b8ad24b99f
2004-12-19 Dhruv Matani <dhruvbird@gmx.net> * docs/html/20_util/allocator.html: Correct link. * docs/html/ext/ballocator_doc.txt: Remove. * docs/html/ext/ballocator_doc.html: Add. From-SVN: r92375
529 lines
20 KiB
HTML
529 lines
20 KiB
HTML
<?xml version="1.0" encoding="ISO-8859-1"?>
|
|
<!DOCTYPE html
|
|
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
|
|
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
|
|
|
|
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
|
|
<head>
|
|
<meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards) and bkoz@gcc.gnu.org (Benjamin Kosnik)" />
|
|
<meta name="KEYWORDS" content="c++, libstdc++, g++, allocator, memory" />
|
|
<meta name="DESCRIPTION" content="Allocators and allocation" />
|
|
<meta name="GENERATOR" content="emacs and ten fingers" />
|
|
<title>Allocators and allocation</title>
|
|
<link rel="StyleSheet" href="../lib3styles.css" type="text/css" />
|
|
<link rel="Start" href="../documentation.html" type="text/html"
|
|
title="GNU C++ Standard Library" />
|
|
<link rel="Bookmark" href="howto.html" type="text/html"
|
|
title="General Utilities" />
|
|
<link rel="Copyright" href="../17_intro/license.html" type="text/html" />
|
|
</head>
|
|
<body>
|
|
|
|
<h1 class="centered"><a name="top">Allocators and allocation</a></h1>
|
|
|
|
<p class="fineprint"><em>
|
|
The latest version of this document is always available at
|
|
<a href="http://gcc.gnu.org/onlinedocs/libstdc++/20_util/allocator.html">
|
|
http://gcc.gnu.org/onlinedocs/libstdc++/20_util/allocator.html</a>.
|
|
</em></p>
|
|
|
|
<p><em>
|
|
To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++-v3 homepage</a>.
|
|
</em></p>
|
|
|
|
<!-- ####################################################### -->
|
|
<hr />
|
|
<p> The C++ Standard encapsulates memory management characteristics
|
|
for strings, container classes, and parts of iostreams in a
|
|
template class called <code>std::allocator</code>.
|
|
</p>
|
|
|
|
<h3 class="left">
|
|
<a name="standard_requirements">Standard requirements</a>
|
|
</h3>
|
|
<p>The C++ standard only gives a few directives in this area:
|
|
</p>
|
|
<ul>
|
|
<li>When you add elements to a container, and the container must allocate
|
|
more memory to hold them, the container makes the request via its
|
|
<code>Allocator</code> template parameter. This includes adding
|
|
chars to the string class, which acts as a regular STL container
|
|
in this respect.
|
|
</li>
|
|
<li>The default <code>Allocator</code> of every container-of-T is
|
|
<code>std::allocator<T></code>.
|
|
</li>
|
|
<li>The interface of the <code>allocator<T></code> class is
|
|
extremely simple. It has about 20 public declarations (nested
|
|
typedefs, member functions, etc), but the two which concern us most
|
|
are:
|
|
<pre>
|
|
T* allocate (size_type n, const void* hint = 0);
|
|
void deallocate (T* p, size_type n);</pre>
|
|
(This is a simplification; the real signatures use nested typedefs.)
|
|
The <code>"n"</code> arguments in both those functions is a
|
|
<em>count</em> of the number of T's to allocate space for,
|
|
<em>not their total size</em>.
|
|
</li>
|
|
<li>"The storage is obtained by calling
|
|
<code>::operator new(size_t)</code>, but it is unspecified when or
|
|
how often this function is called. The use of <code>hint</code>
|
|
is unspecified, but intended as an aid to locality if an
|
|
implementation so desires." [20.4.1.1]/6
|
|
</li>
|
|
</ul>
|
|
|
|
<p> Complete details cam be found in the C++ standard, look in
|
|
[20.4 Memory].
|
|
</p>
|
|
|
|
<h3 class="left">
|
|
<a name="probs_possibilities">Problems and Possibilities</a>
|
|
</h3>
|
|
<p>The easiest way of fulfilling the requirements is to call operator new
|
|
each time a container needs memory, and to call operator delete each
|
|
time the container releases memory. <strong>BUT</strong>
|
|
<a href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00105.html">this
|
|
method is horribly slow</a>.
|
|
</p>
|
|
<p>Or we can keep old memory around, and reuse it in a pool to save time.
|
|
The old libstdc++-v2 used a memory pool, and so do we. As of 3.0,
|
|
<a href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00136.html">it's
|
|
on by default</a>. The pool is shared among all the containers in the
|
|
program: when your program's std::vector<int> gets cut in half
|
|
and frees a bunch of its storage, that memory can be reused by the
|
|
private std::list<WonkyWidget> brought in from a KDE library
|
|
that you linked against. And we don't have to call operators new and
|
|
delete to pass the memory on, either, which is a speed bonus.
|
|
<strong>BUT</strong>...
|
|
</p>
|
|
<p>What about threads? No problem: in a threadsafe environment, the
|
|
memory pool is manipulated atomically, so you can grow a container in
|
|
one thread and shrink it in another, etc. <strong>BUT</strong> what
|
|
if threads in libstdc++-v3 aren't set up properly?
|
|
<a href="../faq/index.html#5_6">That's been answered already</a>.
|
|
</p>
|
|
<p><strong>BUT</strong> what if you want to use your own allocator? What
|
|
if you plan on using a runtime-loadable version of malloc() which uses
|
|
shared telepathic anonymous mmap'd sections serializable over a
|
|
network, so that memory requests <em>should</em> go through malloc?
|
|
And what if you need to debug it?
|
|
</p>
|
|
|
|
<h3 class="left">
|
|
<a name="stdallocator">Implementation details of <code>std::allocator</code></a>
|
|
</h3>
|
|
<p> The implementation of <code> std::allocator</code> has continued
|
|
to evolve through successive releases. Here's a brief history.
|
|
</p>
|
|
|
|
<h5 class="left">
|
|
<a name="30allocator"> 3.0, 3.1, 3.2, 3.3 </a>
|
|
</h5>
|
|
<p> During this period, all allocators were written to the SGI
|
|
style, and all STL containers expected this interface. This
|
|
interface had a traits class called <code>_Alloc_traits</code> that
|
|
attempted to provide more information for compile-time allocation
|
|
selection and optimization. This traits class had another allocator
|
|
wrapper, <code>__simple_alloc<T,A></code>, which was a
|
|
wrapper around another allocator, A, which itself is an allocator
|
|
for instances of T. But wait, there's more:
|
|
<code>__allocator<T,A></code> is another adapter. Many of
|
|
the provided allocator classes were SGI style: such classes can be
|
|
changed to a conforming interface with this wrapper:
|
|
<code>__allocator<T, __alloc></code> is thus the same as
|
|
<code>allocator<T></code>.
|
|
</p>
|
|
|
|
<p> The class <code>std::allocator</code> use the typedef
|
|
<code>__alloc</code> to select an underlying allocator that
|
|
satisfied memory allocation requests. The selection of this
|
|
underlying allocator was not user-configurable.
|
|
</p>
|
|
|
|
<h5 class="left">
|
|
<a name="34allocator"> 3.4 </a>
|
|
</h5>
|
|
<p> For this and later releases, the only allocator interface that
|
|
is support is the standard C++ interface. As such, all STL
|
|
containers have been adjusted, and all external allocators have
|
|
been modified to support this change. Because of this,
|
|
<code>__simple_alloc, __allocator, __alloc, </code> and <code>
|
|
_Alloc_traits</code> have all been removed.
|
|
</p>
|
|
|
|
<p> The class <code>std::allocator</code> just has typedef,
|
|
constructor, and rebind members. It inherits from one of the
|
|
high-speed extension allocators, covered below. Thus, all
|
|
allocation and deallocation depends on the base class.
|
|
</p>
|
|
|
|
<p> The base class that <code>std::allocator</code> is derived from
|
|
is not user-configurable.
|
|
</p>
|
|
|
|
<h5 class="left">
|
|
<a name="benchmarks"> How the default allocation strategy is selected.</a>
|
|
</h5>
|
|
<p> It's difficult to pick an allocation strategy that will provide
|
|
maximum utility, without excessively penalizing some behavior. In
|
|
fact, it's difficult just deciding which typical actions to measure
|
|
for speed.
|
|
</p>
|
|
|
|
<p> Three synthetic benchmarks have been created that provide data
|
|
that is used to compare different C++ allocators. These tests are:
|
|
</p>
|
|
|
|
<ul>
|
|
<li>Insertion. Over multiple iterations, various STL container
|
|
objects have elements inserted to some maximum amount. A variety
|
|
of allocators are tested.
|
|
Test source <a
|
|
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>
|
|
</li>
|
|
|
|
<li>Insertion, clear, and re-insertion in a multi-threaded
|
|
environment. Over multiple iterations, several threads are
|
|
started that insert elements into a STL container, then assign a
|
|
null instance of the same type to clear memory, and then
|
|
re-insert the same number of elements. Several STL containers and
|
|
multiple allocators are tested. This test shows the ability of
|
|
the allocator to reclaim memory on a pre-thread basis, as well as
|
|
measuring thread contention for memory resources.
|
|
Test source
|
|
<a href="http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc%2b%2b-v3/testsuite/performance/20_util/allocator/insert_insert.cc">
|
|
here.</a>
|
|
</li>
|
|
|
|
<li>A threaded producer/consumer model.
|
|
Test source
|
|
<a href="http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc%2b%2b-v3/testsuite/performance/20_util/allocator/producer_consumer.cc">
|
|
here.</a>
|
|
</li>
|
|
</ul>
|
|
|
|
<h5 class="left">
|
|
<a name="forcenew"> Disabling memory caching.</a>
|
|
</h5>
|
|
<p> In use, <code>std::allocator</code> may allocate and deallocate
|
|
using implementation-specified strategies and heuristics. Because of
|
|
this, every call to an allocator object's <code> allocate</code>
|
|
member function may not actually call the global operator new. This
|
|
situation is also duplicated for calls to the <code>
|
|
deallocate</code> member function.
|
|
</p>
|
|
|
|
<p> This can be confusing.
|
|
</p>
|
|
|
|
<p> In particular, this can make debugging memory errors more
|
|
difficult, especially when using third party tools like valgrind or
|
|
debug versions of <code> new</code>.
|
|
</p>
|
|
|
|
<p> There are various ways to solve this problem. One would be to
|
|
use a custom allocator that just called operators <code> new
|
|
</code> and <code> delete</code> directly, for every
|
|
allocation. (See include/ext/new_allocator.h, for instance.)
|
|
However, that option would involve changing source code to use the a
|
|
non-default allocator. Another option is to force the default
|
|
allocator to remove caching and pools, and to directly allocate
|
|
with every call of <code> allocate</code> and directly deallocate
|
|
with every call of <code> deallocate</code>, regardless of
|
|
efficiency. As it turns out, this last option is available,
|
|
although the exact mechanism has evolved with time.
|
|
</p>
|
|
|
|
<p> For GCC releases from 2.95 through the 3.1 series, defining
|
|
<code>__USE_MALLOC</code> on the gcc command line would change the
|
|
default allocation strategy to instead use <code> malloc</code> and
|
|
<code> free</code>. See
|
|
<a href="../23_containers/howto.html#3">this note</a>
|
|
for details as to why this was something needing improvement.
|
|
</p>
|
|
|
|
<p>Starting with GCC 3.2, and continued in the 3.3 series, to
|
|
globally disable memory caching within the library for the
|
|
default allocator, merely set GLIBCPP_FORCE_NEW (at this time,
|
|
with any value) in the system's environment before running the
|
|
program. If your program crashes with GLIBCPP_FORCE_NEW in the
|
|
environment, it likely means that you linked against objects
|
|
built against the older library. Code to support this extension
|
|
is fully compatible with 3.2 code if GLIBCPP_FORCE_NEW is not in
|
|
the environment.
|
|
</p>
|
|
|
|
<p> As it turns out, the 3.4 code base continues to use this
|
|
mechanism, only the environment variable has been changed to
|
|
GLIBCXX_FORCE_NEW.
|
|
</p>
|
|
|
|
<h3 class="left">
|
|
<a name="ext_allocators">Other allocators</a>
|
|
</h3>
|
|
<p> Several other allocators are provided as part of this
|
|
implementation. The location of the extension allocators and their
|
|
names have changed, but in all cases, functionality is
|
|
equivalent. Starting with gcc-3.4, all extension allocators are
|
|
standard style. Before this point, SGI style was the norm. Because of
|
|
this, the number of template arguments also changed. Here's a simple
|
|
chart to track the changes.
|
|
</p>
|
|
|
|
<table title="extension allocators" border="1">
|
|
<tr>
|
|
<th>Allocator (3.4)</th>
|
|
<th>Header (3.4)</th>
|
|
<th>Allocator (3.[0-3])</th>
|
|
<th>Header (3.[0-3])</th>
|
|
</tr>
|
|
<tr>
|
|
<td>__gnu_cxx::new_allocator<T></td>
|
|
<td><ext/new_allocator.h></td>
|
|
<td>std::__new_alloc</td>
|
|
<td><memory></td>
|
|
</tr>
|
|
<tr>
|
|
<td>__gnu_cxx::malloc_allocator<T></td>
|
|
<td><ext/malloc_allocator.h></td>
|
|
<td>std::__malloc_alloc_template<int></td>
|
|
<td><memory></td>
|
|
</tr>
|
|
<tr>
|
|
<td>__gnu_cxx::debug_allocator<T></td>
|
|
<td><ext/debug_allocator.h></td>
|
|
<td>std::debug_alloc<T></td>
|
|
<td><memory></td>
|
|
</tr>
|
|
<tr>
|
|
<td>__gnu_cxx::__pool_alloc<T></td>
|
|
<td><ext/pool_allocator.h></td>
|
|
<td>std::__default_alloc_template<bool,int></td>
|
|
<td><memory></td>
|
|
</tr>
|
|
<tr>
|
|
<td>__gnu_cxx::__mt_alloc<T></td>
|
|
<td><ext/mt_allocator.h></td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
<tr>
|
|
<td>__gnu_cxx::bitmap_allocator<T></td>
|
|
<td><ext/bitmap_allocator.h></td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
</table>
|
|
|
|
<p> Releases after gcc-3.4 have continued to add to the collection
|
|
of available allocators. All of these new allocators are
|
|
standard-style. The following table includes details, along with
|
|
the first released version of GCC that included the extension allocator.
|
|
</p>
|
|
|
|
<table title="more extension allocators" border="1">
|
|
<tr>
|
|
<th>Allocator</th>
|
|
<th>Include</th>
|
|
<th>Version</th>
|
|
</tr>
|
|
<tr>
|
|
<td>__gnu_cxx::array_allocator<T></td>
|
|
<td><ext/array_allocator.h></td>
|
|
<td>4.0.0</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<p>More details on each of these extension allocators follows. </p>
|
|
<ul>
|
|
<li><code>new_allocator</code>
|
|
<p>Simply wraps <code>::operator new</code>
|
|
and <code>::operator delete</code>.
|
|
</p>
|
|
</li>
|
|
<li><code>malloc_allocator</code>
|
|
<p>Simply wraps
|
|
<code>malloc</code> and <code>free</code>. There is also a hook
|
|
for an out-of-memory handler (for new/delete this is taken care of
|
|
elsewhere).
|
|
</p>
|
|
</li>
|
|
<li><code>array_allocator</code>
|
|
<p>Allows allocations of known and fixed sizes using existing
|
|
global or external storage allocated via construction of
|
|
std::tr1::array objects. By using this allocator, fixed size
|
|
containers (including std::string) can be used without
|
|
instances calling <code>::operator new</code> and
|
|
<code>::operator delete</code>. This capability allows the
|
|
use of STL abstractions without runtime complications or
|
|
overhead, even in situations such as program startup. For
|
|
usage examples, please consult the libstdc++ testsuite.
|
|
</p>
|
|
</li>
|
|
<li><code>debug_allocator</code>
|
|
<p> A wrapper around an
|
|
arbitrary allocator A. It passes on slightly increased size
|
|
requests to A, and uses the extra memory to store size information.
|
|
When a pointer is passed to <code>deallocate()</code>, the stored
|
|
size is checked, and assert() is used to guarantee they match.
|
|
</p>
|
|
</li>
|
|
<li><code>__pool_alloc</code>
|
|
<p> A high-performance, single pool allocator. The reusable
|
|
memory is shared among identical instantiations of this type.
|
|
It calls through <code>::operator new</code> to obtain new memory
|
|
when its lists run out. If a client container requests a block
|
|
larger than a certain threshold size, then the pool is bypassed,
|
|
and the allocate/deallocate request is passed to
|
|
<code>::operator new</code> directly. </p>
|
|
|
|
<p> For versions of <code>__pool_alloc</code> after 3.4.0, there is
|
|
only one template parameter, as per the standard.
|
|
</p>
|
|
|
|
<p> Older versions of this class take a boolean template parameter,
|
|
called <code>thr</code>, and an integer template parameter,
|
|
called <code>inst</code>.
|
|
</p>
|
|
|
|
<p>The <code>inst</code> number is used to track additional memory
|
|
pools. The point of the number is to allow multiple
|
|
instantiations of the classes without changing the semantics at
|
|
all. All three of
|
|
</p>
|
|
|
|
<pre>
|
|
typedef __pool_alloc<true,0> normal;
|
|
typedef __pool_alloc<true,1> private;
|
|
typedef __pool_alloc<true,42> also_private;</pre>
|
|
<p>behave exactly the same way. However, the memory pool for each type
|
|
(and remember that different instantiations result in different types)
|
|
remains separate.
|
|
</p>
|
|
<p>The library uses <strong>0</strong> in all its instantiations. If you
|
|
wish to keep separate free lists for a particular purpose, use a
|
|
different number.
|
|
</p>
|
|
<p>The <code>thr</code> boolean determines whether the pool should
|
|
be manipulated atomically or not. When thr=true, the allocator
|
|
is is threadsafe, while thr=false, and is slightly faster but
|
|
unsafe for multiple threads.
|
|
</p>
|
|
|
|
<p>For thread-enabled configurations, the pool is locked with a
|
|
single big lock. In some situations, this implementation detail may
|
|
result in severe performance degredation.
|
|
</p>
|
|
|
|
<p>(Note that the GCC thread abstraction layer allows us to provide safe
|
|
zero-overhead stubs for the threading routines, if threads were
|
|
disabled at configuration time.)
|
|
</p>
|
|
|
|
</li>
|
|
|
|
<li><code>__mt_alloc</code>
|
|
<p>A high-performance
|
|
fixed-size allocator. It has its own documentation, found <a
|
|
href="../ext/mt_allocator.html">here</a>.
|
|
</p>
|
|
</li>
|
|
|
|
<li><code>bitmap_allocator</code>
|
|
<p>A high-performance allocator that uses a bit-map to keep track
|
|
of the used and unused memory locations. It has its own
|
|
documentation, found <a
|
|
href="../ext/ballocator_doc.html">here</a>.
|
|
</p>
|
|
</li>
|
|
</ul>
|
|
|
|
|
|
<h3 class="left">
|
|
<a name="using_custom_allocators">Using a specific allocator</a>
|
|
</h3>
|
|
<p>You can specify different memory management schemes on a
|
|
per-container basis, by overriding the default
|
|
<code>Allocator</code> template parameter. For example, an easy
|
|
(but non-portable) method of specifying that only malloc/free
|
|
should be used instead of the default node allocator is:
|
|
</p>
|
|
<pre>
|
|
std::list <int, __gnu_cxx::malloc_allocator<int> > malloc_list;</pre>
|
|
Likewise, a debugging form of whichever allocator is currently in use:
|
|
<pre>
|
|
std::deque <int, __gnu_cxx::debug_allocator<std::allocator<int> > > debug_deque;</pre>
|
|
|
|
|
|
<h3 class="left">
|
|
<a name="custom_allocators">Writing custom allocators</a>
|
|
</h3>
|
|
<p> Writing a portable C++ allocator would dictate that the
|
|
interface would look much like the one specified for <code>
|
|
std::allocator</code>. Additional member functions, but not
|
|
subtractions, would be permissible.
|
|
</p>
|
|
|
|
<p> Probably the best place to start would be to copy one of the
|
|
extension allocators already shipped with libstdc++: say, <code>
|
|
new_allocator </code>.
|
|
</p>
|
|
|
|
|
|
<h3 class="left">
|
|
<a name="biblio">Bibliography / Further Reading</a>
|
|
</h3>
|
|
<p>
|
|
ISO/IEC 14882:1998 Programming languages - C++ [20.4 Memory]
|
|
</p>
|
|
|
|
<p>
|
|
Austern, Matt, C/C++ Users Journal.
|
|
<a href="http://www.cuj.com/documents/s=8000/cujcexp1812austern/">The Standard Librarian: What Are Allocators Good
|
|
For?</a>
|
|
</p>
|
|
|
|
<p>
|
|
Berger, Emery,
|
|
<a href="http://www.cs.umass.edu/~emery/hoard/"> The Hoard memory allocator </a>
|
|
</p>
|
|
|
|
<p>
|
|
Berger, Emery with Ben Zorn & Kathryn McKinley, OOPSLA 2002
|
|
<a href="http://www.cs.umass.edu/~emery/pubs/berger-oopsla2002.pdf">Reconsidering Custom Memory Allocation</a>
|
|
</p>
|
|
|
|
<p>
|
|
Kreft, Klaus and Angelika Langer, C++ Report, June 1998
|
|
<a href="http://www.langer.camelot.de/Articles/C++Report/Allocators/Allocators.html">Allocator Types</a>
|
|
</p>
|
|
|
|
<p>
|
|
Stroustrup, Bjarne, 19.4 Allocators, The C++ Programming
|
|
Language, Special Edition, Addison Wesley, Inc. 2000
|
|
</p>
|
|
|
|
<p>
|
|
Yen, Felix, <a href="http://home.earthlink.net/~brimar/yalloc/">Yalloc: A Recycling C++ Allocator</a>
|
|
</p>
|
|
|
|
<hr />
|
|
<p>Return <a href="#top">to the top of the page</a> or
|
|
<a href="http://gcc.gnu.org/libstdc++/">to the libstdc++ homepage</a>.
|
|
</p>
|
|
|
|
|
|
<!-- ####################################################### -->
|
|
|
|
<hr />
|
|
<p class="fineprint"><em>
|
|
See <a href="../17_intro/license.html">license.html</a> for copying conditions.
|
|
Comments and suggestions are welcome, and may be sent to
|
|
<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
|
|
</em></p>
|
|
|
|
|
|
</body>
|
|
</html>
|