On Mon, Jun 27, 2011 at 11:17:51AM +0100, Pádraig Brady wrote: > On 27/06/11 08:11, Andrea Righi wrote: > > 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. > > Hmm fair enough. > NOREUSE is meant for specifying you _will_ need the data _once_ > > Isn't this what rsync actually wants though? > I.E. to specify NOREUSE for the file up front > so it would drop from cache automatically as processed, > (if not already in cache). > > I realize that would be a more invasive patch. > > > 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. > > But will that break existing apps (running as root) that expect DONTNEED > to drop cache for a _file_. Perhaps posix_fadvise() is meant to have > process rather than system scope, but that has not been the case until now. The actual problem I think is that apps expect that DONTNEED can be used to drop cache, but this is not written anywhere in the POSIX standard. I would also like to have both functionalities: 1) be sure to drop page cache pages (now there's only a system-wide knob to do this: /proc/sys/vm/drop_caches), 2) give an advice to the kernel that I will not reuse some pages in the future. The standard can only provide 2). If we also want 1) at the file granularity, I think we'd need to introduce something linux specific to avoid having portability problems. -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>