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   8    <meta name="AUTHOR" content="Stefan Olsson &lt;stefan@xapa.se&gt;" />
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  21 <h1 class="centered"><a name="top">A fixed-size, multi-thread optimized allocator</a></h1>
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  34 <hr />
  35 <h3 class="left">
  36   <a name="intro">Introduction</a>
  37 </h3>
  38
  39 <p> The mt allocator [hereinafter referred to simply as "the
  40 allocator"] is a fixed size (power of two) allocator that was
  41 initially developed specifically to suit the needs of multi threaded
  42 applications [hereinafter referred to as an MT application]. Over time
  43 the allocator has evolved and been improved in many ways, in
  44 particular it now also does a good job in single threaded applications
  45 [hereinafter referred to as a ST application]. (Note: In this
  46 document, when referring to single threaded applications this also
  47 includes applications that are compiled with gcc without thread
  48 support enabled. This is accomplished using ifdef's on
  49 __GTHREADS). This allocator is tunable, very flexible, and capable of
  50 high-performance.
  51 </p>
  52
  53 <p>
  54 The aim of this document is to describe - from an application point of
  55 view - the "inner workings" of the allocator.
  56 </p>
  57
  58 <h3 class="left">
  59   <a name="design">Design Overview</a>
  60 </h3>
  61
  62 <p> There are three general components to the allocator: a datum
  63 describing the characteristics of the memory pool, a policy class
  64 containing this pool that links instantiation types to common or
  65 individual pools, and a class inheriting from the policy class that is
  66 the actual allocator.
  67 </p>
  68
  69 <p>The datum describing pools characteristics is
  70 </p>
  71 <pre>
  72   template&lt;bool _Thread&gt;
  73     class __pool
  74 </pre>
  75 <p> This class is parametrized on thread support, and is explicitly
  76 specialized for both multiple threads (with <code>bool==true</code>)
  77 and single threads (via <code>bool==false</code>.) It is possible to
  78 use a custom pool datum instead of the default class that is provided.
  79 </p>
  80
  81 <p> There are two distinct policy classes, each of which can be used
  82 with either type of underlying pool datum.
  83 </p>
  84
  85 <pre>
  86   template&lt;bool _Thread&gt;
  87     struct __common_pool_policy
  88
  89   template&lt;typename _Tp, bool _Thread&gt;
  90     struct __per_type_pool_policy
  91 </pre>
  92
  93 <p> The first policy, <code>__common_pool_policy</code>, implements a
  94 common pool. This means that allocators that are instantiated with
  95 different types, say <code>char</code> and <code>long</code> will both
  96 use the same pool. This is the default policy.
  97 </p>
  98
  99 <p> The second policy, <code>__per_type_pool_policy</code>, implements
 100 a separate pool for each instantiating type. Thus, <code>char</code>
 101 and <code>long</code> will use separate pools. This allows per-type
 102 tuning, for instance.
 103 </p>
 104
 105 <p> Putting this all together, the actual allocator class is
 106 </p>
 107 <pre>
 108   template&lt;typename _Tp, typename _Poolp = __default_policy&gt;
 109     class __mt_alloc : public __mt_alloc_base&lt;_Tp&gt;,  _Poolp
 110 </pre>
 111 <p> This class has the interface required for standard library allocator
 112 classes, namely member functions <code>allocate</code> and
 113 <code>deallocate</code>, plus others.
 114 </p>
 115
 116 <h3 class="left">
 117   <a name="init">Tunable parameters</a>
 118 </h3>
 119
 120 <p>Certain allocation parameters can be modified, or tuned. There
 121 exists a nested <code>struct __pool_base::_Tune</code> that contains all
 122 these parameters, which include settings for
 123 </p>
 124    <ul>
 125      <li>Alignment </li>
 126      <li>Maximum bytes before calling <code>::operator new</code> directly</li>
 127      <li>Minimum bytes</li>
 128      <li>Size of underlying global allocations</li>
 129      <li>Maximum number of supported threads</li>
 130      <li>Migration of deallocations to the global free list</li>
 131      <li>Shunt for global <code>new</code> and <code>delete</code></li>
 132    </ul>
 133 <p>Adjusting parameters for a given instance of an allocator can only
 134 happen before any allocations take place, when the allocator itself is
 135 initialized. For instance:
 136 </p>
 137 <pre>
 138 #include &lt;ext/mt_allocator.h&gt;
 139
 140 struct pod
 141 {
 142   int i;
 143   int j;
 144 };
 145
 146 int main()
 147 {
 148   typedef pod value_type;
 149   typedef __gnu_cxx::__mt_alloc&lt;value_type&gt; allocator_type;
 150   typedef __gnu_cxx::__pool_base::_Tune tune_type;
 151
 152   tune_type t_default;
 153   tune_type t_opt(16, 5120, 32, 5120, 20, 10, false);
 154   tune_type t_single(16, 5120, 32, 5120, 1, 10, false);
 155
 156   tune_type t;
 157   t = allocator_type::_M_get_options();
 158   allocator_type::_M_set_options(t_opt);
 159   t = allocator_type::_M_get_options();
 160
 161   allocator_type a;
 162   allocator_type::pointer p1 = a.allocate(128);
 163   allocator_type::pointer p2 = a.allocate(5128);
 164
 165   a.deallocate(p1, 128);
 166   a.deallocate(p2, 5128);
 167
 168   return 0;
 169 }
 170 </pre>
 171
 172 <h3 class="left">
 173   <a name="init">Initialization</a>
 174 </h3>
 175
 176 <p>
 177 The static variables (pointers to freelists, tuning parameters etc)
 178 are initialized as above, or are set to the global defaults.
 179 </p>
 180
 181 <p>
 182 The very first allocate() call will always call the
 183 _S_initialize_once() function.  In order to make sure that this
 184 function is called exactly once we make use of a __gthread_once call
 185 in MT applications and check a static bool (_S_init) in ST
 186 applications.
 187 </p>
 188
 189 <p>
 190 The _S_initialize() function:
 191 - If the GLIBCXX_FORCE_NEW environment variable is set, it sets the bool
 192   _S_force_new to true and then returns. This will cause subsequent calls to
 193   allocate() to return memory directly from a new() call, and deallocate will
 194   only do a delete() call.
 195 </p>
 196
 197 <p>
 198 - If the GLIBCXX_FORCE_NEW environment variable is not set, both ST and MT
 199   applications will:
 200   - Calculate the number of bins needed. A bin is a specific power of two size
 201     of bytes. I.e., by default the allocator will deal with requests of up to
 202     128 bytes (or whatever the value of _S_max_bytes is when _S_init() is
 203     called). This means that there will be bins of the following sizes
 204     (in bytes): 1, 2, 4, 8, 16, 32, 64, 128.
 205
 206   - Create the _S_binmap array. All requests are rounded up to the next
 207     "large enough" bin. I.e., a request for 29 bytes will cause a block from
 208     the "32 byte bin" to be returned to the application. The purpose of
 209     _S_binmap is to speed up the process of finding out which bin to use.
 210     I.e., the value of _S_binmap[ 29 ] is initialized to 5 (bin 5 = 32 bytes).
 211 </p>
 212 <p>
 213   - Create the _S_bin array. This array consists of bin_records. There will be
 214     as many bin_records in this array as the number of bins that we calculated
 215     earlier. I.e., if _S_max_bytes = 128 there will be 8 entries.
 216     Each bin_record is then initialized:
 217     - bin_record-&gt;first = An array of pointers to block_records. There will be
 218       as many block_records pointers as there are maximum number of threads
 219       (in a ST application there is only 1 thread, in a MT application there
 220       are _S_max_threads).
 221       This holds the pointer to the first free block for each thread in this
 222       bin. I.e., if we would like to know where the first free block of size 32
 223       for thread number 3 is we would look this up by: _S_bin[ 5 ].first[ 3 ]
 224
 225     The above created block_record pointers members are now initialized to
 226     their initial values. I.e. _S_bin[ n ].first[ n ] = NULL;
 227 </p>
 228
 229 <p>
 230 - Additionally a MT application will:
 231   - Create a list of free thread id's. The pointer to the first entry
 232     is stored in _S_thread_freelist_first. The reason for this approach is
 233     that the __gthread_self() call will not return a value that corresponds to
 234     the maximum number of threads allowed but rather a process id number or
 235     something else. So what we do is that we create a list of thread_records.
 236     This list is _S_max_threads long and each entry holds a size_t thread_id
 237     which is initialized to 1, 2, 3, 4, 5 and so on up to _S_max_threads.
 238     Each time a thread calls allocate() or deallocate() we call
 239     _S_get_thread_id() which looks at the value of _S_thread_key which is a
 240     thread local storage pointer. If this is NULL we know that this is a newly
 241     created thread and we pop the first entry from this list and saves the
 242     pointer to this record in the _S_thread_key variable. The next time
 243     we will get the pointer to the thread_record back and we use the
 244     thread_record-&gt;thread_id as identification. I.e., the first thread that
 245     calls allocate will get the first record in this list and thus be thread
 246     number 1 and will then find the pointer to its first free 32 byte block
 247     in _S_bin[ 5 ].first[ 1 ]
 248     When we create the _S_thread_key we also define a destructor
 249     (_S_thread_key_destr) which means that when the thread dies, this
 250     thread_record is returned to the front of this list and the thread id
 251     can then be reused if a new thread is created.
 252     This list is protected by a mutex (_S_thread_freelist_mutex) which is only
 253     locked when records are removed/added to the list.
 254 </p>
 255 <p>
 256   - Initialize the free and used counters of each bin_record:
 257     - bin_record-&gt;free = An array of size_t. This keeps track of the number
 258       of blocks on a specific thread's freelist in each bin. I.e., if a thread
 259       has 12 32-byte blocks on it's freelists and allocates one of these, this
 260       counter would be decreased to 11.
 261
 262     - bin_record-&gt;used = An array of size_t. This keeps track of the number
 263       of blocks currently in use of this size by this thread. I.e., if a thread
 264       has made 678 requests (and no deallocations...) of 32-byte blocks this
 265       counter will read 678.
 266
 267     The above created arrays are now initialized with their initial values.
 268     I.e. _S_bin[ n ].free[ n ] = 0;
 269 </p>
 270 <p>
 271   - Initialize the mutex of each bin_record: The bin_record-&gt;mutex
 272     is used to protect the global freelist. This concept of a global
 273     freelist is explained in more detail in the section "A multi
 274     threaded example", but basically this mutex is locked whenever a
 275     block of memory is retrieved or returned to the global freelist
 276     for this specific bin. This only occurs when a number of blocks
 277     are grabbed from the global list to a thread specific list or when
 278     a thread decides to return some blocks to the global freelist.
 279 </p>
 280
 281 <p> Notes about deallocation. This allocator does not explicitly
 282 release memory. Because of this, memory debugging programs like
 283 valgrind or purify may notice leaks: sorry about this
 284 inconvenience. Operating systems will reclaim allocated memory at
 285 program termination anyway. If sidestepping this kind of noise is
 286 desired, there are three options: use an allocator, like
 287 <code>new_allocator</code> that releases memory while debugging, use
 288 GLIBCXX_FORCE_NEW to bypass the allocator's internal pools, or use a
 289 custom pool datum that releases resources on destruction.</p>
 290
 291 <p>On systems with the function <code>__cxa_atexit</code>, the
 292 allocator can be forced to free all memory allocated before program
 293 termination with the member function
 294 <code>__pool_type::_M_destroy</code>. However, because this member
 295 function relies on the precise and exactly-conforming ordering of
 296 static destructors, including those of a static local
 297 <code>__pool</code> object, it should not be used, ever, on systems
 298 that don't have the necessary underlying support. In addition, in
 299 practice, forcing deallocation can be tricky, as it requires the
 300 <code>__pool</code> object to be fully-constructed before the object
 301 that uses it is fully constructed. For most (but not all) STL
 302 containers, this works, as an instance of the allocator is constructed
 303 as part of a container's constructor. However, this assumption is
 304 implementation-specific, and subject to change. For an example of a
 305 pool that frees memory, see the following
 306     <a href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/ext/mt_allocator/deallocate_local-6.cc?view=markup">
 307     example.</a>
 308 </p>
 309
 310 <h3 class="left">
 311   <a name="st_example">A single threaded example (and a primer for the multi threaded example!)</a>
 312 </h3>
 313
 314 <p>
 315 Let's start by describing how the data on a freelist is laid out in memory.
 316 This is the first two blocks in freelist for thread id 3 in bin 3 (8 bytes):
 317 </p>
 318 <pre>
 319 +----------------+
 320 | next* ---------|--+  (_S_bin[ 3 ].first[ 3 ] points here)
 321 |                |  |
 322 |                |  |
 323 |                |  |
 324 +----------------+  |
 325 | thread_id = 3  |  |
 326 |                |  |
 327 |                |  |
 328 |                |  |
 329 +----------------+  |
 330 | DATA           |  |  (A pointer to here is what is returned to the
 331 |                |  |   the application when needed)
 332 |                |  |
 333 |                |  |
 334 |                |  |
 335 |                |  |
 336 |                |  |
 337 |                |  |
 338 +----------------+  |
 339 +----------------+  |
 340 | next*          |&lt;-+  (If next == NULL it's the last one on the list)
 341 |                |
 342 |                |
 343 |                |
 344 +----------------+
 345 | thread_id = 3  |
 346 |                |
 347 |                |
 348 |                |
 349 +----------------+
 350 | DATA           |
 351 |                |
 352 |                |
 353 |                |
 354 |                |
 355 |                |
 356 |                |
 357 |                |
 358 +----------------+
 359 </pre>
 360
 361 <p>
 362 With this in mind we simplify things a bit for a while and say that there is
 363 only one thread (a ST application). In this case all operations are made to
 364 what is referred to as the global pool - thread id 0 (No thread may be
 365 assigned this id since they span from 1 to _S_max_threads in a MT application).
 366 </p>
 367 <p>
 368 When the application requests memory (calling allocate()) we first look at the
 369 requested size and if this is &gt; _S_max_bytes we call new() directly and return.
 370 </p>
 371 <p>
 372 If the requested size is within limits we start by finding out from which
 373 bin we should serve this request by looking in _S_binmap.
 374 </p>
 375 <p>
 376 A quick look at _S_bin[ bin ].first[ 0 ] tells us if there are any blocks of
 377 this size on the freelist (0). If this is not NULL - fine, just remove the
 378 block that _S_bin[ bin ].first[ 0 ] points to from the list,
 379 update _S_bin[ bin ].first[ 0 ] and return a pointer to that blocks data.
 380 </p>
 381 <p>
 382 If the freelist is empty (the pointer is NULL) we must get memory from the
 383 system and build us a freelist within this memory. All requests for new memory
 384 is made in chunks of _S_chunk_size. Knowing the size of a block_record and
 385 the bytes that this bin stores we then calculate how many blocks we can create
 386 within this chunk, build the list, remove the first block, update the pointer
 387 (_S_bin[ bin ].first[ 0 ]) and return a pointer to that blocks data.
 388 </p>
 389
 390 <p>
 391 Deallocation is equally simple; the pointer is casted back to a block_record
 392 pointer, lookup which bin to use based on the size, add the block to the front
 393 of the global freelist and update the pointer as needed
 394 (_S_bin[ bin ].first[ 0 ]).
 395 </p>
 396
 397 <p>
 398 The decision to add deallocated blocks to the front of the freelist was made
 399 after a set of performance measurements that showed that this is roughly 10%
 400 faster than maintaining a set of "last pointers" as well.
 401 </p>
 402
 403 <h3 class="left">
 404   <a name="mt_example">A multi threaded example</a>
 405 </h3>
 406
 407 <p>
 408 In the ST example we never used the thread_id variable present in each block.
 409 Let's start by explaining the purpose of this in a MT application.
 410 </p>
 411
 412 <p>
 413 The concept of "ownership" was introduced since many MT applications
 414 allocate and deallocate memory to shared containers from different
 415 threads (such as a cache shared amongst all threads). This introduces
 416 a problem if the allocator only returns memory to the current threads
 417 freelist (I.e., there might be one thread doing all the allocation and
 418 thus obtaining ever more memory from the system and another thread
 419 that is getting a longer and longer freelist - this will in the end
 420 consume all available memory).
 421 </p>
 422
 423 <p>
 424 Each time a block is moved from the global list (where ownership is
 425 irrelevant), to a threads freelist (or when a new freelist is built
 426 from a chunk directly onto a threads freelist or when a deallocation
 427 occurs on a block which was not allocated by the same thread id as the
 428 one doing the deallocation) the thread id is set to the current one.
 429 </p>
 430
 431 <p>
 432 What's the use? Well, when a deallocation occurs we can now look at
 433 the thread id and find out if it was allocated by another thread id
 434 and decrease the used counter of that thread instead, thus keeping the
 435 free and used counters correct. And keeping the free and used counters
 436 corrects is very important since the relationship between these two
 437 variables decides if memory should be returned to the global pool or
 438 not when a deallocation occurs.
 439 </p>
 440
 441 <p>
 442 When the application requests memory (calling allocate()) we first
 443 look at the requested size and if this is &gt; _S_max_bytes we call new()
 444 directly and return.
 445 </p>
 446
 447 <p>
 448 If the requested size is within limits we start by finding out from which
 449 bin we should serve this request by looking in _S_binmap.
 450 </p>
 451
 452 <p>
 453 A call to _S_get_thread_id() returns the thread id for the calling thread
 454 (and if no value has been set in _S_thread_key, a new id is assigned and
 455 returned).
 456 </p>
 457
 458 <p>
 459 A quick look at _S_bin[ bin ].first[ thread_id ] tells us if there are
 460 any blocks of this size on the current threads freelist. If this is
 461 not NULL - fine, just remove the block that _S_bin[ bin ].first[
 462 thread_id ] points to from the list, update _S_bin[ bin ].first[
 463 thread_id ], update the free and used counters and return a pointer to
 464 that blocks data.
 465 </p>
 466
 467 <p>
 468 If the freelist is empty (the pointer is NULL) we start by looking at
 469 the global freelist (0). If there are blocks available on the global
 470 freelist we lock this bins mutex and move up to block_count (the
 471 number of blocks of this bins size that will fit into a _S_chunk_size)
 472 or until end of list - whatever comes first - to the current threads
 473 freelist and at the same time change the thread_id ownership and
 474 update the counters and pointers. When the bins mutex has been
 475 unlocked, we remove the block that _S_bin[ bin ].first[ thread_id ]
 476 points to from the list, update _S_bin[ bin ].first[ thread_id ],
 477 update the free and used counters, and return a pointer to that blocks
 478 data.
 479 </p>
 480
 481 <p>
 482 The reason that the number of blocks moved to the current threads
 483 freelist is limited to block_count is to minimize the chance that a
 484 subsequent deallocate() call will return the excess blocks to the
 485 global freelist (based on the _S_freelist_headroom calculation, see
 486 below).
 487 </p>
 488
 489 <p>
 490 However if there isn't any memory on the global pool we need to get
 491 memory from the system - this is done in exactly the same way as in a
 492 single threaded application with one major difference; the list built
 493 in the newly allocated memory (of _S_chunk_size size) is added to the
 494 current threads freelist instead of to the global.
 495 </p>
 496
 497 <p>
 498 The basic process of a deallocation call is simple: always add the
 499 block to the front of the current threads freelist and update the
 500 counters and pointers (as described earlier with the specific check of
 501 ownership that causes the used counter of the thread that originally
 502 allocated the block to be decreased instead of the current threads
 503 counter).
 504 </p>
 505
 506 <p>
 507 And here comes the free and used counters to service. Each time a
 508 deallocation() call is made, the length of the current threads
 509 freelist is compared to the amount memory in use by this thread.
 510 </p>
 511
 512 <p>
 513 Let's go back to the example of an application that has one thread
 514 that does all the allocations and one that deallocates. Both these
 515 threads use say 516 32-byte blocks that was allocated during thread
 516 creation for example.  Their used counters will both say 516 at this
 517 point. The allocation thread now grabs 1000 32-byte blocks and puts
 518 them in a shared container. The used counter for this thread is now
 519 1516.
 520 </p>
 521
 522 <p>
 523 The deallocation thread now deallocates 500 of these blocks. For each
 524 deallocation made the used counter of the allocating thread is
 525 decreased and the freelist of the deallocation thread gets longer and
 526 longer. But the calculation made in deallocate() will limit the length
 527 of the freelist in the deallocation thread to _S_freelist_headroom %
 528 of it's used counter.  In this case, when the freelist (given that the
 529 _S_freelist_headroom is at it's default value of 10%) exceeds 52
 530 (516/10) blocks will be returned to the global pool where the
 531 allocating thread may pick them up and reuse them.
 532 </p>
 533
 534 <p>
 535 In order to reduce lock contention (since this requires this bins
 536 mutex to be locked) this operation is also made in chunks of blocks
 537 (just like when chunks of blocks are moved from the global freelist to
 538 a threads freelist mentioned above). The "formula" used can probably
 539 be improved to further reduce the risk of blocks being "bounced back
 540 and forth" between freelists.
 541 </p>
 542
 543 <hr />
 544 <p>Return <a href="#top">to the top of the page</a> or
 545    <a href="http://gcc.gnu.org/libstdc++/">to the libstdc++ homepage</a>.
 546 </p>
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