What's the downside of having one API to pre-allocate memory to be used by the GC, and a second API to suspend/resume GC operations? When you run out of pre-allocated memory, it will resume GC operations automatically.

I'm naively thinking, the performance bottleneck is not with tracking allocations but constantly freeing them and then reallocating. Let the GC track allocations, but prevent it from doing anything else so long as it is under the pre-allocated memory limit for the process. When resumed, it will free unreferenced memory. That way, the program can suspend GC before a performance sensitive block and resume it afterwards. API's don't need to change, because the change at all that way.