lkml.org 
[lkml]   [2011]   [Jun]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    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


    \
     
     \ /
      Last update: 2011-06-27 09:17    [W:0.025 / U:60.448 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site