diff options
| author | bkoz <bkoz@138bc75d-0d04-0410-961f-82ee72b054a4> | 2004-02-12 01:11:48 +0000 |
|---|---|---|
| committer | bkoz <bkoz@138bc75d-0d04-0410-961f-82ee72b054a4> | 2004-02-12 01:11:48 +0000 |
| commit | 612345a0f72cdea886e4086112170bb61a6f2407 (patch) | |
| tree | 5caf25dbf9a9f9aa18a5e5af6a075584352c60f1 /libstdc++-v3/docs | |
| parent | 866768aac52f3d2f415cf60c95b0a591c2130956 (diff) | |
| download | gcc-612345a0f72cdea886e4086112170bb61a6f2407.tar.gz | |
2004-02-11 Stefan Olsson <stefan@xapa.se>
* docs/html/ext/mt_allocator.html: New.
2004-02-11 Benjamin Kosnik <bkoz@redhat.com>
* docs/html/20_util/allocator.html: New file, consolidate
allocator information here. Revamp.
* docs/html/documentation.html: Change links.
* docs/html/20_util/howto.html: Same.
* docs/html/ext/howto.html: Same.
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@77687 138bc75d-0d04-0410-961f-82ee72b054a4
Diffstat (limited to 'libstdc++-v3/docs')
| -rw-r--r-- | libstdc++-v3/docs/html/20_util/allocator.html | 468 | ||||
| -rw-r--r-- | libstdc++-v3/docs/html/20_util/howto.html | 2 | ||||
| -rw-r--r-- | libstdc++-v3/docs/html/documentation.html | 9 | ||||
| -rw-r--r-- | libstdc++-v3/docs/html/ext/howto.html | 244 | ||||
| -rw-r--r-- | libstdc++-v3/docs/html/ext/mt_allocator.html | 414 |
5 files changed, 888 insertions, 249 deletions
diff --git a/libstdc++-v3/docs/html/20_util/allocator.html b/libstdc++-v3/docs/html/20_util/allocator.html new file mode 100644 index 00000000000..43aaae7e079 --- /dev/null +++ b/libstdc++-v3/docs/html/20_util/allocator.html @@ -0,0 +1,468 @@ +<?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="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<bool, int></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> +</table> + + <p>More details on each of these 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>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> 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>(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> + </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> diff --git a/libstdc++-v3/docs/html/20_util/howto.html b/libstdc++-v3/docs/html/20_util/howto.html index 13d0c8e0855..9597707a8be 100644 --- a/libstdc++-v3/docs/html/20_util/howto.html +++ b/libstdc++-v3/docs/html/20_util/howto.html @@ -219,7 +219,7 @@ <hr /> <h2><a name="5">Memory allocators</a></h2> <p>The available free store ("heap") management classes are - described <a href="../ext/howto.html">here</a>. + described <a href="allocator.html">here</a>. </p> <p>Return <a href="#top">to top of page</a> or <a href="../faq/index.html">to the FAQ</a>. diff --git a/libstdc++-v3/docs/html/documentation.html b/libstdc++-v3/docs/html/documentation.html index ceb60764e10..54bd59e8745 100644 --- a/libstdc++-v3/docs/html/documentation.html +++ b/libstdc++-v3/docs/html/documentation.html @@ -126,8 +126,8 @@ <li><a href="18_support/howto.html#2">Implementation properties</a></li> <li><a href="18_support/howto.html#3">Start and Termination</a></li> <li><a href="18_support/howto.html#4">Verbose <code>terminate</code></a></li> - <li><a href="18_support/howto.html#6">Dynamic memory management</a></li> - <li><a href="18_support/howto.html#7">RTTI, the ABI, and demangling</a></li> + <li><a href="18_support/howto.html#5">Dynamic memory management</a></li> + <li><a href="18_support/howto.html#6">RTTI, the ABI, and demangling</a></li> </ul> </li> @@ -145,7 +145,7 @@ <li><a href="20_util/howto.html#2"><code>auto_ptr</code> inside container classes</a></li> <li><a href="20_util/howto.html#3">Functors</a></li> <li><a href="20_util/howto.html#4">Pairs</a></li> - <li><a href="20_util/howto.html#5">Memory allocators</a></li> + <li><a href="20_util/allocator.html">Allocators and allocation</a></li> </ul> </li> @@ -226,8 +226,7 @@ <ul> <li><a href="ext/howto.html#1">Ropes and trees and hashes, oh my!</a></li> <li><a href="ext/howto.html#2">Added members and types</a></li> - <li><a href="ext/howto.html#3">Allocators (versions 3.0, 3.1, 3.2, 3.3)</a></li> - <li><a href="ext/howto.html#6">Allocators (version 3.4)</a></li> + <li><a href="ext/mt_allocator.html"><code>__mt_alloc</code> </a></li> <li><a href="ext/howto.html#4">Compile-time checks</a></li> <li><a href="ext/howto.html#5">LWG Issues</a></li> <li><a href="ext/../18_support/howto.html#5">Demangling</a></li> diff --git a/libstdc++-v3/docs/html/ext/howto.html b/libstdc++-v3/docs/html/ext/howto.html index 2ce76ee9db3..3e5c35c476c 100644 --- a/libstdc++-v3/docs/html/ext/howto.html +++ b/libstdc++-v3/docs/html/ext/howto.html @@ -48,8 +48,7 @@ <ul> <li><a href="#1">Ropes and trees and hashes, oh my!</a></li> <li><a href="#2">Added members and types</a></li> - <li><a href="#3">Allocators (versions 3.0, 3.1, 3.2, 3.3)</a></li> - <li><a href="#6">Allocators (version 3.4)</a></li> + <li><a href="mt_allocator.html"><code>__mt_alloc</code> </a></li> <li><a href="#4">Compile-time checks</a></li> <li><a href="#5">LWG Issues</a></li> <li><a href="../18_support/howto.html#5">Demangling</a></li> @@ -141,247 +140,6 @@ </p> <hr /> -<h2><a name="3">Allocators (versions 3.0, 3.1, 3.2, 3.3)</a></h2> - <p>Thread-safety, space efficiency, high speed, portability... this is a - mess. Where to begin? - </p> - <h3>The Rules</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 - char's 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 simplicifcation; 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> - <h3>Problems and Possibilities</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> - <p>Well then: - </p> - <h3>Available allocators in namespace std</h3> - <p>First I'll describe the situation as it exists for the code which - was released in GCC 3.1 and 3.2. Then I'll describe the differences - for 3.0. The allocator classes also have source documentation, - which is described <a href="../documentation.html#4">here</a> (you - will need to retrieve the maintainer-level docs, as almost none of - these entities are in the ISO standard). - </p> - <p>As a general rule of thumb, users are not allowed to use names which - begin with an underscore. This means that to be portable between - compilers, none of the following may be used in your program directly. - (If you decide to be unportable, then you're free do do what you want, - but it's not our fault if stuff breaks.) They are presented here for - information for maintainers and contributors in addition to users. - </p> - <p>These classes are always available: - </p> - <ul> - <li><code>__new_alloc</code> simply wraps <code>::operator new</code> - and <code>::operator delete</code>. - </li> - <li><code>__malloc_alloc_template<int inst></code> 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). The <code>inst</code> parameter is described below. - This class was called <code>malloc_alloc</code> in earlier versions. - </li> - <li><code>allocator<T></code> has already been described; it is - The Standard Allocator for instances of T. It uses the internal - <code>__alloc</code> typedef (see below) to satisy its requests. - </li> - <li><code>__simple_alloc<T,A></code> is a wrapper around another - allocator, A, which itself is an allocator for instances of T. - This is primarily used in an internal "allocator traits" - class which helps encapsulate the different styles of allocators. - </li> - <li><code>__debug_alloc<A></code> is also 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. - </li> - <li><code>__allocator<T,A></code> is an adaptor. Many of these - allocator classes have a consistent yet non-standard interface. - 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>. - </li> - </ul> - <p>Normally, - <code> __default_alloc_template<bool thr, int inst> </code> - is also available. This is the high-speed pool, called the default - node allocator. The reusable memory is shared among identical - instantiations of - this type. It calls through <code>__new_alloc</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>__new_alloc</code> directly. - </p> - <p>Its <code>inst</code> parameter is described below. The - <code>thr</code> boolean determines whether the pool should be - manipulated atomically or not. Two typedefs are provided: - <code>__alloc</code> is defined as this node allocator with thr=true, - and therefore is threadsafe, while <code>__single_client_alloc</code> - defines thr=false, and is slightly faster but unsafe for multiple - threads. - </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. In this situation, - <code>__alloc</code> should not be noticably slower than - <code>__single_client_alloc</code>.) - </p> - <p>[Another threadsafe allocator where each thread keeps its own free - list, so that no locking is needed, might be described here.] - </p> - <h3>A cannon to swat a fly:<code> __USE_MALLOC</code></h3> - <p>If you've already read <a href="../23_containers/howto.html#3">this - advice</a> but still think you remember how to use this macro from - SGI STL days. We have removed it in gcc 3.3. See next section - for the new way to get the same effect. - </p> - <h3>Globally disabling memory caching:<code> GLIBCXX_FORCE_NEW</code></h3> - <p>Starting with gcc 3.3, if you want to globally disable memory - caching within the library for the default allocator (i.e. - the one you get for all library objects when you do not specify - which one to use), merely set GLIBCXX_FORCE_NEW (at this time, - with any value) into your environment before running the - program. You will obtain a similar effect without having to - recompile your entire program and the entire library (the new - operator in gcc is a light wrapper around malloc). If your - program crashes with GLIBCXX_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 GLIBCXX_FORCE_NEW is not in the - environment. Prior to GCC 3.4, this variable was spelt - GLIBCPP_FORCE_NEW. - </p> - <h3>Writing your own allocators</h3> - <p>Depending on your application (a specific program, a generic library, - etc), allocator classes tend to be one of two styles: "SGI" - or "standard". See the comments in stl_alloc.h for more - information on this crucial difference. - </p> - <p>At the bottom of that header is a helper type, - <code>_Alloc_traits</code>, and various specializations of it. This - allows the container classes to make possible compile-time - optimizations based on features of the allocator. You should provide - a specialization of this type for your allocator (doing so takes only - two or three statements). - </p> - <h3>Using non-default allocators</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 nonportable) - method of specifying that only malloc/free should be used instead of - the default node allocator is: - </p> - <pre> - std::list <my_type, std::__malloc_alloc_template<0> > my_malloc_based_list;</pre> - Likewise, a debugging form of whichever allocator is currently in use: - <pre> - std::deque <my_type, std::__debug_alloc<std::__alloc> > debug_deque;</pre> - <h3><code>inst</code></h3> - <p>The <code>__malloc_alloc_template</code> and - <code>__default_alloc_template</code> classes take an integer parameter, - called inst here. This number is completely unused. - </p> - <p>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 __default_alloc_template<true,0> normal; - typedef __default_alloc_template<true,1> private; - typedef __default_alloc_template<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> - <h3>3.0.x</h3> - <p>For 3.0.x, many of the names were incorrectly <em>not</em> prefixed - with underscores. So symbols such as "std::single_client_alloc" - are present. Be very careful to not depend on these names any more - than you would depend on implementation-only names. - </p> - <p>Certain macros like <code>_NOTHREADS</code> and <code>__STL_THREADS</code> - can affect the 3.0.x allocators. Do not use them. Those macros have - been completely removed for 3.1. - </p> - <p>Return <a href="#top">to top of page</a> or - <a href="../faq/index.html">to the FAQ</a>. - </p> - -<hr /> -<h2><a name="6">Allocators (version 3.4)</a></h2> - <p>Changes are coming... - </p> - <p>If you plan on writing your own allocators, - <a href="../documentation.html#4">source documentation</a> is - available. You'll need to get the "maintainers" collection - in order to see the helper classes and extra notes. - </p> - <p>Return <a href="#top">to top of page</a> or - <a href="../faq/index.html">to the FAQ</a>. - </p> - -<hr /> <h2><a name="4">Compile-time checks</a></h2> <p>Currently libstdc++-v3 uses the concept checkers from the Boost library to perform <a href="../19_diagnostics/howto.html#3">optional diff --git a/libstdc++-v3/docs/html/ext/mt_allocator.html b/libstdc++-v3/docs/html/ext/mt_allocator.html new file mode 100644 index 00000000000..93a5bfb8ee0 --- /dev/null +++ b/libstdc++-v3/docs/html/ext/mt_allocator.html @@ -0,0 +1,414 @@ +<?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="Stefan Olsson <stefan@xapa.se>" /> + <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>A fixed-size, multi-thread optimized allocator</title> +<link rel="StyleSheet" href="../lib3styles.css" type="text/css" /> +<link rel="Copyright" href="../17_intro/license.html" type="text/html" /> +</head> +<body> + +<h1 class="centered"><a name="top">A fixed-size, multi-thread optimized allocator</a></h1> + +<p class="fineprint"><em> + The latest version of this document is always available at + <a href="http://gcc.gnu.org/onlinedocs/libstdc++/ext/mt_allocator.html"> + http://gcc.gnu.org/onlinedocs/libstdc++/ext/mt_allocator.html</a>. +</em></p> + +<p><em> + To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++-v3 homepage</a>. +</em></p> + +<!-- ####################################################### --> +<hr /> +<h3 class="left"> + <a name="intro">Introduction</a> +</h3> + +<p> The mt allocator [hereinafter referred to simply as "the +allocator"] is a fixed size (power of two) allocator that was +initially developed specifically to suit the needs of multi threaded +applications [hereinafter referred to as an MT application]. Over time +the allocator has evolved and been improved in many ways, one of the +being that it now also does a good job in single threaded applications +[hereinafter referred to as a ST application]. (Note: In this +document, when referring to single threaded applications this also +includes applications that are compiled with gcc without thread +support enabled. This is accomplished using ifdef's on __GTHREADS) +</p> + +<p> +The aim of this document is to describe - from a application point of +view - the "inner workings" of the allocator. +</p> + + +<h3 class="left"> + <a name="init">Initialization</a> +</h3> + +<p> +The static variables (pointers to freelists, tuning parameters etc) +are initialized to their default values at file scope, i.e.: +</p> + +<pre> + template<typename _Tp> size_t + __mt_alloc<_Tp>::_S_freelist_headroom = 10; +</pre> + +<p> +The very first allocate() call will always call the _S_init() function. +In order to make sure that this function is called exactly once we make use +of a __gthread_once (with _S_once_mt and _S_init as arguments) call in MT +applications and check a static bool (_S_initialized) in ST applications. +</p> + +<p> +The _S_init() function: +- If the GLIBCXX_FORCE_NEW environment variable is set, it sets the bool + _S_force_new to true and then returns. This will cause subsequent calls to + allocate() to return memory directly from a new() call, and deallocate will + only do a delete() call. +</p> + +<p> +- If the GLIBCXX_FORCE_NEW environment variable is not set, both ST and MT + applications will: + - Calculate the number of bins needed. A bin is a specific power of two size + of bytes. I.e., by default the allocator will deal with requests of up to + 128 bytes (or whatever the value of _S_max_bytes is when _S_init() is + called). This means that there will be bins of the following sizes + (in bytes): 1, 2, 4, 8, 16, 32, 64, 128. + + - Create the _S_binmap array. All requests are rounded up to the next + "large enough" bin. I.e., a request for 29 bytes will cause a block from + the "32 byte bin" to be returned to the application. The purpose of + _S_binmap is to speed up the process of finding out which bin to use. + I.e., the value of _S_binmap[ 29 ] is initialized to 5 (bin 5 = 32 bytes). +</p> +<p> + - Create the _S_bin array. This array consists of bin_records. There will be + as many bin_records in this array as the number of bins that we calculated + earlier. I.e., if _S_max_bytes = 128 there will be 8 entries. + Each bin_record is then initialized: + - bin_record->first = An array of pointers to block_records. There will be + as many block_records pointers as there are maximum number of threads + (in a ST application there is only 1 thread, in a MT application there + are _S_max_threads). + This holds the pointer to the first free block for each thread in this + bin. I.e., if we would like to know where the first free block of size 32 + for thread number 3 is we would look this up by: _S_bin[ 5 ].first[ 3 ] + - bin_record->last = See above, the only difference being that this points + to the last record on the same freelist. + + The above created block_record pointers members are now initialized to + their initial values. I.e. _S_bin[ n ].first[ n ] = NULL; +</p> + +<p> +- Additionally a MT application will: + - Create a list of free thread id's. The pointer to the first entry + is stored in _S_thread_freelist_first. The reason for this approach is + that the __gthread_self() call will not return a value that corresponds to + the maximum number of threads allowed but rather a process id number or + something else. So what we do is that we create a list of thread_records. + This list is _S_max_threads long and each entry holds a size_t thread_id + which is initialized to 1, 2, 3, 4, 5 and so on up to _S_max_threads. + Each time a thread calls allocate() or deallocate() we call + _S_get_thread_id() which looks at the value of _S_thread_key which is a + thread local storage pointer. If this is NULL we know that this is a newly + created thread and we pop the first entry from this list and saves the + pointer to this record in the _S_thread_key variable. The next time + we will get the pointer to the thread_record back and we use the + thread_record->thread_id as identification. I.e., the first thread that + calls allocate will get the first record in this list and thus be thread + number 1 and will then find the pointer to its first free 32 byte block + in _S_bin[ 5 ].first[ 1 ] + When we create the _S_thread_key we also define a destructor + (_S_thread_key_destr) which means that when the thread dies, this + thread_record is returned to the front of this list and the thread id + can then be reused if a new thread is created. + This list is protected by a mutex (_S_thread_freelist_mutex) which is only + locked when records are removed/added to the list. +</p> +<p> + - Initialize the free and used counters of each bin_record: + - bin_record->free = An array of size_t. This keeps track of the number + of blocks on a specific thread's freelist in each bin. I.e., if a thread + has 12 32-byte blocks on it's freelists and allocates one of these, this + counter would be decreased to 11. + + - bin_record->used = An array of size_t. This keeps track of the number + of blocks currently in use of this size by this thread. I.e., if a thread + has made 678 requests (and no deallocations...) of 32-byte blocks this + counter will read 678. + + The above created arrays are now initialized with their initial values. + I.e. _S_bin[ n ].free[ n ] = 0; +</p> +<p> + - Initialize the mutex of each bin_record: + The bin_record->mutex is used to protect the global freelist. This concept + of a global freelist is explained in more detail in the section + "A multi threaded example", but basically this mutex is locked whenever + a block of memory is retrieved or returned to the global freelist for this + specific bin. This only occurs when a number of blocks are grabbed from the + global list to a thread specific list or when a thread decides to return + some blocks to the global freelist. +</p> + +<h3 class="left"> + <a name="st_example">A single threaded example (and a primer for the multi threaded example!)</a> +</h3> + +<p> +Let's start by describing how the data on a freelist is laid out in memory. +This is the first two blocks in freelist for thread id 3 in bin 3 (8 bytes): +</p> +<pre> ++----------------+ +| next* ---------|--+ (_S_bin[ 3 ].first[ 3 ] points here) +| | | +| | | +| | | ++----------------+ | +| thread_id = 3 | | +| | | +| | | +| | | ++----------------+ | +| DATA | | (A pointer to here is what is returned to the +| | | the application when needed) +| | | +| | | +| | | +| | | +| | | +| | | ++----------------+ | ++----------------+ | +| next* |<-+ (If next == NULL it's the last one on the list and +| | then the _S_bin[ 3 ].last[ 3 ] pointer points to +| | here as well) +| | ++----------------+ +| thread_id = 3 | +| | +| | +| | ++----------------+ +| DATA | +| | +| | +| | +| | +| | +| | +| | ++----------------+ +</pre> + +<p> +With this in mind we simplify things a bit for a while and say that there is +only one thread (a ST application). In this case all operations are made to +what is referred to as the global pool - thread id 0 (No thread may be +assigned this id since they span from 1 to _S_max_threads in a MT application). +</p> +<p> +When the application requests memory (calling allocate()) we first look at the +requested size and if this is > _S_max_bytes we call new() directly and return. +</p> +<p> +If the requested size is within limits we start by finding out from which +bin we should serve this request by looking in _S_binmap. +</p> +<p> +A quick look at _S_bin[ bin ].first[ 0 ] tells us if there are any blocks of +this size on the freelist (0). If this is not NULL - fine, just remove the +block that _S_bin[ bin ].first[ 0 ] points to from the list, +update _S_bin[ bin ].first[ 0 ] and return a pointer to that blocks data. +</p> +<p> +If the freelist is empty (the pointer is NULL) we must get memory from the +system and build us a freelist within this memory. All requests for new memory +is made in chunks of _S_chunk_size. Knowing the size of a block_record and +the bytes that this bin stores we then calculate how many blocks we can create +within this chunk, build the list, remove the first block, update the pointers +(_S_bin[ bin ].first[ 0 ] and _S_bin[ bin ].last[ 0 ]) and return a pointer +to that blocks data. +</p> + +<p> +Deallocation is equally simple; the pointer is casted back to a block_record +pointer, lookup which bin to use based on the size, add the block to the end +of the global freelist (with the next pointer set to NULL) and update the +pointers as needed (_S_bin[ bin ].first[ 0 ] and _S_bin[ bin ].last[ 0 ]). +</p> + +<h3 class="left"> + <a name="mt_example">A multi threaded example</a> +</h3> + +<p> +In the ST example we never used the thread_id variable present in each block. +Let's start by explaining the purpose of this in a MT application. +</p> + +<p> +The concept of "ownership" was introduced since many MT applications +allocate and deallocate memory to shared containers from different +threads (such as a cache shared amongst all threads). This introduces +a problem if the allocator only returns memory to the current threads +freelist (I.e., there might be one thread doing all the allocation and +thus obtaining ever more memory from the system and another thread +that is getting a longer and longer freelist - this will in the end +consume all available memory). +</p> + +<p> +Each time a block is moved from the global list (where ownership is +irrelevant), to a threads freelist (or when a new freelist is built +from a chunk directly onto a threads freelist or when a deallocation +occurs on a block which was not allocated by the same thread id as the +one doing the deallocation) the thread id is set to the current one. +</p> + +<p> +What's the use? Well, when a deallocation occurs we can now look at +the thread id and find out if it was allocated by another thread id +and decrease the used counter of that thread instead, thus keeping the +free and used counters correct. And keeping the free and used counters +corrects is very important since the relationship between these two +variables decides if memory should be returned to the global pool or +not when a deallocation occurs. +</p> + +<p> +When the application requests memory (calling allocate()) we first +look at the requested size and if this is > _S_max_bytes we call new() +directly and return. +</p> + +<p> +If the requested size is within limits we start by finding out from which +bin we should serve this request by looking in _S_binmap. +</p> + +<p> +A call to _S_get_thread_id() returns the thread id for the calling thread +(and if no value has been set in _S_thread_key, a new id is assigned and +returned). +</p> + +<p> +A quick look at _S_bin[ bin ].first[ thread_id ] tells us if there are +any blocks of this size on the current threads freelist. If this is +not NULL - fine, just remove the block that _S_bin[ bin ].first[ +thread_id ] points to from the list, update _S_bin[ bin ].first[ +thread_id ], update the free and used counters and return a pointer to +that blocks data. +</p> + +<p> +If the freelist is empty (the pointer is NULL) we start by looking at +the global freelist (0). If there are blocks available on the global +freelist we lock this bins mutex and move up to block_count (the +number of blocks of this bins size that will fit into a _S_chunk_size) +or until end of list - whatever comes first - to the current threads +freelist and at the same time change the thread_id ownership and +update the counters and pointers. When the bins mutex has been +unlocked, we remove the block that _S_bin[ bin ].first[ thread_id ] +points to from the list, update _S_bin[ bin ].first[ thread_id ], +update the free and used counters, and return a pointer to that blocks +data. +</p> + +<p> +The reason that the number of blocks moved to the current threads +freelist is limited to block_count is to minimize the chance that a +subsequent deallocate() call will return the excess blocks to the +global freelist (based on the _S_freelist_headroom calculation, see +below). +</p> + +<p> +However if there isn't any memory on the global pool we need to get +memory from the system - this is done in exactly the same way as in a +single threaded application with one major difference; the list built +in the newly allocated memory (of _S_chunk_size size) is added to the +current threads freelist instead of to the global. +</p> + +<p> +The basic process of a deallocation call is simple: always add the +block to the end of the current threads freelist and update the +counters and pointers (as described earlier with the specific check of +ownership that causes the used counter of the thread that originally +allocated the block to be decreased instead of the current threads +counter). +</p> + +<p> +And here comes the free and used counters to service. Each time a +deallocation() call is made, the length of the current threads +freelist is compared to the amount memory in use by this thread. +</p> + +<p> +Let's go back to the example of an application that has one thread +that does all the allocations and one that deallocates. Both these +threads use say 516 32-byte blocks that was allocated during thread +creation for example. Their used counters will both say 516 at this +point. The allocation thread now grabs 1000 32-byte blocks and puts +them in a shared container. The used counter for this thread is now +1516. +</p> + +<p> +The deallocation thread now deallocates 500 of these blocks. For each +deallocation made the used counter of the allocating thread is +decreased and the freelist of the deallocation thread gets longer and +longer. But the calculation made in deallocate() will limit the length +of the freelist in the deallocation thread to _S_freelist_headroom % +of it's used counter. In this case, when the freelist (given that the +_S_freelist_headroom is at it's default value of 10%) exceeds 52 +(516/10) blocks will be returned to the global pool where the +allocating thread may pick them up and reuse them. +</p> + +<p> +In order to reduce lock contention (since this requires this bins +mutex to be locked) this operation is also made in chunks of blocks +(just like when chunks of blocks are moved from the global freelist to +a threads freelist mentioned above). The "formula" used can probably +be improved to further reduce the risk of blocks being "bounced back +and forth" between freelists. +</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> |