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A Custom Block Allocator for Speeding Up VC++ STL

, 30 Oct 2006 CPOL
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A block allocator for use with STL containers that greatly improves speed in programs doing massive data insertions and extractions.


block_allocator is a custom STL allocator for use with STL as implemented in Microsoft VC++. Rather than doing allocations on a per-node basis, block_allocator allocates memory in fixed sized chunks, and delivers portions of these chunks as requested. Typical speed improvements of 40% have been obtained with respect to the default allocator. The size of the chunks, set by the user, should not be too little (reduced speed improvements) nor too large (memory wasted). Experiment and see what sizes fit best to your application.

block_allocator can substitute for the default allocator in the following containers:

  • list,
  • set,
  • multiset,
  • map,
  • multimap,
and WON'T work with other containers such as vector or queue. Note however that vector and queue already perform allocation in chunks. The usage of block_allocator is fairly simple, for instance:
// block allocated list of ints with chunks of 1024 elements
std::list<int,block_allocator<int,1024> > l;
Normal containers and block allocated containers can coexist without problems.

Compatibility mode with MSVC++ 6.0/7.0

Due to limitations of the standard library provided with these compilers, the mode of usage explained above does not work here. To circumvent this problem one must proceed as follows: For each of the containers supported, there's an associated block allocated container derived from it thru use of block_allocator. You have to define an activating macro for each container to be defined prior to the inclusion of blockallocator.h:

  • list -> block_allocated_list (macro DEFINE_BLOCK_ALLOCATED_LIST),
  • set -> block_allocated_set (macro DEFINE_BLOCK_ALLOCATED_SET),
  • multiset -> block_allocated_multiset (macro DEFINE_BLOCK_ALLOCATED_MULTISET),
  • map -> block_allocated_map (macro DEFINE_BLOCK_ALLOCATED_MAP),
  • multimap -> block_allocated_multimap (macro DEFINE_BLOCK_ALLOCATED_MULTIMAP),

To use block allocation based STL in your application, define the corresponding activating macro, include blockallocator.h and then change your declarations as follows:

  • list<type> -> block_allocated_list<type,chunk_size>
  • set<key> -> block_allocated_set<key,chunk_size>
  • multiset<key> -> block_allocated_multiset<key,chunk_size>
  • map<key,type> -> block_allocated_map<key,type,chunk_size>
  • multimap<key,type> -> block_allocated_multimap<key,type,chunk_size>

where chunk_size is the size of the chunks. You can enter too the other optional template parameters (see MSVC++ STL docs for more info).

The MSVC++ 6.0/7.0 compatibility mode can also be used in MSVC++ 7.1, so you need not modify your block_allocator-related code when porting legacy code to 7.1.

Multithreading issues

Each block allocated container instance uses its own block_allocator, so no multithreading problems should arise as long as your program conveniently protects their containers for concurrent access (or if no two threads access the same container instance). This is the same scenario posed by regular STL classes (remember operations on containers are not guarded by CRITICAL_SECTIONs or anything similar), so the moral of it all is: If your program was multithread safe without block_allocator, it'll continue to be with it.

Version history

  • 29th Feb, 2000 - 1.1
    • Initial release in CodeProject.
  • 22nd Mar, 2001 - 1.2
    • Included definitions for operator== and operator!=. The lack of these caused linking errors when invoking list::swap() and similar methods. The funny thing about it is that no one ever reported this seemingly important bug, so either swap() is not that much used or not that many people use block_allocator!
  • 25th Oct, 2006 - 1.3
    • block_allocator now works with MSVC++ 7.1 and 8.0. Thanks to James May for helping with testing this new version of the code.
  • 30th Oct, 2006 - 1.4
    • Fixed some typedefs incorrectly made private in block_allocated_list, block_allocated_set, etc.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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Comments and Discussions

GeneralRe: why do you need to define MSVC_STL_list_node? PinmemberJoaquín M López Muñoz25-Aug-04 5:11 
GeneralRe: why do you need to define MSVC_STL_list_node? PinsussAnonymous26-Aug-04 10:03 
GeneralRe: why do you need to define MSVC_STL_list_node? PinsussAnonymous26-Aug-04 10:05 
GeneralRe: why do you need to define MSVC_STL_list_node? PinmemberJoaquín M López Muñoz26-Aug-04 10:48 
don't be mad at me, just want the real answer..
Not at all, I love discussing this stuff. Please thrash me till you're satisfied Poke tongue | ;-P
I'll answer in reverse order:
I also searched my MSDN help that is VC6.0, NOT .NET the std::list class declaration is the same as in .NET.
Yes, the interface of std::list is the same, what changes is the internal implementation with respect to how the allocator is used. Please keep reading.
VC++ 6.0 does not understand member template classes, so 'template struct rebind { typedef allocator other; };' is useless. Good to know. So, if our allocators have to work with VC++ 6.0 and Dinkumware, we need to implement a member function called '_Charalloc()'...
This is also correct and it is the key point of this discussion. Consider the definition
typed std::list<int,std::allocator<int> > my_list;
So, according to what the article says, the implementation in MSVC 6.0 of my_list allocates its nodes by calling
where n is not sizeof(int) but rather the size of an internal node. So, the allocator does not know in advance how large the nodes requested will be (though they'll always be the same size for a given container and element). And the problem is that block_allocator needs to know this size at compile-time so that it can arrange its block structure in an efficient manner. To remedy this, the definition
is basically the same (it derives from, actually) as
std::list<type,block_allocator<type,MSVC_STL_list_node<type>,chunk_size> >
and block_allocator uses the type MSVC_STL_list_node<type> only to gain knowledge, at compile-time, of the size of the nodes it'll be requested. That's it.
It's a little cumbersome, so if I haven't made myself completely clear please tell me so.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo
GeneralRe: why do you need to define MSVC_STL_list_node? PinsussAnonymous26-Aug-04 11:51 
GeneralRe: why do you need to define MSVC_STL_list_node? PinmemberJoaquín M López Muñoz26-Aug-04 12:32 
GeneralRe: why do you need to define MSVC_STL_list_node? PinsussAnonymous26-Aug-04 21:54 
GeneralRe: why do you need to define MSVC_STL_list_node? PinmemberJoaquín M López Muñoz28-Aug-04 10:45 
GeneralRe: why do you need to define MSVC_STL_list_node? PinsussAnonymous7-Sep-04 9:46 
Generalblock_allocated_list::sort() does not return or produces incorrect result PinsussPeteH16-Apr-03 19:50 

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