Implementation
Prev | Chapter 20. The mt_allocator | Next

## Implementation

### Tunable Parameters

Certain allocation parameters can be modified, or tuned. There exists a nested `struct __pool_base::_Tune` that contains all these parameters, which include settings for

- Alignment

- Maximum bytes before calling `::operator new` directly

- Minimum bytes

- Size of underlying global allocations

- Maximum number of supported threads

- Migration of deallocations to the global free list

- Shunt for global `new` and `delete`

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:

    #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;
    }

### Initialization

The static variables (pointers to freelists, tuning parameters etc) are initialized as above, or are set to the global defaults.

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.

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.

- 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).

- 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;

- 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.

- 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;

- 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.

### Deallocation Notes

Notes about deallocation. This allocator does not explicitly release memory back to the OS, but keeps its own freelists instead. 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 `new_allocator` 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.

On systems with the function `__cxa_atexit`, the allocator can be forced to free all memory allocated before program termination with the member function `__pool_type::_M_destroy`. However, because this member function relies on the precise and exactly-conforming ordering of static destructors, including those of a static local `__pool` 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 `__pool` 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 example.

Prev | Up | Next
Design Issues | Home | Single Thread Example