On Thu, Jun 23, 2011 at 03:14:57PM +0100, Pádraig Brady wrote: > On 23/06/11 14:57, Andrea Righi wrote: > > On Thu, Jun 23, 2011 at 12:14:21PM +0100, Pádraig 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@xxxxxxxxxxxxxxx> > >> > >> 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>