Compelling examples of custom C++ allocators?

What are some really good reasons to ditch std::allocator in favor of a custom solution? Have you run across any situations where it was absolutely necessary for correctness, performance, scalability, etc? Any really clever examples?

Custom allocators have always been a feature of the Standard Library that I haven't had much need for. I was just wondering if anyone here on SO could provide some compelling examples to justify their existence.

Solution 1:

As I mention here, I've seen Intel TBB's custom STL allocator significantly improve performance of a multithreaded app simply by changing a single

std::vector<T>

to

std::vector<T,tbb::scalable_allocator<T> >

(this is a quick and convenient way of switching the allocator to use TBB's nifty thread-private heaps; see page 7 in this document)

Solution 2:

One area where custom allocators can be useful is game development, especially on game consoles, as they have only a small amount of memory and no swap. On such systems you want to make sure that you have tight control over each subsystem, so that one uncritical system can't steal the memory from a critical one. Other things like pool allocators can help to reduce memory fragmentation. You can find a long, detailed paper on the topic at:

EASTL -- Electronic Arts Standard Template Library