Memory Allocation

Intel® Threading Building Blocks (Intel® TBB) provides two memory allocator templates that are similar to the STL template class std::allocator. These two templates, scalable_allocator<T> and cache_aligned_allocator<T>, address critical issues in parallel programming as follows:

  • Scalability. Problems of scalability arise when using memory allocators originally designed for serial programs, on threads that might have to compete for a single shared pool in a way that allows only one thread to allocate at a time. Use the memory allocator template scalable_allocator<T> to avoid such scalability bottlenecks. This template can improve the performance of programs that rapidly allocate and free memory.

  • False sharing. Problems of sharing arise when two threads access different words that share the same cache line. The problem is that a cache line is the unit of information interchange between processor caches. If one processor modifies a cache line and another processor reads (or writes) the same cache line, the cache line must be moved from one processor to the other, even if the two processors are dealing with different words within the line. False sharing can hurt performance because cache lines can take hundreds of clocks to move.

Use the class cache_aligned_allocator<T> to always allocate on a cache line. Two objects allocated by cache_aligned_allocator are guaranteed to not have false sharing. If an object is allocated by cache_aligned_allocator and another object is allocated some other way, there is no guarantee. The interface to cache_aligned_allocator is identical to std::allocator, so you can use it as the allocator argument to STL template classes.

The following code shows how to declare an STL vector that uses cache_aligned_allocator for allocation:

std::vector<int,cache_aligned_allocator<int> >;

Tip

The functionality of cache_aligned_allocator<T> comes at some cost in space, because it must allocate at least one cache line’s worth of memory, even for a small object. So use cache_aligned_allocator<T> only if false sharing is likely to be a real problem.

The scalable memory allocator incorporates McRT technology developed by Intel’s PSL  CTG team.

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