nmalloc doesn't cache VA for allocations > 8KB
Recently x86-64's default C library allocator was switched back to nmalloc
from dmalloc for fragmentation concerns exposed by xulrunner. nmalloc doesn't
cache address space in userland for buffers larger than 8KB, which mean
hitting mmap allocations for any of those requests. Some limited form of
VA caching should improve performance in general and mitigate some-to-all
of the performance hit going back from dmalloc to nmalloc.