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 -- 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>