a1340af76f
2005-09-01 Benjamin Kosnik <bkoz@redhat.com>
* testsuite/lib/libstdc++.exp (check_v3_target_cxa_atexit): New.
* testsuite/lib/dg-options.exp (dg-require-cxa-atexit): New.
* testsuite/ext/mt_allocator/deallocate_local-6.cc: New.
* testsuite/ext/mt_allocator/deallocate_local-8.cc: New.
* testsuite/ext/mt_allocator/deallocate_local_thread-5.cc: New.
* testsuite/ext/mt_allocator/deallocate_local_thread-7.cc: New.
* docs/html/ext/mt_allocator.html: Add link to examples.
* testsuite/testsuite_allocator.h: Tweak.
* testsuite/ext/mt_allocator/deallocate_global-2.cc: Same.
* testsuite/ext/mt_allocator/deallocate_global-4.cc: Same.
* testsuite/ext/mt_allocator/deallocate_global_thread-1.cc: Same.
* testsuite/ext/mt_allocator/deallocate_global_thread-3.cc: Same.
* testsuite/ext/mt_allocator/deallocate_local-2.cc: Same.
* testsuite/ext/mt_allocator/deallocate_local-4.cc: Same.
* testsuite/ext/mt_allocator/deallocate_local_thread-1.cc: Same.
* testsuite/ext/mt_allocator/deallocate_local_thread-3.cc: Same.
* testsuite/ext/new_allocator/deallocate_global.cc: Same.
* testsuite/ext/new_allocator/deallocate_local.cc: Same.
From-SVN: r103721
559 lines
22 KiB
HTML
559 lines
22 KiB
HTML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<head>
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<meta name="AUTHOR" content="Stefan Olsson <stefan@xapa.se>" />
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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>A fixed-size, multi-thread optimized allocator</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" title="Extensions" />
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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">A fixed-size, multi-thread optimized allocator</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++/ext/mt_allocator.html">
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http://gcc.gnu.org/onlinedocs/libstdc++/ext/mt_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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<h3 class="left">
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<a name="intro">Introduction</a>
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</h3>
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<p> The mt allocator [hereinafter referred to simply as "the
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allocator"] is a fixed size (power of two) allocator that was
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initially developed specifically to suit the needs of multi threaded
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applications [hereinafter referred to as an MT application]. Over time
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the allocator has evolved and been improved in many ways, in
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particular it now also does a good job in single threaded applications
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[hereinafter referred to as a ST application]. (Note: In this
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document, when referring to single threaded applications this also
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includes applications that are compiled with gcc without thread
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support enabled. This is accomplished using ifdef's on
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__GTHREADS). This allocator is tunable, very flexible, and capable of
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high-performance.
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</p>
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<p>
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The aim of this document is to describe - from a application point of
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view - the "inner workings" of the allocator.
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</p>
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<h3 class="left">
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<a name="design">Design Overview</a>
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</h3>
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<p> There are three general components to the allocator: a datum
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describing the characteristics of the memory pool, a policy class
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containing this pool that links instantiation types to common or
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individual pools, and a class inheriting from the policy class that is
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the actual allocator.
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</p>
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<p>The datum describing pools characteristics is
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<pre>
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template<bool _Thread>
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class __pool
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</pre>
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This class is parametrized on thread support, and is explicitly
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specialized for both multiple threads (with <code>bool==true</code>)
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and single threads (via <code>bool==false</code>.) It is possible to
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use a custom pool datum instead of the default class that is provided.
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</p>
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<p> There are two distinct policy classes, each of which can be used
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with either type of underlying pool datum.
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</p>
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<pre>
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template<bool _Thread>
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struct __common_pool_policy
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template<typename _Tp, bool _Thread>
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struct __per_type_pool_policy
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</pre>
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<p> The first policy, <code>__common_pool_policy</code>, implements a
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common pool. This means that allocators that are instantiated with
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different types, say <code>char</code> and <code>long</code> will both
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use the same pool. This is the default policy.
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</p>
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<p> The second policy, <code>__per_type_pool_policy</code>, implements
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a separate pool for each instantiating type. Thus, <code>char</code>
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and <code>long</code> will use separate pools. This allows per-type
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tuning, for instance.
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</p>
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<p> Putting this all together, the actual allocator class is
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<pre>
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template<typename _Tp, typename _Poolp = __default_policy>
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class __mt_alloc : public __mt_alloc_base<_Tp>, _Poolp
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</pre>
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This class has the interface required for standard library allocator
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classes, namely member functions <code>allocate</code> and
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<code>deallocate</code>, plus others.
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</p>
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<h3 class="left">
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<a name="init">Tunable parameters</a>
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</h3>
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<p>Certain allocation parameters can be modified, or tuned. There
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exists a nested <pre>struct __pool_base::_Tune</pre> that contains all
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these parameters, which include settings for
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</p>
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<ul>
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<li>Alignment </li>
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<li>Maximum bytes before calling <code>::operator new</code> directly</li>
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<li>Minimum bytes</li>
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<li>Size of underlying global allocations</li>
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<li>Maximum number of supported threads</li>
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<li>Migration of deallocations to the global free list</li>
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<li>Shunt for global <code>new</code> and <code>delete</code></li>
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</ul>
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<p>Adjusting parameters for a given instance of an allocator can only
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happen before any allocations take place, when the allocator itself is
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initialized. For instance:
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</p>
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<pre>
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#include <ext/mt_allocator.h>
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struct pod
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{
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int i;
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int j;
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};
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int main()
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{
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typedef pod value_type;
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typedef __gnu_cxx::__mt_alloc<value_type> allocator_type;
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typedef __gnu_cxx::__pool_base::_Tune tune_type;
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tune_type t_default;
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tune_type t_opt(16, 5120, 32, 5120, 20, 10, false);
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tune_type t_single(16, 5120, 32, 5120, 1, 10, false);
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tune_type t;
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t = allocator_type::_M_get_options();
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allocator_type::_M_set_options(t_opt);
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t = allocator_type::_M_get_options();
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allocator_type a;
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allocator_type::pointer p1 = a.allocate(128);
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allocator_type::pointer p2 = a.allocate(5128);
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a.deallocate(p1, 128);
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a.deallocate(p2, 5128);
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return 0;
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}
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</pre>
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<h3 class="left">
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<a name="init">Initialization</a>
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</h3>
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<p>
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The static variables (pointers to freelists, tuning parameters etc)
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are initialized as above, or are set to the global defaults.
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</p>
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<p>
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The very first allocate() call will always call the
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_S_initialize_once() function. In order to make sure that this
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function is called exactly once we make use of a __gthread_once call
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in MT applications and check a static bool (_S_init) in ST
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applications.
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</p>
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<p>
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The _S_initialize() function:
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- If the GLIBCXX_FORCE_NEW environment variable is set, it sets the bool
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_S_force_new to true and then returns. This will cause subsequent calls to
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allocate() to return memory directly from a new() call, and deallocate will
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only do a delete() call.
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</p>
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<p>
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- If the GLIBCXX_FORCE_NEW environment variable is not set, both ST and MT
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applications will:
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- Calculate the number of bins needed. A bin is a specific power of two size
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of bytes. I.e., by default the allocator will deal with requests of up to
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128 bytes (or whatever the value of _S_max_bytes is when _S_init() is
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called). This means that there will be bins of the following sizes
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(in bytes): 1, 2, 4, 8, 16, 32, 64, 128.
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- Create the _S_binmap array. All requests are rounded up to the next
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"large enough" bin. I.e., a request for 29 bytes will cause a block from
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the "32 byte bin" to be returned to the application. The purpose of
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_S_binmap is to speed up the process of finding out which bin to use.
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I.e., the value of _S_binmap[ 29 ] is initialized to 5 (bin 5 = 32 bytes).
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</p>
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<p>
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- Create the _S_bin array. This array consists of bin_records. There will be
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as many bin_records in this array as the number of bins that we calculated
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earlier. I.e., if _S_max_bytes = 128 there will be 8 entries.
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Each bin_record is then initialized:
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- bin_record->first = An array of pointers to block_records. There will be
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as many block_records pointers as there are maximum number of threads
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(in a ST application there is only 1 thread, in a MT application there
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are _S_max_threads).
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This holds the pointer to the first free block for each thread in this
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bin. I.e., if we would like to know where the first free block of size 32
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for thread number 3 is we would look this up by: _S_bin[ 5 ].first[ 3 ]
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The above created block_record pointers members are now initialized to
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their initial values. I.e. _S_bin[ n ].first[ n ] = NULL;
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</p>
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<p>
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- Additionally a MT application will:
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- Create a list of free thread id's. The pointer to the first entry
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is stored in _S_thread_freelist_first. The reason for this approach is
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that the __gthread_self() call will not return a value that corresponds to
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the maximum number of threads allowed but rather a process id number or
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something else. So what we do is that we create a list of thread_records.
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This list is _S_max_threads long and each entry holds a size_t thread_id
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which is initialized to 1, 2, 3, 4, 5 and so on up to _S_max_threads.
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Each time a thread calls allocate() or deallocate() we call
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_S_get_thread_id() which looks at the value of _S_thread_key which is a
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thread local storage pointer. If this is NULL we know that this is a newly
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created thread and we pop the first entry from this list and saves the
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pointer to this record in the _S_thread_key variable. The next time
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we will get the pointer to the thread_record back and we use the
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thread_record->thread_id as identification. I.e., the first thread that
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calls allocate will get the first record in this list and thus be thread
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number 1 and will then find the pointer to its first free 32 byte block
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in _S_bin[ 5 ].first[ 1 ]
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When we create the _S_thread_key we also define a destructor
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(_S_thread_key_destr) which means that when the thread dies, this
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thread_record is returned to the front of this list and the thread id
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can then be reused if a new thread is created.
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This list is protected by a mutex (_S_thread_freelist_mutex) which is only
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locked when records are removed/added to the list.
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</p>
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<p>
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- Initialize the free and used counters of each bin_record:
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- bin_record->free = An array of size_t. This keeps track of the number
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of blocks on a specific thread's freelist in each bin. I.e., if a thread
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has 12 32-byte blocks on it's freelists and allocates one of these, this
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counter would be decreased to 11.
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- bin_record->used = An array of size_t. This keeps track of the number
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of blocks currently in use of this size by this thread. I.e., if a thread
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has made 678 requests (and no deallocations...) of 32-byte blocks this
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counter will read 678.
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The above created arrays are now initialized with their initial values.
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I.e. _S_bin[ n ].free[ n ] = 0;
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</p>
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<p>
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- Initialize the mutex of each bin_record: The bin_record->mutex
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is used to protect the global freelist. This concept of a global
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freelist is explained in more detail in the section "A multi
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threaded example", but basically this mutex is locked whenever a
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block of memory is retrieved or returned to the global freelist
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for this specific bin. This only occurs when a number of blocks
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are grabbed from the global list to a thread specific list or when
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a thread decides to return some blocks to the global freelist.
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</p>
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<p> Notes about deallocation. This allocator does not explicitly
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release memory. Because of this, memory debugging programs like
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valgrind or purify may notice leaks: sorry about this
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inconvenience. Operating systems will reclaim allocated memory at
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program termination anyway. If sidestepping this kind of noise is
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desired, there are three options: use an allocator, like
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<code>new_allocator</code> that releases memory while debugging, use
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GLIBCXX_FORCE_NEW to bypass the allocator's internal pools, or use a
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custom pool datum that releases resources on destruction.</p>
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<p>On systems with the function <code>__cxa_atexit</code>, the
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allocator can be forced to free all memory allocated before program
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termination with the member function
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<code>__pool_type::_M_destroy</code>. However, because this member
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function relies on the precise and exactly-conforming ordering of
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static destructors, including those of a static local
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<code>__pool</code> object, it should not be used, ever, on systems
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that don't have the necessary underlying support. In addition, in
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practice, forcing deallocation can be tricky, as it requires the
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<code>__pool</code> object to be fully-constructed before the object
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that uses it is fully constructed. For most (but not all) STL
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containers, this works, as an instance of the allocator is constructed
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as part of a container's constructor. However, this assumption is
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implementation-specific, and subject to change. For an example of a
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pool that frees memory, see the following
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<a href="http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc%2b%2b-v3/testsuite/ext/mt_allocator/deallocate_local-6.cc">
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example.</a>
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</p>
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<h3 class="left">
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<a name="st_example">A single threaded example (and a primer for the multi threaded example!)</a>
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</h3>
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<p>
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Let's start by describing how the data on a freelist is laid out in memory.
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This is the first two blocks in freelist for thread id 3 in bin 3 (8 bytes):
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</p>
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<pre>
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+----------------+
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| next* ---------|--+ (_S_bin[ 3 ].first[ 3 ] points here)
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+----------------+ |
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| thread_id = 3 | |
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+----------------+ |
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| DATA | | (A pointer to here is what is returned to the
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| | | the application when needed)
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+----------------+ |
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+----------------+ |
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| next* |<-+ (If next == NULL it's the last one on the list)
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+----------------+
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| thread_id = 3 |
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+----------------+
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| DATA |
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+----------------+
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</pre>
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<p>
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With this in mind we simplify things a bit for a while and say that there is
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only one thread (a ST application). In this case all operations are made to
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what is referred to as the global pool - thread id 0 (No thread may be
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assigned this id since they span from 1 to _S_max_threads in a MT application).
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</p>
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<p>
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When the application requests memory (calling allocate()) we first look at the
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requested size and if this is > _S_max_bytes we call new() directly and return.
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</p>
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<p>
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If the requested size is within limits we start by finding out from which
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bin we should serve this request by looking in _S_binmap.
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</p>
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<p>
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A quick look at _S_bin[ bin ].first[ 0 ] tells us if there are any blocks of
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this size on the freelist (0). If this is not NULL - fine, just remove the
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block that _S_bin[ bin ].first[ 0 ] points to from the list,
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update _S_bin[ bin ].first[ 0 ] and return a pointer to that blocks data.
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</p>
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<p>
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If the freelist is empty (the pointer is NULL) we must get memory from the
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system and build us a freelist within this memory. All requests for new memory
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is made in chunks of _S_chunk_size. Knowing the size of a block_record and
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the bytes that this bin stores we then calculate how many blocks we can create
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within this chunk, build the list, remove the first block, update the pointer
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(_S_bin[ bin ].first[ 0 ]) and return a pointer to that blocks data.
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</p>
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<p>
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Deallocation is equally simple; the pointer is casted back to a block_record
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pointer, lookup which bin to use based on the size, add the block to the front
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of the global freelist and update the pointer as needed
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(_S_bin[ bin ].first[ 0 ]).
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</p>
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<p>
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The decision to add deallocated blocks to the front of the freelist was made
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after a set of performance measurements that showed that this is roughly 10%
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faster than maintaining a set of "last pointers" as well.
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</p>
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<h3 class="left">
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<a name="mt_example">A multi threaded example</a>
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</h3>
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<p>
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In the ST example we never used the thread_id variable present in each block.
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Let's start by explaining the purpose of this in a MT application.
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</p>
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<p>
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The concept of "ownership" was introduced since many MT applications
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allocate and deallocate memory to shared containers from different
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threads (such as a cache shared amongst all threads). This introduces
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a problem if the allocator only returns memory to the current threads
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freelist (I.e., there might be one thread doing all the allocation and
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thus obtaining ever more memory from the system and another thread
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that is getting a longer and longer freelist - this will in the end
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consume all available memory).
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</p>
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<p>
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Each time a block is moved from the global list (where ownership is
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irrelevant), to a threads freelist (or when a new freelist is built
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from a chunk directly onto a threads freelist or when a deallocation
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occurs on a block which was not allocated by the same thread id as the
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one doing the deallocation) the thread id is set to the current one.
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</p>
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<p>
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What's the use? Well, when a deallocation occurs we can now look at
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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 front 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>
|
|
|
|
|
|
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|
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|
|
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|
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See <a href="../17_intro/license.html">license.html</a> for copying conditions.
|
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Comments and suggestions are welcome, and may be sent to
|
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<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
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