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 cheers, Pádraig. -- 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>