Re: differences between MADV_FREE and MADV_DONTNEED

[Nicholas Miell <nmiell@comcast.net>]

On Mon, 2006-01-16 at 16:24 -0800, Suleiman Souhlal wrote:
> Eric W. Biederman wrote:
> > As I recall the logic with DONTNEED was to mark the mapping of
> > the page clean so the page didn't need to be swapped out, it could
> > just be dropped.
> > 
> > That is why they anonymous and the file backed cases differ.
> > 
> > Part of the point is to avoid the case of swapping the pages out if
> > the application doesn't care what is on them anymore.
> 
> Well, imho, MADV_DONTNEED should mean "I won't need this anytime soon", 
> and MADV_FREE "I will never need this again".
> 

POSIX doesn't have a madvise(), but it does have a posix_madvise(), with
flags defined as follows:

POSIX_MADV_NORMAL
   Specifies that the application has no advice to give on its behavior
with respect to the specified range. It is the default characteristic if
no advice is given for a range of memory.
POSIX_MADV_SEQUENTIAL
   Specifies that the application expects to access the specified range
sequentially from lower addresses to higher addresses.
POSIX_MADV_RANDOM
   Specifies that the application expects to access the specified range
in a random order.
POSIX_MADV_WILLNEED
   Specifies that the application expects to access the specified range
in the near future.
POSIX_MADV_DONTNEED
   Specifies that the application expects that it will not access the
specified range in the near future.

Note that glibc forwards posix_madvise() directly to madvise(2), which
means that right now, POSIX conformant apps which use
posix_madvise(addr, len, POSIX_MADV_DONTNEED) are silently corrupting
data on Linux systems.

-- 
Nicholas Miell <nmiell@comcast.net>