On Wed, Aug 04, 2010 at 03:56:10PM -0700, Andrew Morton wrote: > On Wed, 4 Aug 2010 15:38:29 +0100 > Mel Gorman <mel@xxxxxxxxx> wrote: > > > Commenting on the series "Reduce writeback from page reclaim context V6" > > Andrew Morton noted; > > > > direct-reclaim wants to write a dirty page because that page is in the > > zone which the caller wants to allocate from! Telling the flusher threads > > to perform generic writeback will sometimes cause them to just gum the > > disk up with pages from different zones, making it even harder/slower to > > allocate a page from the zones we're interested in, no? > > > > On the machines used to test the series, there were relatively few zones > > and only one BDI so the scenario describes is a possibility. This series is > > a very early prototype series aimed at mitigating the problem. > > > > Patch 1 adds wakeup_flusher_threads_pages() which takes a list of pages > > from page reclaim. Each inode belonging to a page on the list is marked > > I_DIRTY_RECLAIM. When the flusher thread wakes, inodes with this tag are > > unconditionally moved to the wb->b_io list for writing. > > > > Patch 2 notes that writing back inodes does not necessarily write back > > pages belonging to the zone page reclaim is concerned with. In response, it > > adds a zone and counter to wb_writeback_work. As pages from the target zone > > are written, the zone-specific counter is updated. When the flusher thread > > then checks the zone counters if a specific zone is being targeted. While > > more pages may be written than necessary, the assumption is that the pages > > need cleaning eventually, the inode must be relatively old to have pages at > > the end of the LRU, the IO will be relatively efficient due to less random > > seeks and that pages from the target zone will still be cleaned. > > > > Testing did not show any significant differences in terms of reducing dirty > > file pages being written back but the lack of multiple BDIs and NUMA nodes in > > the test rig is a problem. Maybe someone else has access to a more suitable > > test rig. > > > > Any comment as to the suitability for such a direction? > > um. Might work. Isn't pretty though. > No, it's not. > But until we can demonstrate the problem or someone reports it, we > probably have more important issues to be looking at ;) I think that a > better approach is to try to trigger this problem as we develop and > test reclaim. That's a reasonable plan as we'll know for sure if this is the right direction or not. I'll put the patches on the back-burner for now and hopefully someone will remember them if a bug is reported about large stalls under memory pressure but that is specific to a machine with many nodes and many disks. > And if we _can't_ demonstrate it, work out why the heck > not - either the code's smarter than we thought it was or the test is > no good. > It's always possible that we won't be able to demonstrate it because the right file pages are getting cleaned more often than not by the time reclaim happens :/ -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
