A Custom Block Allocator for Speeding Up VC++ STL

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Introduction

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:

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// block allocated list of ints with chunks of 1024 elements
std::list > 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 -> block_allocated_list
• set -> block_allocated_set
• multiset -> block_allocated_multiset
• map -> block_allocated_map
• multimap -> block_allocated_multimap

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

• 29^th Feb, 2000 - 1.1
  • Initial release in CodeProject.
• 22^nd 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!
• 25^th 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.
• 30^th Oct, 2006 - 1.4
  • Fixed some typedefs incorrectly made private in block_allocated_list, block_allocated_set, etc.

License

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

About the Author

Joaquín M López Muñoz

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