[gosu]

Probably the fact that it eagerly coalesces newly freed blocks where high performance allocators like tcmalloc and jemalloc use slabs of fixed-sized blocks which are never coalesced. Coalescing will load adjacent blocks into cache, and it probably implies doubly linked lists where slab allocators can get away with cache-friendlier single links or bitmaps etc. (Because you probably have to remove a block from the middle of a free list if you've just coalesced it with a newly freed block). It turns out cache footprint is key for speed and scalability in allocators, so that's a big problem.

I'm embarassed to link this twice in a row now, but at one point I played around with the perf of a couple allocators, including glibc's ptmalloc. I wound up answering exactly this question, at least for some simple use cases: http://www.andrew.cmu.edu/user/apodolsk/418/finalreport.html.

Comments in ptmalloc sources say they basically stick to the algorithm described here: http://gee.cs.oswego.edu/dl/html/malloc.html.