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<head>
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<meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards) and bkoz@gcc.gnu.org (Benjamin Kosnik)" />
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<meta name="KEYWORDS" content="c++, libstdc++, g++, allocator, memory" />
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<meta name="DESCRIPTION" content="Allocators and allocation" />
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<meta name="GENERATOR" content="emacs and ten fingers" />
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<title>Allocators and allocation</title>
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<link rel="StyleSheet" href="../lib3styles.css" type="text/css" />
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<link rel="Start" href="../documentation.html" type="text/html"
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title="GNU C++ Standard Library" />
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<link rel="Bookmark" href="howto.html" type="text/html"
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title="General Utilities" />
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<link rel="Copyright" href="../17_intro/license.html" type="text/html" />
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</head>
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<body>
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<h1 class="centered"><a name="top">Allocators and allocation</a></h1>
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<p class="fineprint"><em>
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The latest version of this document is always available at
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<a href="http://gcc.gnu.org/onlinedocs/libstdc++/20_util/allocator.html">
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http://gcc.gnu.org/onlinedocs/libstdc++/20_util/allocator.html</a>.
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</em></p>
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<p><em>
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To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++-v3 homepage</a>.
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</em></p>
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<!-- ####################################################### -->
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<hr />
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<p> The C++ Standard encapsulates memory management characteristics
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for strings, container classes, and parts of iostreams in a
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template class called <code>std::allocator</code>.
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</p>
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<h3 class="left">
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<a name="standard_requirements">Standard requirements</a>
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</h3>
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<p>The C++ standard only gives a few directives in this area:
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</p>
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<ul>
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<li>When you add elements to a container, and the container must allocate
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more memory to hold them, the container makes the request via its
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<code>Allocator</code> template parameter. This includes adding
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chars to the string class, which acts as a regular STL container
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in this respect.
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</li>
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<li>The default <code>Allocator</code> of every container-of-T is
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<code>std::allocator<T></code>.
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</li>
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<li>The interface of the <code>allocator<T></code> class is
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extremely simple. It has about 20 public declarations (nested
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typedefs, member functions, etc), but the two which concern us most
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are:
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<pre>
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T* allocate (size_type n, const void* hint = 0);
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void deallocate (T* p, size_type n);</pre>
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(This is a simplification; the real signatures use nested typedefs.)
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The <code>"n"</code> arguments in both those functions is a
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<em>count</em> of the number of T's to allocate space for,
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<em>not their total size</em>.
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</li>
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<li>"The storage is obtained by calling
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<code>::operator new(size_t)</code>, but it is unspecified when or
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how often this function is called. The use of <code>hint</code>
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is unspecified, but intended as an aid to locality if an
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implementation so desires." [20.4.1.1]/6
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</li>
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</ul>
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<p> Complete details cam be found in the C++ standard, look in
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[20.4 Memory].
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</p>
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<h3 class="left">
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<a name="probs_possibilities">Problems and Possibilities</a>
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</h3>
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<p>The easiest way of fulfilling the requirements is to call operator new
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each time a container needs memory, and to call operator delete each
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time the container releases memory. <strong>BUT</strong>
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<a href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00105.html">this
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method is horribly slow</a>.
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</p>
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<p>Or we can keep old memory around, and reuse it in a pool to save time.
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The old libstdc++-v2 used a memory pool, and so do we. As of 3.0,
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<a href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00136.html">it's
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on by default</a>. The pool is shared among all the containers in the
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program: when your program's std::vector<int> gets cut in half
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and frees a bunch of its storage, that memory can be reused by the
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private std::list<WonkyWidget> brought in from a KDE library
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that you linked against. And we don't have to call operators new and
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delete to pass the memory on, either, which is a speed bonus.
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<strong>BUT</strong>...
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</p>
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<p>What about threads? No problem: in a threadsafe environment, the
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memory pool is manipulated atomically, so you can grow a container in
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one thread and shrink it in another, etc. <strong>BUT</strong> what
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if threads in libstdc++-v3 aren't set up properly?
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<a href="../faq/index.html#5_6">That's been answered already</a>.
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</p>
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<p><strong>BUT</strong> what if you want to use your own allocator? What
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if you plan on using a runtime-loadable version of malloc() which uses
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shared telepathic anonymous mmap'd sections serializable over a
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network, so that memory requests <em>should</em> go through malloc?
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And what if you need to debug it?
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</p>
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<h3 class="left">
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<a name="stdallocator">Implementation details of <code>std::allocator</code></a>
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</h3>
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<p> The implementation of <code> std::allocator</code> has continued
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to evolve through successive releases. Here's a brief history.
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</p>
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<h5 class="left">
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<a name="30allocator"> 3.0, 3.1, 3.2, 3.3 </a>
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</h5>
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<p> During this period, all allocators were written to the SGI
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style, and all STL containers expected this interface. This
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interface had a traits class called <code>_Alloc_traits</code> that
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attempted to provide more information for compile-time allocation
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selection and optimization. This traits class had another allocator
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wrapper, <code>__simple_alloc<T,A></code>, which was a
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wrapper around another allocator, A, which itself is an allocator
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for instances of T. But wait, there's more:
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<code>__allocator<T,A></code> is another adapter. Many of
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the provided allocator classes were SGI style: such classes can be
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changed to a conforming interface with this wrapper:
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<code>__allocator<T, __alloc></code> is thus the same as
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<code>allocator<T></code>.
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</p>
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<p> The class <code>std::allocator</code> use the typedef
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<code>__alloc</code> to select an underlying allocator that
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satisfied memory allocation requests. The selection of this
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underlying allocator was not user-configurable.
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</p>
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<h5 class="left">
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<a name="34allocator"> 3.4 </a>
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</h5>
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<p> For this and later releases, the only allocator interface that
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is support is the standard C++ interface. As such, all STL
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containers have been adjusted, and all external allocators have
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been modified to support this change. Because of this,
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<code>__simple_alloc, __allocator, __alloc, </code> and <code>
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_Alloc_traits</code> have all been removed.
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</p>
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<p> The class <code>std::allocator</code> just has typedef,
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constructor, and rebind members. It inherits from one of the
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high-speed extension allocators, covered below. Thus, all
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allocation and deallocation depends on the base class.
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</p>
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<p> The base class that <code>std::allocator</code> is derived from
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is not user-configurable.
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</p>
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<h5 class="left">
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<a name="benchmarks"> How the default allocation strategy is selected.</a>
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</h5>
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<p> It's difficult to pick an allocation strategy that will provide
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maximum utility, without excessively penalizing some behavior. In
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fact, it's difficult just deciding which typical actions to measure
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for speed.
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</p>
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<p> Three synthetic benchmarks have been created that provide data
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that is used to compare different C++ allocators. These tests are:
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</p>
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<ul>
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<li>Insertion. Over multiple iterations, various STL container
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objects have elements inserted to some maximum amount. A variety
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of allocators are tested.
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Test source <a
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href="http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc%2b%2b-v3/testsuite/performance/20_util/allocator/insert.cc?only_with_tag=MAIN">here.</a>
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</li>
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<li>Insertion, clear, and re-insertion in a multi-threaded
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environment. Over multiple iterations, several threads are
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started that insert elements into a STL container, then assign a
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null instance of the same type to clear memory, and then
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re-insert the same number of elements. Several STL containers and
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multiple allocators are tested. This test shows the ability of
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the allocator to reclaim memory on a pre-thread basis, as well as
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measuring thread contention for memory resources.
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Test source
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<a href="http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc%2b%2b-v3/testsuite/performance/20_util/allocator/insert_insert.cc">
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here.</a>
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</li>
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<li>A threaded producer/consumer model.
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Test source
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<a href="http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc%2b%2b-v3/testsuite/performance/20_util/allocator/producer_consumer.cc">
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here.</a>
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</li>
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</ul>
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<h5 class="left">
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<a name="forcenew"> Disabling memory caching.</a>
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</h5>
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<p> In use, <code>std::allocator</code> may allocate and deallocate
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using implementation-specified strategies and heuristics. Because of
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this, every call to an allocator object's <code> allocate</code>
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member function may not actually call the global operator new. This
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situation is also duplicated for calls to the <code>
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deallocate</code> member function.
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</p>
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<p> This can be confusing.
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</p>
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<p> In particular, this can make debugging memory errors more
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difficult, especially when using third party tools like valgrind or
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debug versions of <code> new</code>.
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</p>
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<p> There are various ways to solve this problem. One would be to
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use a custom allocator that just called operators <code> new
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</code> and <code> delete</code> directly, for every
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allocation. (See include/ext/new_allocator.h, for instance.)
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However, that option would involve changing source code to use the a
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non-default allocator. Another option is to force the default
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allocator to remove caching and pools, and to directly allocate
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with every call of <code> allocate</code> and directly deallocate
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with every call of <code> deallocate</code>, regardless of
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efficiency. As it turns out, this last option is available,
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although the exact mechanism has evolved with time.
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</p>
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<p> For GCC releases from 2.95 through the 3.1 series, defining
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<code>__USE_MALLOC</code> on the gcc command line would change the
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default allocation strategy to instead use <code> malloc</code> and
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<code> free</code>. See
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<a href="../23_containers/howto.html#3">this note</a>
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for details as to why this was something needing improvement.
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</p>
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<p>Starting with GCC 3.2, and continued in the 3.3 series, to
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globally disable memory caching within the library for the
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default allocator, merely set GLIBCPP_FORCE_NEW (at this time,
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with any value) in the system's environment before running the
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program. If your program crashes with GLIBCPP_FORCE_NEW in the
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environment, it likely means that you linked against objects
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built against the older library. Code to support this extension
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is fully compatible with 3.2 code if GLIBCPP_FORCE_NEW is not in
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the environment.
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</p>
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<p> As it turns out, the 3.4 code base continues to use this
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mechanism, only the environment variable has been changed to
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GLIBCXX_FORCE_NEW.
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</p>
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<h3 class="left">
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<a name="ext_allocators">Other allocators</a>
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</h3>
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<p> Several other allocators are provided as part of this
|
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implementation. The location of the extension allocators and their
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names have changed, but in all cases, functionality is
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equivalent. Starting with gcc-3.4, all extension allocators are
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standard style. Before this point, SGI style was the norm. Because of
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this, the number of template arguments also changed. Here's a simple
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chart to track the changes.
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</p>
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<table title="extension allocators" border="1">
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<tr>
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<th>Allocator (3.4)</th>
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|
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<th>Header (3.4)</th>
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<th>Allocator (3.[0-3])</th>
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<th>Header (3.[0-3])</th>
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</tr>
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<tr>
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<td>__gnu_cxx::new_allocator<T></td>
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<td><ext/new_allocator.h></td>
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<td>std::__new_alloc</td>
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<td><memory></td>
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</tr>
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<tr>
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<td>__gnu_cxx::malloc_allocator<T></td>
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<td><ext/malloc_allocator.h></td>
|
| 290 |
|
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<td>std::__malloc_alloc_template<int></td>
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<td><memory></td>
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|
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</tr>
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<tr>
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<td>__gnu_cxx::debug_allocator<T></td>
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<td><ext/debug_allocator.h></td>
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<td>std::debug_alloc<T></td>
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<td><memory></td>
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</tr>
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<tr>
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<td>__gnu_cxx::__pool_alloc<T></td>
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<td><ext/pool_allocator.h></td>
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<td>std::__default_alloc_template<bool,int></td>
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<td><memory></td>
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</tr>
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<tr>
|
| 306 |
|
|
<td>__gnu_cxx::__mt_alloc<T></td>
|
| 307 |
|
|
<td><ext/mt_allocator.h></td>
|
| 308 |
|
|
<td></td>
|
| 309 |
|
|
<td></td>
|
| 310 |
|
|
</tr>
|
| 311 |
|
|
<tr>
|
| 312 |
|
|
<td>__gnu_cxx::bitmap_allocator<T></td>
|
| 313 |
|
|
<td><ext/bitmap_allocator.h></td>
|
| 314 |
|
|
<td></td>
|
| 315 |
|
|
<td></td>
|
| 316 |
|
|
</tr>
|
| 317 |
|
|
</table>
|
| 318 |
|
|
|
| 319 |
|
|
<p> Releases after gcc-3.4 have continued to add to the collection
|
| 320 |
|
|
of available allocators. All of these new allocators are
|
| 321 |
|
|
standard-style. The following table includes details, along with
|
| 322 |
|
|
the first released version of GCC that included the extension allocator.
|
| 323 |
|
|
</p>
|
| 324 |
|
|
|
| 325 |
|
|
<table title="more extension allocators" border="1">
|
| 326 |
|
|
<tr>
|
| 327 |
|
|
<th>Allocator</th>
|
| 328 |
|
|
<th>Include</th>
|
| 329 |
|
|
<th>Version</th>
|
| 330 |
|
|
</tr>
|
| 331 |
|
|
<tr>
|
| 332 |
|
|
<td>__gnu_cxx::array_allocator<T></td>
|
| 333 |
|
|
<td><ext/array_allocator.h></td>
|
| 334 |
|
|
<td>4.0.0</td>
|
| 335 |
|
|
</tr>
|
| 336 |
|
|
</table>
|
| 337 |
|
|
|
| 338 |
|
|
<p>More details on each of these extension allocators follows. </p>
|
| 339 |
|
|
<ul>
|
| 340 |
|
|
<li><code>new_allocator</code>
|
| 341 |
|
|
<p>Simply wraps <code>::operator new</code>
|
| 342 |
|
|
and <code>::operator delete</code>.
|
| 343 |
|
|
</p>
|
| 344 |
|
|
</li>
|
| 345 |
|
|
<li><code>malloc_allocator</code>
|
| 346 |
|
|
<p>Simply wraps
|
| 347 |
|
|
<code>malloc</code> and <code>free</code>. There is also a hook
|
| 348 |
|
|
for an out-of-memory handler (for new/delete this is taken care of
|
| 349 |
|
|
elsewhere).
|
| 350 |
|
|
</p>
|
| 351 |
|
|
</li>
|
| 352 |
|
|
<li><code>array_allocator</code>
|
| 353 |
|
|
<p>Allows allocations of known and fixed sizes using existing
|
| 354 |
|
|
global or external storage allocated via construction of
|
| 355 |
|
|
std::tr1::array objects. By using this allocator, fixed size
|
| 356 |
|
|
containers (including std::string) can be used without
|
| 357 |
|
|
instances calling <code>::operator new</code> and
|
| 358 |
|
|
<code>::operator delete</code>. This capability allows the
|
| 359 |
|
|
use of STL abstractions without runtime complications or
|
| 360 |
|
|
overhead, even in situations such as program startup. For
|
| 361 |
|
|
usage examples, please consult the libstdc++ testsuite.
|
| 362 |
|
|
</p>
|
| 363 |
|
|
</li>
|
| 364 |
|
|
<li><code>debug_allocator</code>
|
| 365 |
|
|
<p> A wrapper around an
|
| 366 |
|
|
arbitrary allocator A. It passes on slightly increased size
|
| 367 |
|
|
requests to A, and uses the extra memory to store size information.
|
| 368 |
|
|
When a pointer is passed to <code>deallocate()</code>, the stored
|
| 369 |
|
|
size is checked, and assert() is used to guarantee they match.
|
| 370 |
|
|
</p>
|
| 371 |
|
|
</li>
|
| 372 |
|
|
<li><code>__pool_alloc</code>
|
| 373 |
|
|
<p> A high-performance, single pool allocator. The reusable
|
| 374 |
|
|
memory is shared among identical instantiations of this type.
|
| 375 |
|
|
It calls through <code>::operator new</code> to obtain new memory
|
| 376 |
|
|
when its lists run out. If a client container requests a block
|
| 377 |
|
|
larger than a certain threshold size, then the pool is bypassed,
|
| 378 |
|
|
and the allocate/deallocate request is passed to
|
| 379 |
|
|
<code>::operator new</code> directly. </p>
|
| 380 |
|
|
|
| 381 |
|
|
<p> For versions of <code>__pool_alloc</code> after 3.4.0, there is
|
| 382 |
|
|
only one template parameter, as per the standard.
|
| 383 |
|
|
</p>
|
| 384 |
|
|
|
| 385 |
|
|
<p> Older versions of this class take a boolean template parameter,
|
| 386 |
|
|
called <code>thr</code>, and an integer template parameter,
|
| 387 |
|
|
called <code>inst</code>.
|
| 388 |
|
|
</p>
|
| 389 |
|
|
|
| 390 |
|
|
<p>The <code>inst</code> number is used to track additional memory
|
| 391 |
|
|
pools. The point of the number is to allow multiple
|
| 392 |
|
|
instantiations of the classes without changing the semantics at
|
| 393 |
|
|
all. All three of
|
| 394 |
|
|
</p>
|
| 395 |
|
|
|
| 396 |
|
|
<pre>
|
| 397 |
|
|
typedef __pool_alloc<true,0> normal;
|
| 398 |
|
|
typedef __pool_alloc<true,1> private;
|
| 399 |
|
|
typedef __pool_alloc<true,42> also_private;</pre>
|
| 400 |
|
|
<p>behave exactly the same way. However, the memory pool for each type
|
| 401 |
|
|
(and remember that different instantiations result in different types)
|
| 402 |
|
|
remains separate.
|
| 403 |
|
|
</p>
|
| 404 |
|
|
<p>The library uses <strong>0</strong> in all its instantiations. If you
|
| 405 |
|
|
wish to keep separate free lists for a particular purpose, use a
|
| 406 |
|
|
different number.
|
| 407 |
|
|
</p>
|
| 408 |
|
|
<p>The <code>thr</code> boolean determines whether the pool should
|
| 409 |
|
|
be manipulated atomically or not. When thr=true, the allocator
|
| 410 |
|
|
is is threadsafe, while thr=false, and is slightly faster but
|
| 411 |
|
|
unsafe for multiple threads.
|
| 412 |
|
|
</p>
|
| 413 |
|
|
|
| 414 |
|
|
<p>For thread-enabled configurations, the pool is locked with a
|
| 415 |
|
|
single big lock. In some situations, this implementation detail may
|
| 416 |
|
|
result in severe performance degredation.
|
| 417 |
|
|
</p>
|
| 418 |
|
|
|
| 419 |
|
|
<p>(Note that the GCC thread abstraction layer allows us to provide safe
|
| 420 |
|
|
zero-overhead stubs for the threading routines, if threads were
|
| 421 |
|
|
disabled at configuration time.)
|
| 422 |
|
|
</p>
|
| 423 |
|
|
|
| 424 |
|
|
</li>
|
| 425 |
|
|
|
| 426 |
|
|
<li><code>__mt_alloc</code>
|
| 427 |
|
|
<p>A high-performance
|
| 428 |
|
|
fixed-size allocator. It has its own documentation, found <a
|
| 429 |
|
|
href="../ext/mt_allocator.html">here</a>.
|
| 430 |
|
|
</p>
|
| 431 |
|
|
</li>
|
| 432 |
|
|
|
| 433 |
|
|
<li><code>bitmap_allocator</code>
|
| 434 |
|
|
<p>A high-performance allocator that uses a bit-map to keep track
|
| 435 |
|
|
of the used and unused memory locations. It has its own
|
| 436 |
|
|
documentation, found <a
|
| 437 |
|
|
href="../ext/ballocator_doc.html">here</a>.
|
| 438 |
|
|
</p>
|
| 439 |
|
|
</li>
|
| 440 |
|
|
</ul>
|
| 441 |
|
|
|
| 442 |
|
|
|
| 443 |
|
|
<h3 class="left">
|
| 444 |
|
|
<a name="using_custom_allocators">Using a specific allocator</a>
|
| 445 |
|
|
</h3>
|
| 446 |
|
|
<p>You can specify different memory management schemes on a
|
| 447 |
|
|
per-container basis, by overriding the default
|
| 448 |
|
|
<code>Allocator</code> template parameter. For example, an easy
|
| 449 |
|
|
(but non-portable) method of specifying that only malloc/free
|
| 450 |
|
|
should be used instead of the default node allocator is:
|
| 451 |
|
|
</p>
|
| 452 |
|
|
<pre>
|
| 453 |
|
|
std::list <int, __gnu_cxx::malloc_allocator<int> > malloc_list;</pre>
|
| 454 |
|
|
Likewise, a debugging form of whichever allocator is currently in use:
|
| 455 |
|
|
<pre>
|
| 456 |
|
|
std::deque <int, __gnu_cxx::debug_allocator<std::allocator<int> > > debug_deque;</pre>
|
| 457 |
|
|
|
| 458 |
|
|
|
| 459 |
|
|
<h3 class="left">
|
| 460 |
|
|
<a name="custom_allocators">Writing custom allocators</a>
|
| 461 |
|
|
</h3>
|
| 462 |
|
|
<p> Writing a portable C++ allocator would dictate that the
|
| 463 |
|
|
interface would look much like the one specified for <code>
|
| 464 |
|
|
std::allocator</code>. Additional member functions, but not
|
| 465 |
|
|
subtractions, would be permissible.
|
| 466 |
|
|
</p>
|
| 467 |
|
|
|
| 468 |
|
|
<p> Probably the best place to start would be to copy one of the
|
| 469 |
|
|
extension allocators already shipped with libstdc++: say, <code>
|
| 470 |
|
|
new_allocator </code>.
|
| 471 |
|
|
</p>
|
| 472 |
|
|
|
| 473 |
|
|
|
| 474 |
|
|
<h3 class="left">
|
| 475 |
|
|
<a name="biblio">Bibliography / Further Reading</a>
|
| 476 |
|
|
</h3>
|
| 477 |
|
|
<p>
|
| 478 |
|
|
ISO/IEC 14882:1998 Programming languages - C++ [20.4 Memory]
|
| 479 |
|
|
</p>
|
| 480 |
|
|
|
| 481 |
|
|
<p>
|
| 482 |
|
|
Austern, Matt, C/C++ Users Journal.
|
| 483 |
|
|
<a href="http://www.cuj.com/documents/s=8000/cujcexp1812austern/">The Standard Librarian: What Are Allocators Good
|
| 484 |
|
|
For?</a>
|
| 485 |
|
|
</p>
|
| 486 |
|
|
|
| 487 |
|
|
<p>
|
| 488 |
|
|
Berger, Emery,
|
| 489 |
|
|
<a href="http://www.cs.umass.edu/~emery/hoard/"> The Hoard memory allocator </a>
|
| 490 |
|
|
</p>
|
| 491 |
|
|
|
| 492 |
|
|
<p>
|
| 493 |
|
|
Berger, Emery with Ben Zorn & Kathryn McKinley, OOPSLA 2002
|
| 494 |
|
|
<a href="http://www.cs.umass.edu/~emery/pubs/berger-oopsla2002.pdf">Reconsidering Custom Memory Allocation</a>
|
| 495 |
|
|
</p>
|
| 496 |
|
|
|
| 497 |
|
|
<p>
|
| 498 |
|
|
Kreft, Klaus and Angelika Langer, C++ Report, June 1998
|
| 499 |
|
|
<a href="http://www.langer.camelot.de/Articles/C++Report/Allocators/Allocators.html">Allocator Types</a>
|
| 500 |
|
|
</p>
|
| 501 |
|
|
|
| 502 |
|
|
<p>
|
| 503 |
|
|
Stroustrup, Bjarne, 19.4 Allocators, The C++ Programming
|
| 504 |
|
|
Language, Special Edition, Addison Wesley, Inc. 2000
|
| 505 |
|
|
</p>
|
| 506 |
|
|
|
| 507 |
|
|
<p>
|
| 508 |
|
|
Yen, Felix, <a href="http://home.earthlink.net/~brimar/yalloc/">Yalloc: A Recycling C++ Allocator</a>
|
| 509 |
|
|
</p>
|
| 510 |
|
|
|
| 511 |
|
|
<hr />
|
| 512 |
|
|
<p>Return <a href="#top">to the top of the page</a> or
|
| 513 |
|
|
<a href="http://gcc.gnu.org/libstdc++/">to the libstdc++ homepage</a>.
|
| 514 |
|
|
</p>
|
| 515 |
|
|
|
| 516 |
|
|
|
| 517 |
|
|
<!-- ####################################################### -->
|
| 518 |
|
|
|
| 519 |
|
|
<hr />
|
| 520 |
|
|
<p class="fineprint"><em>
|
| 521 |
|
|
See <a href="../17_intro/license.html">license.html</a> for copying conditions.
|
| 522 |
|
|
Comments and suggestions are welcome, and may be sent to
|
| 523 |
|
|
<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
|
| 524 |
|
|
</em></p>
|
| 525 |
|
|
|
| 526 |
|
|
|
| 527 |
|
|
</body>
|
| 528 |
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</html>
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