More aggressive page purging

[Mike Hommey]

On Thu, May 10, 2012 at 11:19:07AM -0700, Jason Evans wrote:
> I don't see the point in double purge on OS X (or FreeBSD, which
> behaves similarly), assuming that the kernel actually does the right
> think under memory pressure.  All it does is make 'top' output easier
> to interpret,  yet jemalloc provides adequate statistics to accurately
> assess the application's actual active memory footprint (and Firefox
> even exposes those statistics, right?).  IMO MADV_FREE is a feature,
> not a bug.  Indeed, on Linux, MADV_DONTNEED is a performance headache
> that we've had to work around in various ways at Facebook, with mixed
> success.

BTW, madvise(MADV_FREE) shows up pretty high in profiling on OSX under
some circumstances with Firefox, so even that semantic has performance
headaches.

Mike