I remember I did a memory allocator incompletely, and it did increase the speed of my application, but then debugging memory problem become less ideal because valgrind couldn't tell about memory boundaries any more. Make sure you put proper guard in debugging to trigger segfault :)
Valgrind comes with C macros for exactly that kind of use case: you can inform it about the state of your custom allocator, i.e. which parts of the heap are available/forbidden when this differs from its own tracking of the system calls and malloc usage.
