Measuring RSS on Mac when MADV_FREE is used

[Justin Lebar]

At Mozilla, we recently ported our jemalloc fork to MacOS.  It works
great, except we've noticed that our RSS measurements are now
basically useless, because memory freed with madvise(MADV_FREE) is
counted against our process's RSS until the system starts running low
on memory and steals those pages away from us.

As far as I can tell, upstream jemalloc has the same behavior.

In contrast, MADV_DONTNEED on Linux releases memory (almost) immediately.

I don't think what MacOS is doing here is so unreasonable; the problem
isn't so much that it's holding onto this memory, but rather that I
now can't meaningfully measure my process's RSS via the operating
system.  (The program can compute this value, of course, by measuring
how much memory it's MADV_FREE'd.)

Do you guys know of any way I can force MacOS to drop my MADV_FREE'd
pages, short of making a large allocation in another process (which
might have other side-effects, like paging out things I want to keep
in memory)?  Or is there a way to directly measure how many
non-MADV_FREE'd pages my process is using?  I'm pretty stumped.

Regards,
-Justin