Table of Contents
Memory contains three general areas. First, function and operator
calls via new and delete
operator or member function calls. Second, allocation via
allocator. And finally, smart pointer and
intelligent pointer abstractions.
Memory management for Standard Library entities is encapsulated in a
class template called allocator. The
allocator abstraction is used throughout the
library in string, container classes,
algorithms, and parts of iostreams. This class, and base classes of
it, are the superset of available free store (“heap”)
management classes.
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, which is usually aliased to allocator_type. This includes adding chars to the string class, which acts as a regular STL container in this respect.
The default Allocator argument of every
container-of-T is 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);
The n arguments in both those
functions is a count of the number of
T's to allocate space for, not their
total size.
(This is a simplification; the real signatures use nested typedefs.)
The storage is obtained by calling ::operator
new, but it is unspecified when or how
often this function is called. The use of the
hint is unspecified, but intended as an
aid to locality if an implementation so
desires. [20.4.1.1]/6
Complete details can be found in the C++ standard, look in
[20.4 Memory].
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. This method may be slower
than caching the allocations and re-using previously-allocated
memory, but has the advantage of working correctly across a wide
variety of hardware and operating systems, including large
clusters. The __gnu_cxx::new_allocator
implements the simple operator new and operator delete semantics,
while __gnu_cxx::malloc_allocator
implements much the same thing, only with the C language functions
std::malloc and free.
Another approach is to use intelligence within the allocator
class to cache allocations. This extra machinery can take a variety
of forms: a bitmap index, an index into an exponentially increasing
power-of-two-sized buckets, or simpler fixed-size pooling cache.
The cache 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 operators
new and delete are not
always called to pass the memory on, either, which is a speed
bonus. Examples of allocators that use these techniques are
__gnu_cxx::bitmap_allocator,
__gnu_cxx::pool_allocator, and
__gnu_cxx::__mt_alloc.
Depending on the implementation techniques used, the underlying
operating system, and compilation environment, scaling caching
allocators can be tricky. In particular, order-of-destruction and
order-of-creation for memory pools may be difficult to pin down
with certainty, which may create problems when used with plugins
or loading and unloading shared objects in memory. As such, using
caching allocators on systems that do not support
abi::__cxa_atexit is not recommended.
The only allocator interface that is supported is the standard C++ interface. As such, all STL containers have been adjusted, and all external allocators have been modified to support this change.
The class 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 allocator is derived from
may not be user-configurable.
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 for sequence and associative containers.
Insertion and erasure in a multi-threaded environment.
This test shows the ability of the allocator to reclaim memory on a per-thread basis, as well as measuring thread contention for memory resources. Test source here.
A threaded producer/consumer model.
Test source for sequence and associative containers.
The current default choice for
allocator is
__gnu_cxx::new_allocator.
In use, 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
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 also available.
To globally disable memory caching within the library for the
default allocator, merely set
GLIBCXX_FORCE_NEW (with any value) in the
system's environment before running the program. If your program
crashes with GLIBCXX_FORCE_NEW in the
environment, it likely means that you linked against objects
built against the older library (objects which might still using the
cached allocations...).
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 or free
should be used instead of the default node allocator is:
std::list <int, __gnu_cxx::malloc_allocator<int> > malloc_list;Likewise, a debugging form of whichever allocator is currently in use:
std::deque <int, __gnu_cxx::debug_allocator<std::allocator<int> > > debug_deque;
Writing a portable C++ allocator would dictate that the interface
would look much like the one specified for
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: say a simple one like
new_allocator.
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.
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 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.
throw_allocator
Includes memory tracking and marking abilities as well as hooks for throwing exceptions at configurable intervals (including random, all, none).
__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.
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 thread-safe, while thr =
false, 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 degradation.
(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 with exponentially-increasing allocations. 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.