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Extensions"/><link rel="prev" href="bk01pt03ch19s07.html" title="Diagnostics"/><link rel="next" href="bitmap_allocator.html" title="bitmap_allocator"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 20. Allocators</th></tr><tr><td align="left"><a accesskey="p" href="bk01pt03ch19s07.html">Prev</a> </td><th width="60%" align="center">Part III. + Extensions + +</th><td align="right"> <a accesskey="n" href="bitmap_allocator.html">Next</a></td></tr></table><hr/></div><div class="chapter" title="Chapter 20. Allocators"><div class="titlepage"><div><div><h2 class="title"><a id="manual.ext.allocator"/>Chapter 20. Allocators</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl><dt><span class="section"><a href="ext_allocators.html#manual.ext.allocator.mt">mt_allocator</a></span></dt><dd><dl><dt><span class="section"><a href="ext_allocators.html#allocator.mt.intro">Intro</a></span></dt><dt><span class="section"><a href="ext_allocators.html#allocator.mt.design_issues">Design Issues</a></span></dt><dt><span class="section"><a href="ext_allocators.html#allocator.mt.impl">Implementation</a></span></dt><dt><span class="section"><a href="ext_allocators.html#allocator.mt.example_single">Single Thread Example</a></span></dt><dt><span class="section"><a href="ext_allocators.html#allocator.mt.example_multi">Multiple Thread Example</a></span></dt></dl></dd><dt><span class="section"><a href="bitmap_allocator.html">bitmap_allocator</a></span></dt><dd><dl><dt><span class="section"><a href="bitmap_allocator.html#allocator.bitmap.design">Design</a></span></dt><dt><span class="section"><a href="bitmap_allocator.html#allocator.bitmap.impl">Implementation</a></span></dt></dl></dd></dl></div><div class="section" title="mt_allocator"><div class="titlepage"><div><div><h2 class="title"><a id="manual.ext.allocator.mt"/>mt_allocator</h2></div></div></div><p> +</p><div class="section" title="Intro"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.mt.intro"/>Intro</h3></div></div></div><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, in + particular 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). This allocator is tunable, very flexible, and capable + of high-performance. +</p><p> + The aim of this document is to describe - from an application point of + view - the "inner workings" of the allocator. +</p></div><div class="section" title="Design Issues"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.mt.design_issues"/>Design Issues</h3></div></div></div><div class="section" title="Overview"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.mt.overview"/>Overview</h4></div></div></div><p> There are three general components to the allocator: a datum +describing the characteristics of the memory pool, a policy class +containing this pool that links instantiation types to common or +individual pools, and a class inheriting from the policy class that is +the actual allocator. +</p><p>The datum describing pools characteristics is +</p><pre class="programlisting"> + template<bool _Thread> + class __pool +</pre><p> This class is parametrized on thread support, and is explicitly +specialized for both multiple threads (with <code class="code">bool==true</code>) +and single threads (via <code class="code">bool==false</code>.) It is possible to +use a custom pool datum instead of the default class that is provided. +</p><p> There are two distinct policy classes, each of which can be used +with either type of underlying pool datum. +</p><pre class="programlisting"> + template<bool _Thread> + struct __common_pool_policy + + template<typename _Tp, bool _Thread> + struct __per_type_pool_policy +</pre><p> The first policy, <code class="code">__common_pool_policy</code>, implements a +common pool. This means that allocators that are instantiated with +different types, say <code class="code">char</code> and <code class="code">long</code> will both +use the same pool. This is the default policy. +</p><p> The second policy, <code class="code">__per_type_pool_policy</code>, implements +a separate pool for each instantiating type. Thus, <code class="code">char</code> +and <code class="code">long</code> will use separate pools. This allows per-type +tuning, for instance. +</p><p> Putting this all together, the actual allocator class is +</p><pre class="programlisting"> + template<typename _Tp, typename _Poolp = __default_policy> + class __mt_alloc : public __mt_alloc_base<_Tp>, _Poolp +</pre><p> This class has the interface required for standard library allocator +classes, namely member functions <code class="code">allocate</code> and +<code class="code">deallocate</code>, plus others. +</p></div></div><div class="section" title="Implementation"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.mt.impl"/>Implementation</h3></div></div></div><div class="section" title="Tunable Parameters"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.mt.tune"/>Tunable Parameters</h4></div></div></div><p>Certain allocation parameters can be modified, or tuned. There +exists a nested <code class="code">struct __pool_base::_Tune</code> that contains all +these parameters, which include settings for +</p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>Alignment</p></li><li class="listitem"><p>Maximum bytes before calling <code class="code">::operator new</code> directly</p></li><li class="listitem"><p>Minimum bytes</p></li><li class="listitem"><p>Size of underlying global allocations</p></li><li class="listitem"><p>Maximum number of supported threads</p></li><li class="listitem"><p>Migration of deallocations to the global free list</p></li><li class="listitem"><p>Shunt for global <code class="code">new</code> and <code class="code">delete</code></p></li></ul></div><p>Adjusting parameters for a given instance of an allocator can only +happen before any allocations take place, when the allocator itself is +initialized. For instance: +</p><pre class="programlisting"> +#include <ext/mt_allocator.h> + +struct pod +{ + int i; + int j; +}; + +int main() +{ + typedef pod value_type; + typedef __gnu_cxx::__mt_alloc<value_type> allocator_type; + typedef __gnu_cxx::__pool_base::_Tune tune_type; + + tune_type t_default; + tune_type t_opt(16, 5120, 32, 5120, 20, 10, false); + tune_type t_single(16, 5120, 32, 5120, 1, 10, false); + + tune_type t; + t = allocator_type::_M_get_options(); + allocator_type::_M_set_options(t_opt); + t = allocator_type::_M_get_options(); + + allocator_type a; + allocator_type::pointer p1 = a.allocate(128); + allocator_type::pointer p2 = a.allocate(5128); + + a.deallocate(p1, 128); + a.deallocate(p2, 5128); + + return 0; +} +</pre></div><div class="section" title="Initialization"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.mt.init"/>Initialization</h4></div></div></div><p> +The static variables (pointers to freelists, tuning parameters etc) +are initialized as above, or are set to the global defaults. +</p><p> +The very first allocate() call will always call the +_S_initialize_once() function. In order to make sure that this +function is called exactly once we make use of a __gthread_once call +in MT applications and check a static bool (_S_init) in ST +applications. +</p><p> +The _S_initialize() 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 ] + + 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 or 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></div><div class="section" title="Deallocation Notes"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.mt.deallocation"/>Deallocation Notes</h4></div></div></div><p> Notes about deallocation. This allocator does not explicitly +release memory. Because of this, memory debugging programs like +valgrind or purify may notice leaks: sorry about this +inconvenience. Operating systems will reclaim allocated memory at +program termination anyway. If sidestepping this kind of noise is +desired, there are three options: use an allocator, like +<code class="code">new_allocator</code> that releases memory while debugging, use +GLIBCXX_FORCE_NEW to bypass the allocator's internal pools, or use a +custom pool datum that releases resources on destruction. +</p><p> + On systems with the function <code class="code">__cxa_atexit</code>, the +allocator can be forced to free all memory allocated before program +termination with the member function +<code class="code">__pool_type::_M_destroy</code>. However, because this member +function relies on the precise and exactly-conforming ordering of +static destructors, including those of a static local +<code class="code">__pool</code> object, it should not be used, ever, on systems +that don't have the necessary underlying support. In addition, in +practice, forcing deallocation can be tricky, as it requires the +<code class="code">__pool</code> object to be fully-constructed before the object +that uses it is fully constructed. For most (but not all) STL +containers, this works, as an instance of the allocator is constructed +as part of a container's constructor. However, this assumption is +implementation-specific, and subject to change. For an example of a +pool that frees memory, see the following + <a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc++-v3/testsuite/ext/mt_allocator/deallocate_local-6.cc?view=markup"> + example.</a> +</p></div></div><div class="section" title="Single Thread Example"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.mt.example_single"/>Single Thread Example</h3></div></div></div><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 class="programlisting"> ++----------------+ +| 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) +| | +| | +| | ++----------------+ +| 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 pointer +(_S_bin[ bin ].first[ 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 front +of the global freelist and update the pointer as needed +(_S_bin[ bin ].first[ 0 ]). +</p><p> +The decision to add deallocated blocks to the front of the freelist was made +after a set of performance measurements that showed that this is roughly 10% +faster than maintaining a set of "last pointers" as well. +</p></div><div class="section" title="Multiple Thread Example"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.mt.example_multi"/>Multiple Thread Example</h3></div></div></div><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 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></div></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="bk01pt03ch19s07.html">Prev</a> </td><td align="center"><a accesskey="u" href="extensions.html">Up</a></td><td align="right"> <a accesskey="n" href="bitmap_allocator.html">Next</a></td></tr><tr><td align="left" valign="top">Diagnostics </td><td align="center"><a accesskey="h" href="../spine.html">Home</a></td><td align="right" valign="top"> bitmap_allocator</td></tr></table></div></body></html> |