Re: [PATCH RFC] fadvise: move active pages to inactive list with POSIX_FADV_DONTNEED

[Andrea Righi <andrea@betterlinux.com>]

On Thu, Jun 23, 2011 at 03:14:57PM +0100, Padraig Brady wrote:
> On 23/06/11 14:57, Andrea Righi wrote:
> > On Thu, Jun 23, 2011 at 12:14:21PM +0100, Padraig Brady wrote:
> >> On 22/06/11 22:51, 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_fadvise(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.
> >>>
> >>> With the following solution when posix_fadvise(POSIX_FADV_DONTNEED) is
> >>> called for an active page instead of removing it from the page cache it
> >>> is added to the tail of the inactive list. Otherwise, if it's already in
> >>> the inactive list the page is removed from the page cache.
> >>>
> >>> 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_fadvise(POSIX_FADV_DONTNEED). If the page was also touched by
> >>> other processes 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.
> >>>
> >>> Testcase:
> >>>
> >>>   - create a 1GB file called "zero"
> >>>   - run md5sum zero to read all the pages in page cache (this is to
> >>>     simulate the user activity on this file)
> >>>   - run "rsync zero zero_copy" (rsync is patched with [3])
> >>>   - re-run md5sum zero (user activity on the working set) and measure
> >>>     the time to complete this command
> >>>
> >>> The test has been performed using 3.0.0-rc4 vanilla and with this patch
> >>> applied (3.0.0-rc4-fadvise).
> >>>
> >>> Results:
> >>>                   avg elapsed time      block:block_bio_queue
> >>>  3.0.0-rc4                  4.127s                      8,214
> >>>  3.0.0-rc4-fadvise          2.146s                          0
> >>>
> >>> In the first case the file is evicted from page cache completely and we
> >>> must re-read it from the disk. In the second case the file is still in
> >>> page cache (in the inactive list) and we don't need any other additional
> >>> I/O operation.
> >>>
> >>> [1] http://marc.info/?l=rsync&m=128885034930933&w=2
> >>> [2] https://lkml.org/lkml/2011/2/20/57
> >>> [3] http://lists.samba.org/archive/rsync/2010-November/025827.html
> >>>
> >>> Signed-off-by: Andrea Righi <andrea@betterlinux.com>
> >>
> >> Hmm, What if you do want to evict it from the cache for testing purposes?
> >> Perhaps this functionality should be associated with POSIX_FADV_NOREUSE?
> >> dd has been recently modified to support invalidating the cache for a file,
> >> and it uses POSIX_FADV_DONTNEED for that.
> >> http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=5f311553
> > 
> > I don't have any objection to associate POSIX_FADV_NOREUSE to this
> > functionality. Actually maintaining a specific functionality to drop
> > file cache pages can be useful, indeed.
> > 
> > However, I'm not sure if POSIX_FADV_NOREUSE or POSIX_FADV_DONTNEED
> > either are suitable.
> > 
> > According to the standard:
> > 
> >  POSIX_FADV_NOREUSE = data will be accessed only once
> >  POSIX_FADV_DONTNEED = data will not be accessed in the near future
> 
> 
> > So, associating the "drop the page cache" semantic sounds like an
> > implementation detail and applications shouldn't implicitly rely on this
> > behaviour.
> 
> Well the "standard" really is what has been implemented up to now.
> POSIX_FADV_NOREUSE currently does nothing so, associating this
> new behavior with it seems less problematic for user space.

mmmh.. yes, we would also respect backward compatibility. The behaviour
of invalidate_mapping_pages() would remain the same: drop page cache if
possible.

> Also the names fit pretty well I think.
> 
>   POSIX_FADV_DONTNEED = drop if possible
>   POSIX_FADV_NOREUSE = current app won't reuse so reduce cache eligibility

Agreed.

-Andrea

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