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SubjectRe: [PATCH v3 0/2] fadvise: support POSIX_FADV_NOREUSE
On Mon, Jun 27, 2011 at 12:04:41PM +0900, KOSAKI Motohiro wrote:
> (2011/06/24 22:49), Andrea Righi wrote:
> > There were some reported problems in the past about trashing page cache
> > when a backup software (i.e., rsync) touches a huge amount of pages (see
> > for example [1]).
> >
> > This problem has been almost fixed by the Minchan Kim's patch [2] and a
> > proper use of fadvise() in the backup software. For example this patch
> > set [3] has been proposed for inclusion in rsync.
> >
> > However, there can be still other similar trashing problems: when the
> > backup software reads all the source files, some of them may be part of
> > the actual working set of the system. When a POSIX_FADV_DONTNEED is
> > performed _all_ pages are evicted from pagecache, both the working set
> > and the use-once pages touched only by the backup software.
> >
> > A previous proposal [4] tried to resolve this problem being less
> > agressive in invalidating active pages, moving them to the inactive list
> > intead of just evict them from the page cache.
> >
> > However, this approach changed completely the old behavior of
> > invalidate_mapping_pages(), that is not only used by fadvise.
> >
> > The new solution maps POSIX_FADV_NOREUSE to the less-agressive page
> > invalidation policy.
> >
> > With POSIX_FADV_NOREUSE active pages are moved to the tail of the
> > inactive list, and pages in the inactive list are just removed from page
> > cache. Pages mapped by other processes or unevictable pages are not
> > touched at all.
> >
> > In this way if the backup was the only user of a page, that page will be
> > immediately removed from the page cache by calling POSIX_FADV_NOREUSE.
> > If the page was also touched by other tasks it'll be moved to the
> > inactive list, having another chance of being re-added to the working
> > set, or simply reclaimed when memory is needed.
> >
> > In conclusion, now userspace applications that want to drop some page
> > cache pages can choose between the following advices:
> >
> > POSIX_FADV_DONTNEED = drop page cache if possible
> > POSIX_FADV_NOREUSE = reduce page cache eligibility
>
> Eeek.
>
> Your POSIX_FADV_NOREUSE is very different from POSIX definition.
> POSIX says,
>
> POSIX_FADV_NOREUSE
> Specifies that the application expects to access the specified data once and then
> not reuse it thereafter.
>
> IfI understand correctly, it designed for calling _before_ data access
> and to be expected may prevent lru activation. But your NORESE is designed
> for calling _after_ data access. Big difference might makes a chance of
> portability issue.

You're right. NOREUSE is designed to implement drop behind policy.

I'll post a new patch that will plug this logic in DONTNEED (like the
presious version), but without breaking the old /proc/sys/vm/drop_caches
behavior.

Thanks,
-Andrea


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