[millstone]

A C++ program that copies arounds lots of temporary std::strings is an example of unnecessary allocations.

The thing about optimizing allocators is that you very quickly run out of ways to make them faster without a space tradeoff. Notice the first optimizations detailed in the article was to retain a 'free page' instead of returning it to the kernel: this speeds up allocation and deallocation, but increases memory usage. And if one makes it faster, why not two, or three, or fifty?

The same is true for that slab-style allocator. The basic idea is to separate memory into separate pools of different sizes, and only allocate from the pool for the given size. If that pool is full, enlarge it, even if other pools have enough space to satisfy the allocation. That's wasted memory!

So we have to balance speed against memory usage. The right balance is domain-specific: that Java app running on a monster server can afford to waste lots of memory to speed up allocations, while the allocator on my iPhone needs to be much more mindful of its space overhead.