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Allocators and allocation

The latest version of this document is always available at http://gcc.gnu.org/onlinedocs/libstdc++/20_util/allocator.html.

To the libstdc++-v3 homepage.

The C++ Standard encapsulates memory management characteristics for strings, container classes, and parts of iostreams in a template class called std::allocator.

Standard requirements

The C++ standard only gives a few directives in this area:

• When you add elements to a container, and the container must allocate more memory to hold them, the container makes the request via its Allocator template parameter. This includes adding chars to the string class, which acts as a regular STL container in this respect.
• The default Allocator of every container-of-T is std::allocator<T>.
• The interface of the allocator<T> class is extremely simple. It has about 20 public declarations (nested typedefs, member functions, etc), but the two which concern us most are:

        T*    allocate   (size_type n, const void* hint = 0);
        void  deallocate (T* p, size_type n);

  (This is a simplification; the real signatures use nested typedefs.) The "n" arguments in both those functions is a count of the number of T's to allocate space for, not their total size.
• "The storage is obtained by calling ::operator new(size_t), but it is unspecified when or how often this function is called. The use of hint is unspecified, but intended as an aid to locality if an implementation so desires." [20.4.1.1]/6

Complete details cam be found in the C++ standard, look in [20.4 Memory].

Problems and Possibilities

The easiest way of fulfilling the requirements is to call operator new each time a container needs memory, and to call operator delete each time the container releases memory. BUT this method is horribly slow.

Or we can keep old memory around, and reuse it in a pool to save time. The old libstdc++-v2 used a memory pool, and so do we. As of 3.0, it's on by default. The pool is shared among all the containers in the program: when your program's std::vector<int> gets cut in half and frees a bunch of its storage, that memory can be reused by the private std::list<WonkyWidget> brought in from a KDE library that you linked against. And we don't have to call operators new and delete to pass the memory on, either, which is a speed bonus. BUT...

What about threads? No problem: in a threadsafe environment, the memory pool is manipulated atomically, so you can grow a container in one thread and shrink it in another, etc. BUT what if threads in libstdc++-v3 aren't set up properly? That's been answered already.

BUT what if you want to use your own allocator? What if you plan on using a runtime-loadable version of malloc() which uses shared telepathic anonymous mmap'd sections serializable over a network, so that memory requests should go through malloc? And what if you need to debug it?

Implementation details of std::allocator

The implementation of std::allocator has continued to evolve through successive releases. Here's a brief history.

3.0, 3.1, 3.2, 3.3

During this period, all allocators were written to the SGI style, and all STL containers expected this interface. This interface had a traits class called _Alloc_traits that attempted to provide more information for compile-time allocation selection and optimization. This traits class had another allocator wrapper, __simple_alloc<T,A>, which was a wrapper around another allocator, A, which itself is an allocator for instances of T. But wait, there's more: __allocator<T,A> is another adapter. Many of the provided allocator classes were SGI style: such classes can be changed to a conforming interface with this wrapper: __allocator<T, __alloc> is thus the same as allocator<T>.

The class std::allocator use the typedef __alloc to select an underlying allocator that satisfied memory allocation requests. The selection of this underlying allocator was not user-configurable.

3.4

For this and later releases, the only allocator interface that is support is the standard C++ interface. As such, all STL containers have been adjusted, and all external allocators have been modified to support this change. Because of this, __simple_alloc, __allocator, __alloc, and _Alloc_traits have all been removed.

The class std::allocator just has typedef, constructor, and rebind members. It inherits from one of the high-speed extension allocators, covered below. Thus, all allocation and deallocation depends on the base class.

The base class that std::allocator is derived from is not user-configurable.

How the default allocation strategy is selected.

It's difficult to pick an allocation strategy that will provide maximum utility, without excessively penalizing some behavior. In fact, it's difficult just deciding which typical actions to measure for speed.

Three synthetic benchmarks have been created that provide data that is used to compare different C++ allocators. These tests are:

• Insertion. Over multiple iterations, various STL container objects have elements inserted to some maximum amount. A variety of allocators are tested. Test source here.
• Insertion, clear, and re-insertion in a multi-threaded environment. Over multiple iterations, several threads are started that insert elements into a STL container, then assign a null instance of the same type to clear memory, and then re-insert the same number of elements. Several STL containers and multiple allocators are tested. This test shows the ability of the allocator to reclaim memory on a pre-thread basis, as well as measuring thread contention for memory resources. Test source here.
• A threaded producer/consumer model. Test source here.

Disabling memory caching.

In use, std::allocator may allocate and deallocate using implementation-specified strategies and heuristics. Because of this, every call to an allocator object's allocate member function may not actually call the global operator new. This situation is also duplicated for calls to the deallocate member function.

This can be confusing.

In particular, this can make debugging memory errors more difficult, especially when using third party tools like valgrind or debug versions of new.

There are various ways to solve this problem. One would be to use a custom allocator that just called operators new and delete directly, for every allocation. (See include/ext/new_allocator.h, for instance.) However, that option would involve changing source code to use the a non-default allocator. Another option is to force the default allocator to remove caching and pools, and to directly allocate with every call of allocate and directly deallocate with every call of deallocate, regardless of efficiency. As it turns out, this last option is available, although the exact mechanism has evolved with time.

For GCC releases from 2.95 through the 3.1 series, defining __USE_MALLOC on the gcc command line would change the default allocation strategy to instead use malloc and free. See this note for details as to why this was something needing improvement.

Starting with GCC 3.2, and continued in the 3.3 series, to globally disable memory caching within the library for the default allocator, merely set GLIBCPP_FORCE_NEW (at this time, with any value) in the system's environment before running the program. If your program crashes with GLIBCPP_FORCE_NEW in the environment, it likely means that you linked against objects built against the older library. Code to support this extension is fully compatible with 3.2 code if GLIBCPP_FORCE_NEW is not in the environment.

As it turns out, the 3.4 code base continues to use this mechanism, only the environment variable has been changed to GLIBCXX_FORCE_NEW.

Other allocators

Several other allocators are provided as part of this implementation. The location of the extension allocators and their names have changed, but in all cases, functionality is equivalent. Starting with gcc-3.4, all extension allocators are standard style. Before this point, SGI style was the norm. Because of this, the number of template arguments also changed. Here's a simple chart to track the changes.

Releases after gcc-3.4 have continued to add to the collection of available allocators. All of these new allocators are standard-style. The following table includes details, along with the first released version of GCC that included the extension allocator.

More details on each of these extension allocators follows.

• new_allocator

  Simply wraps ::operator new and ::operator delete.

• malloc_allocator

  Simply wraps malloc and free. There is also a hook for an out-of-memory handler (for new/delete this is taken care of elsewhere).

• array_allocator

  Allows allocations of known and fixed sizes using existing global or external storage allocated via construction of std::tr1::array objects. By using this allocator, fixed size containers (including std::string) can be used without instances calling ::operator new and ::operator delete. This capability allows the use of STL abstractions without runtime complications or overhead, even in situations such as program startup. For usage examples, please consult the libstdc++ testsuite.

• debug_allocator

  A wrapper around an arbitrary allocator A. It passes on slightly increased size requests to A, and uses the extra memory to store size information. When a pointer is passed to deallocate(), the stored size is checked, and assert() is used to guarantee they match.

• __pool_alloc

  A high-performance, single pool allocator. The reusable memory is shared among identical instantiations of this type. It calls through ::operator new to obtain new memory when its lists run out. If a client container requests a block larger than a certain threshold size, then the pool is bypassed, and the allocate/deallocate request is passed to ::operator new directly.

  For versions of __pool_alloc after 3.4.0, there is only one template parameter, as per the standard.

  Older versions of this class take a boolean template parameter, called thr, and an integer template parameter, called inst.

  The inst number is used to track additional memory pools. The point of the number is to allow multiple instantiations of the classes without changing the semantics at all. All three of

      typedef  __pool_alloc<true,0>    normal;
      typedef  __pool_alloc<true,1>    private;
      typedef  __pool_alloc<true,42>   also_private;

  behave exactly the same way. However, the memory pool for each type (and remember that different instantiations result in different types) remains separate.

  The library uses 0 in all its instantiations. If you wish to keep separate free lists for a particular purpose, use a different number.

  The thr boolean determines whether the pool should be manipulated atomically or not. When thr=true, the allocator is is threadsafe, while thr=false, and is slightly faster but unsafe for multiple threads.

  For thread-enabled configurations, the pool is locked with a single big lock. In some situations, this implementation detail may result in severe performance degredation.

  (Note that the GCC thread abstraction layer allows us to provide safe zero-overhead stubs for the threading routines, if threads were disabled at configuration time.)

• __mt_alloc

  A high-performance fixed-size allocator. It has its own documentation, found here.

• bitmap_allocator

  A high-performance allocator that uses a bit-map to keep track of the used and unused memory locations. It has its own documentation, found here.

Using a specific allocator

You can specify different memory management schemes on a per-container basis, by overriding the default Allocator template parameter. For example, an easy (but non-portable) method of specifying that only malloc/free should be used instead of the default node allocator is:

    std::list <int, __gnu_cxx::malloc_allocator<int> >  malloc_list;

    std::deque <int, __gnu_cxx::debug_allocator<std::allocator<int> > >  debug_deque;

Writing custom allocators

Writing a portable C++ allocator would dictate that the interface would look much like the one specified for std::allocator. Additional member functions, but not subtractions, would be permissible.

Probably the best place to start would be to copy one of the extension allocators already shipped with libstdc++: say, new_allocator .

Bibliography / Further Reading

ISO/IEC 14882:1998 Programming languages - C++ [20.4 Memory]

Austern, Matt, C/C++ Users Journal. The Standard Librarian: What Are Allocators Good For?

Berger, Emery, The Hoard memory allocator

Berger, Emery with Ben Zorn & Kathryn McKinley, OOPSLA 2002 Reconsidering Custom Memory Allocation

Kreft, Klaus and Angelika Langer, C++ Report, June 1998 Allocator Types

Stroustrup, Bjarne, 19.4 Allocators, The C++ Programming Language, Special Edition, Addison Wesley, Inc. 2000

Yen, Felix, Yalloc: A Recycling C++ Allocator

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