Jemalloc stats

[Mike Hommey]

Hi,

In Firefox, we're tracking some of the stats provided by our fork of
jemalloc. One of them is:
- HeapCommitted - Memory mapped by the heap allocator that is committed,
  i.e. in physical memory or paged to disk.  When heap-committed is
  larger than heap-allocated, the difference between the two values is
  likely due to external fragmentation; that is, the allocator allocated
  a large block of memory and is unable to decommit it because a small
  part of that block is currently in use.

It would seem like this could match stats.active, but i'm not entirely
sure it is the case. In particular, we don't count madvised pages in
that metric, but it would seem stats.active does, although I haven't dug
deep enough yet.

Mike