Using madvise for ld.so

[Andrew Morton]

Michael Matz <matz at kde.org> wrote:
>
> Hi Andrew,
> 
> On Tue, 16 Mar 2004, Andrew Morton wrote:
> 
> > > On Tue, 16 Mar 2004 21:00:26 +0100, Dirk Mueller <mueller at kde.org> wrote:
> > > > might be a stupid question, but how did you "clear the cache" ?
> > > 
> > > the usual "use memory till oom" thing (included in an earlier email iirc)
> > > i wish the kernel guys would sort this out which a proper solution.
> > 
> > In 2.6 you can run posix_fadvise(fd, start, length, POSIX_FADV_DONTNEED).
> > 
> > I've attached a little proggy which uses fadvise to shoot down some or all
> > of a file's pagecache.
> 
> This is only for one file, though.  I.e. one would need to do /proc 
> parsing magic to really clean the whole file cache (for all files not open 
> anymore).  I think the "alloc all mem, touch all pages" approach is 
> easier ;)

True.

You want to kill all pagecache to simulate cold startup?  No, we don't have
a way of doing that apart from the force-a-swapstorm approach which you're
using.  That's more valid anyway, because it will also cause reclaim of the
inode and directory entry caches.

> Which reminds me of a question I asked myself: If I open the file, fadvise 
> a WILLNEED on it, all its pages are scheduled for IO.  Completion is not 
> waited for.  So, if the program then exits, the file gets closed, and 
> assuming that was the last ref to that inode: will the not yet completed 
> IO be canceled or will it still be completed?

The I/O will not be cancelled.