Re: madvise (MADV_FREE)

[Jamie Lokier <jamie.lokier@cern.ch>]

William J. Earl wrote:
>      I have been asked by some application people to have free() use
> MADV_DONTNEED or the equivalent in selected cases, specifically when
> the memory allocated is large, in order to free up the physical and
> virtual (swap space) memory for other uses.  If the application uses
> very large chunks of memory, giving it back entirely is a win.  The
> application could be recoded to do its own mmap() of /dev/zero and
> munmap(), but would prefer that this behavior be automatic.  Of course,
> MADV_DONTNEED does not apply in the case of mmap()/munmap() of /dev/zero,
> but it is not implausible to give up virtual memory.  Note that
> I am not claiming one should do anything of the sort for small
> allocations.

Take a look at Glibc's malloc/free, which is the only one we care about
for Linux.  Glibc's malloc uses mmap() of /dev/zero for large
allocations automatically.  You can change the threshold if you like.

However, assuming this was not the case, even your application would
benefit more from MADV_FREE than MADV_DONTNEED.  MADV_DONTNEED forces a
non-trivial minimum recycling cost, whereas MADV_FREE allows the cost to
be balanced between the kernel and the application, according to the
current paging situation.

-- Jamie
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