More aggressive page purging

[Mike Hommey]

On Fri, May 11, 2012 at 04:16:16PM -0700, Jason Evans wrote:
> jemalloc maintains 'allocated', 'active', and 'mapped' statistics.
> 'active' ignores a bit of internally allocated metadata memory, but it
> is very close to being RSS minus madvise'd memory minus non-malloc
> memory (libraries, other mapped files, etc.).

'active' doesn't track e.g. huge allocations that are only partially
used. e.g malloc(5M) will report 8M of active.

Mike