
 147       data structures are not necessarily the best choice for every use case. In
 148       order to maximise the throughput of an application one should consider high-performance
 149       concurrent data structures <a href="#ftn.lockfree.introduction___motivation.f1" class="footnote" name="lockfree.introduction___motivation.f1"><sup class="footnote">[5]</sup></a>.
 150     </p>
 151 <p>
 152       Lock-free data structures will be a better choice in order to optimize the
 153       latency of a system or to avoid priority inversion, which may be necessary
 154       in real-time applications. In general we advise to consider if lock-free data
 155       structures are necessary or if concurrent data structures are sufficient. In
 156       any case we advice to perform benchmarks with different data structures for
 157       a specific workload.
 158     </p>
 159 <h3>
 160 <a name="lockfree.introduction___motivation.h3"></a>
 161       <span class="phrase"><a name="lockfree.introduction___motivation.sources_of_blocking_behavior"></a></span><a class="link" href="lockfree.html#lockfree.introduction___motivation.sources_of_blocking_behavior">Sources
 162       of Blocking Behavior</a>
 163     </h3>
 164 <p>
 165       Apart from locks and mutexes (which we are not using in <code class="literal">boost.lockfree</code>
 166       anyway), there are three other aspects, that could violate lock-freedom:
 167     </p>
 168 <div class="variablelist">
 169 <p class="title"><b></b></p>
 170 <dl class="variablelist">
 171 <dt><span class="term">Atomic Operations</span></dt>
 172 <dd><p>
 173             Some architectures do not provide the necessary atomic operations in
 174             natively in hardware. If this is not the case, they are emulated in software
 175             using spinlocks, which by itself is blocking.
 176           </p></dd>
 177 <dt><span class="term">Memory Allocations</span></dt>
 178 <dd><p>
 179             Allocating memory from the operating system is not lock-free. This makes
 180             it impossible to implement true dynamically-sized non-blocking data structures.
 181             The node-based data structures of <code class="literal">boost.lockfree</code> use
 182             a memory pool to allocate the internal nodes. If this memory pool is
 183             exhausted, memory for new nodes has to be allocated from the operating
 184             system. However all data structures of <code class="literal">boost.lockfree</code>
 185             can be configured to avoid memory allocations (instead the specific calls
 186             will fail). This is especially useful for real-time systems that require
 187             lock-free memory allocations.
 188           </p></dd>
 189 <dt><span class="term">Exception Handling</span></dt>
 190 <dd><p>
 191             The C++ exception handling does not give any guarantees about its real-time
 192             behavior. We therefore do not encourage the use of exceptions and exception
 193             handling in lock-free code.
 194           </p></dd>
 195 </dl>
 196 </div>
 197 <h3>
 198 <a name="lockfree.introduction___motivation.h4"></a>
 199       <span class="phrase"><a name="lockfree.introduction___motivation.data_structures"></a></span><a class="link" href="lockfree.html#lockfree.introduction___motivation.data_structures">Data
 200       Structures</a>
 201     </h3>
 202 <p>
 203       <code class="literal">boost.lockfree</code> implements three lock-free data structures:
 204     </p>
 205 <div class="variablelist">
 206 <p class="title"><b></b></p>
 207 <dl class="variablelist">
 208 <dt><span class="term"><code class="computeroutput"><a class="link" href="boost/lockfree/queue.html" title="Class template queue">boost::lockfree::queue</a></code></span></dt>
 209 <dd><p>
 210             a lock-free multi-produced/multi-consumer queue
 211           </p></dd>
 212 <dt><span class="term"><code class="computeroutput"><a class="link" href="boost/lockfree/stack.html" title="Class template stack">boost::lockfree::stack</a></code></span></dt>
 213 <dd><p>
 214             a lock-free multi-produced/multi-consumer stack
 215           </p></dd>
 216 <dt><span class="term"><code class="computeroutput"><a class="link" href="boost/lockfree/spsc_queue.html" title="Class template spsc_queue">boost::lockfree::spsc_queue</a></code></span></dt>
 217 <dd><p>
 218             a wait-free single-producer/single-consumer queue (commonly known as
 219             ringbuffer)
 220           </p></dd>
 221 </dl>
 222 </div>
 223 <h4>
 224 <a name="lockfree.introduction___motivation.h5"></a>
 225       <span class="phrase"><a name="lockfree.introduction___motivation.data_structure_configuration"></a></span><a class="link" href="lockfree.html#lockfree.introduction___motivation.data_structure_configuration">Data
 226       Structure Configuration</a>
 227     </h4>
 228 <p>
 229       The data structures can be configured with <a href="../../libs/parameter/doc/html/index.html" target="_top">Boost.Parameter</a>-style
 230       templates:
 231     </p>
 232 <div class="variablelist">
 233 <p class="title"><b></b></p>
 234 <dl class="variablelist">
 235 <dt><span class="term"><code class="computeroutput"><a class="link" href="boost/lockfree/fixed_sized.html" title="Struct template fixed_sized">boost::lockfree::fixed_sized</a></code></span></dt>
 236 <dd><p>
 237             Configures the data structure as <span class="bold"><strong>fixed sized</strong></span>.
 238             The internal nodes are stored inside an array and they are addressed
 239             by array indexing. This limits the possible size of the queue to the
 240             number of elements that can be addressed by the index type (usually 2**16-2),
 241             but on platforms that lack double-width compare-and-exchange instructions,
 242             this is the best way to achieve lock-freedom.
 243           </p></dd>
 244 <dt><span class="term"><code class="computeroutput"><a class="link" href="boost/lockfree/capacity.html" title="Struct template capacity">boost::lockfree::capacity</a></code></span></dt>
 245 <dd><p>
 246             Sets the <span class="bold"><strong>capacity</strong></span> of a data structure
 247             at compile-time. This implies that a data structure is fixed-sized.
 248           </p></dd>
 249 <dt><span class="term"><code class="computeroutput"><a class="link" href="boost/lockfree/allocator.html" title="Struct template allocator">boost::lockfree::allocator</a></code></span></dt>
 250 <dd><p>
 251             Defines the allocator. <code class="literal">boost.lockfree</code> supports stateful
 252             allocator and is compatible with <a href="../../libs/interprocess/index.html" target="_top">Boost.Interprocess</a>
 253             allocators.
 254           </p></dd>
 255 </dl>
 256 </div>
 257 </div>
 258 <div class="footnotes">
 259 <br><hr style="width:100; text-align:left;margin-left: 0">
 260 <div id="ftn.lockfree.introduction___motivation.f0" class="footnote"><p><a href="#lockfree.introduction___motivation.f0" class="para"><sup class="para">[4] </sup></a>
 261         Spinlocks do not directly interact with the operating system either. However
 262         it is possible that the owning thread is preempted by the operating system,
 263         which violates the lock-free property.
 264       </p></div>
 265 <div id="ftn.lockfree.introduction___motivation.f1" class="footnote"><p><a href="#lockfree.introduction___motivation.f1" class="para"><sup class="para">[5] </sup></a>
 266         <a href="http://threadingbuildingblocks.org/" target="_top">Intel's Thread Building
 267         Blocks library</a> provides many efficient concurrent data structures,
 268         which are not necessarily lock-free.
 269       </p></div>
 270 </div>
 271 </div>
 272 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 273 <td align="left"><p><small>Last revised: October 30, 2014 at 10:20:18 GMT</small></p></td>
 274 <td align="right"><div class="copyright-footer"></div></td>
 275 </tr></table>
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