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<div class="chapter">
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<div><h2 class="title">
<a name="lockfree"></a>Chapter 18. Boost.Lockfree</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Tim</span> <span class="surname">Blechmann</span>
</h3></div></div>
<div><p class="copyright">Copyright © 2008-2011 Tim
Blechmann</p></div>
<div><div class="legalnotice">
<a name="lockfree.legal"></a><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></div>
</div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="lockfree.html#lockfree.introduction___motivation">Introduction &
Motivation</a></span></dt>
<dt><span class="section"><a href="lockfree/examples.html">Examples</a></span></dt>
<dt><span class="section"><a href="lockfree/rationale.html">Rationale</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="lockfree/rationale.html#lockfree.rationale.data_structures">Data Structures</a></span></dt>
<dt><span class="section"><a href="lockfree/rationale.html#lockfree.rationale.memory_management">Memory Management</a></span></dt>
<dt><span class="section"><a href="lockfree/rationale.html#lockfree.rationale.aba_prevention">ABA Prevention</a></span></dt>
<dt><span class="section"><a href="lockfree/rationale.html#lockfree.rationale.interprocess_support">Interprocess
Support</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="lockfree/reference.html">Reference</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="lockfree/reference.html#header.boost.lockfree.policies_hpp">Header <boost/lockfree/policies.hpp></a></span></dt>
<dt><span class="section"><a href="lockfree/reference.html#header.boost.lockfree.queue_hpp">Header <boost/lockfree/queue.hpp></a></span></dt>
<dt><span class="section"><a href="lockfree/reference.html#header.boost.lockfree.spsc_queue_hpp">Header <boost/lockfree/spsc_queue.hpp></a></span></dt>
<dt><span class="section"><a href="lockfree/reference.html#header.boost.lockfree.stack_hpp">Header <boost/lockfree/stack.hpp></a></span></dt>
</dl></dd>
<dt><span class="section"><a href="lockfree/appendices.html">Appendices</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="lockfree/appendices.html#lockfree.appendices.supported_platforms___compilers">Supported
Platforms & Compilers</a></span></dt>
<dt><span class="section"><a href="lockfree/appendices.html#lockfree.appendices.future_developments">Future Developments</a></span></dt>
<dt><span class="section"><a href="lockfree/appendices.html#lockfree.appendices.references">References</a></span></dt>
</dl></dd>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="lockfree.introduction___motivation"></a><a class="link" href="lockfree.html#lockfree.introduction___motivation" title="Introduction & Motivation">Introduction &
Motivation</a>
</h2></div></div></div>
<h3>
<a name="lockfree.introduction___motivation.h0"></a>
<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
& Terminology</a>
</h3>
<p>
The term <span class="bold"><strong>non-blocking</strong></span> denotes concurrent data
structures, which do not use traditional synchronization primitives like guards
to ensure thread-safety. Maurice Herlihy and Nir Shavit (compare <a href="http://books.google.com/books?id=pFSwuqtJgxYC" target="_top">"The
Art of Multiprocessor Programming"</a>) distinguish between 3 types
of non-blocking data structures, each having different properties:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
data structures are <span class="bold"><strong>wait-free</strong></span>, if every
concurrent operation is guaranteed to be finished in a finite number of
steps. It is therefore possible to give worst-case guarantees for the number
of operations.
</li>
<li class="listitem">
data structures are <span class="bold"><strong>lock-free</strong></span>, if some
concurrent operations are guaranteed to be finished in a finite number
of steps. While it is in theory possible that some operations never make
any progress, it is very unlikely to happen in practical applications.
</li>
<li class="listitem">
data structures are <span class="bold"><strong>obstruction-free</strong></span>,
if a concurrent operation is guaranteed to be finished in a finite number
of steps, unless another concurrent operation interferes.
</li>
</ul></div>
<p>
Some data structures can only be implemented in a lock-free manner, if they
are used under certain restrictions. The relevant aspects for the implementation
of <code class="literal">boost.lockfree</code> are the number of producer and consumer
threads. <span class="bold"><strong>Single-producer</strong></span> (<span class="bold"><strong>sp</strong></span>)
or <span class="bold"><strong>multiple producer</strong></span> (<span class="bold"><strong>mp</strong></span>)
means that only a single thread or multiple concurrent threads are allowed
to add data to a data structure. <span class="bold"><strong>Single-consumer</strong></span>
(<span class="bold"><strong>sc</strong></span>) or <span class="bold"><strong>Multiple-consumer</strong></span>
(<span class="bold"><strong>mc</strong></span>) denote the equivalent for the removal
of data from the data structure.
</p>
<h3>
<a name="lockfree.introduction___motivation.h1"></a>
<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
of Non-Blocking Data Structures</a>
</h3>
<p>
Non-blocking data structures do not rely on locks and mutexes to ensure thread-safety.
The synchronization is done completely in user-space without any direct interaction
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
(a low-priority thread needs to wait for a high-priority thread).
</p>
<p>
Instead of relying on guards, non-blocking data structures require <span class="bold"><strong>atomic operations</strong></span> (specific CPU instructions executed
without interruption). This means that any thread either sees the state before
or after the operation, but no intermediate state can be observed. Not all
hardware supports the same set of atomic instructions. If it is not available
in hardware, it can be emulated in software using guards. However this has
the obvious drawback of losing the lock-free property.
</p>
<h3>
<a name="lockfree.introduction___motivation.h2"></a>
<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
of Non-Blocking Data Structures</a>
</h3>
<p>
When discussing the performance of non-blocking data structures, one has to
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
'wait-free' only mention the upper bound of an operation. Therefore lock-free
data structures are not necessarily the best choice for every use case. In
order to maximise the throughput of an application one should consider high-performance
concurrent data structures <a href="#ftn.lockfree.introduction___motivation.f1" class="footnote" name="lockfree.introduction___motivation.f1"><sup class="footnote">[5]</sup></a>.
</p>
<p>
Lock-free data structures will be a better choice in order to optimize the
latency of a system or to avoid priority inversion, which may be necessary
in real-time applications. In general we advise to consider if lock-free data
structures are necessary or if concurrent data structures are sufficient. In
any case we advice to perform benchmarks with different data structures for
a specific workload.
</p>
<h3>
<a name="lockfree.introduction___motivation.h3"></a>
<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
of Blocking Behavior</a>
</h3>
<p>
Apart from locks and mutexes (which we are not using in <code class="literal">boost.lockfree</code>
anyway), there are three other aspects, that could violate lock-freedom:
</p>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">Atomic Operations</span></dt>
<dd><p>
Some architectures do not provide the necessary atomic operations in
natively in hardware. If this is not the case, they are emulated in software
using spinlocks, which by itself is blocking.
</p></dd>
<dt><span class="term">Memory Allocations</span></dt>
<dd><p>
Allocating memory from the operating system is not lock-free. This makes
it impossible to implement true dynamically-sized non-blocking data structures.
The node-based data structures of <code class="literal">boost.lockfree</code> use
a memory pool to allocate the internal nodes. If this memory pool is
exhausted, memory for new nodes has to be allocated from the operating
system. However all data structures of <code class="literal">boost.lockfree</code>
can be configured to avoid memory allocations (instead the specific calls
will fail). This is especially useful for real-time systems that require
lock-free memory allocations.
</p></dd>
<dt><span class="term">Exception Handling</span></dt>
<dd><p>
The C++ exception handling does not give any guarantees about its real-time
behavior. We therefore do not encourage the use of exceptions and exception
handling in lock-free code.
</p></dd>
</dl>
</div>
<h3>
<a name="lockfree.introduction___motivation.h4"></a>
<span class="phrase"><a name="lockfree.introduction___motivation.data_structures"></a></span><a class="link" href="lockfree.html#lockfree.introduction___motivation.data_structures">Data
Structures</a>
</h3>
<p>
<code class="literal">boost.lockfree</code> implements three lock-free data structures:
</p>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<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>
<dd><p>
a lock-free multi-produced/multi-consumer queue
</p></dd>
<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>
<dd><p>
a lock-free multi-produced/multi-consumer stack
</p></dd>
<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>
<dd><p>
a wait-free single-producer/single-consumer queue (commonly known as
ringbuffer)
</p></dd>
</dl>
</div>
<h4>
<a name="lockfree.introduction___motivation.h5"></a>
<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
Structure Configuration</a>
</h4>
<p>
The data structures can be configured with <a href="../../libs/parameter/doc/html/index.html" target="_top">Boost.Parameter</a>-style
templates:
</p>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<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>
<dd><p>
Configures the data structure as <span class="bold"><strong>fixed sized</strong></span>.
The internal nodes are stored inside an array and they are addressed
by array indexing. This limits the possible size of the queue to the
number of elements that can be addressed by the index type (usually 2**16-2),
but on platforms that lack double-width compare-and-exchange instructions,
this is the best way to achieve lock-freedom.
</p></dd>
<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>
<dd><p>
Sets the <span class="bold"><strong>capacity</strong></span> of a data structure
at compile-time. This implies that a data structure is fixed-sized.
</p></dd>
<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>
<dd><p>
Defines the allocator. <code class="literal">boost.lockfree</code> supports stateful
allocator and is compatible with <a href="../../libs/interprocess/index.html" target="_top">Boost.Interprocess</a>
allocators.
</p></dd>
</dl>
</div>
</div>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.lockfree.introduction___motivation.f0" class="footnote"><p><a href="#lockfree.introduction___motivation.f0" class="para"><sup class="para">[4] </sup></a>
Spinlocks do not directly interact with the operating system either. However
it is possible that the owning thread is preempted by the operating system,
which violates the lock-free property.
</p></div>
<div id="ftn.lockfree.introduction___motivation.f1" class="footnote"><p><a href="#lockfree.introduction___motivation.f1" class="para"><sup class="para">[5] </sup></a>
<a href="http://threadingbuildingblocks.org/" target="_top">Intel's Thread Building
Blocks library</a> provides many efficient concurrent data structures,
which are not necessarily lock-free.
</p></div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"><p><small>Last revised: December 14, 2015 at 09:19:03 GMT</small></p></td>
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