Messages in this thread | | | Subject | Re: [PATCH 1/4] Properly account for the number of page cache pages zone_reclaim() can reclaim | From | Ram Pai <> | Date | Wed, 10 Jun 2009 15:42:59 -0700 |
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On Wed, 2009-06-10 at 14:41 +0100, Mel Gorman wrote: > On Wed, Jun 10, 2009 at 07:59:44PM +0800, Wu Fengguang wrote: > > On Wed, Jun 10, 2009 at 06:31:53PM +0800, Mel Gorman wrote: > > > On Wed, Jun 10, 2009 at 09:19:39AM +0800, Wu Fengguang wrote: > > > > On Wed, Jun 10, 2009 at 01:01:41AM +0800, Mel Gorman wrote: > > > > > On NUMA machines, the administrator can configure zone_reclaim_mode that > > > > > is a more targetted form of direct reclaim. On machines with large NUMA > > > > > distances for example, a zone_reclaim_mode defaults to 1 meaning that clean > > > > > unmapped pages will be reclaimed if the zone watermarks are not being met. > > > > > > > > > > There is a heuristic that determines if the scan is worthwhile but the > > > > > problem is that the heuristic is not being properly applied and is basically > > > > > assuming zone_reclaim_mode is 1 if it is enabled. > > > > > > > > > > Historically, once enabled it was depending on NR_FILE_PAGES which may > > > > > include swapcache pages that the reclaim_mode cannot deal with. Patch > > > > > vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch by > > > > > Kosaki Motohiro noted that zone_page_state(zone, NR_FILE_PAGES) included > > > > > pages that were not file-backed such as swapcache and made a calculation > > > > > based on the inactive, active and mapped files. This is far superior > > > > > when zone_reclaim==1 but if RECLAIM_SWAP is set, then NR_FILE_PAGES is a > > > > > reasonable starting figure. > > > > > > > > > > This patch alters how zone_reclaim() works out how many pages it might be > > > > > able to reclaim given the current reclaim_mode. If RECLAIM_SWAP is set > > > > > in the reclaim_mode it will either consider NR_FILE_PAGES as potential > > > > > candidates or else use NR_{IN}ACTIVE}_PAGES-NR_FILE_MAPPED to discount > > > > > swapcache and other non-file-backed pages. If RECLAIM_WRITE is not set, > > > > > then NR_FILE_DIRTY number of pages are not candidates. If RECLAIM_SWAP is > > > > > not set, then NR_FILE_MAPPED are not. > > > > > > > > > > Signed-off-by: Mel Gorman <mel@csn.ul.ie> > > > > > Acked-by: Christoph Lameter <cl@linux-foundation.org> > > > > > --- > > > > > mm/vmscan.c | 52 ++++++++++++++++++++++++++++++++++++++-------------- > > > > > 1 files changed, 38 insertions(+), 14 deletions(-) > > > > > > > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > > > > index 2ddcfc8..2bfc76e 100644 > > > > > --- a/mm/vmscan.c > > > > > +++ b/mm/vmscan.c > > > > > @@ -2333,6 +2333,41 @@ int sysctl_min_unmapped_ratio = 1; > > > > > */ > > > > > int sysctl_min_slab_ratio = 5; > > > > > > > > > > +static inline unsigned long zone_unmapped_file_pages(struct zone *zone) > > > > > +{ > > > > > + return zone_page_state(zone, NR_INACTIVE_FILE) + > > > > > + zone_page_state(zone, NR_ACTIVE_FILE) - > > > > > + zone_page_state(zone, NR_FILE_MAPPED); > > > > > > > > This may underflow if too many tmpfs pages are mapped. > > > > > > > > > > You're right. This is also a bug now in mmotm for patch > > > vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch which > > > is where I took this code out of and didn't think deeply enough about. > > > Well spotted. > > > > > > Should this be something like? > > > > > > static unsigned long zone_unmapped_file_pages(struct zone *zone) > > > { > > > unsigned long file_mapped = zone_page_state(zone, NR_FILE_MAPPED); > > > unsigned long file_lru = zone_page_state(zone, NR_INACTIVE_FILE) > > > zone_page_state(zone, NR_ACTIVE_FILE); > > > > > > return (file_lru > file_mapped) ? (file_lru - file_mapped) : 0; > > > } > > > > > > ? > > > > > > If that returns 0, it does mean that there are very few pages that the > > > current reclaim_mode is going to be able to deal with so even if the > > > count is not perfect, it should be good enough for what we need it for. > > > > Agreed. We opt to give up direct zone reclaim than to risk busy looping ;) > > > > Yep. Those busy loops doth chew up the CPU time, heat the planet and > wear out Ye Olde Bugzilla with the wailing of unhappy users :) > > > > > > +} > > > > > + > > > > > +/* Work out how many page cache pages we can reclaim in this reclaim_mode */ > > > > > +static inline long zone_pagecache_reclaimable(struct zone *zone) > > > > > +{ > > > > > + long nr_pagecache_reclaimable; > > > > > + long delta = 0; > > > > > + > > > > > + /* > > > > > + * If RECLAIM_SWAP is set, then all file pages are considered > > > > > + * potentially reclaimable. Otherwise, we have to worry about > > > > > + * pages like swapcache and zone_unmapped_file_pages() provides > > > > > + * a better estimate > > > > > + */ > > > > > + if (zone_reclaim_mode & RECLAIM_SWAP) > > > > > + nr_pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES); > > > > > + else > > > > > + nr_pagecache_reclaimable = zone_unmapped_file_pages(zone); > > > > > + > > > > > + /* If we can't clean pages, remove dirty pages from consideration */ > > > > > + if (!(zone_reclaim_mode & RECLAIM_WRITE)) > > > > > + delta += zone_page_state(zone, NR_FILE_DIRTY); > > > > > + > > > > > + /* Beware of double accounting */ > > > > > > > > The double accounting happens for NR_FILE_MAPPED but not > > > > NR_FILE_DIRTY(dirty tmpfs pages won't be accounted), > > > > > > I should have taken that out. In an interim version, delta was altered > > > more than once in a way that could have caused underflow. > > > > > > > so this comment > > > > is more suitable for zone_unmapped_file_pages(). But the double > > > > accounting does affects this abstraction. So a more reasonable > > > > sequence could be to first substract NR_FILE_DIRTY and then > > > > conditionally substract NR_FILE_MAPPED? > > > > > > The end result is the same I believe and I prefer having the > > > zone_unmapped_file_pages() doing just that and nothing else because it's > > > in line with what zone_lru_pages() does. > > > > OK. > > > > > > Or better to introduce a new counter NR_TMPFS_MAPPED to fix this mess? > > > > > > > > > > I considered such a counter and dismissed it but maybe it merits wider discussion. > > > > > > My problem with it is that it would affect the pagecache add/remove hot paths > > > and a few other sites and increase the amount of accouting we do within a > > > zone. It seemed unjustified to help a seldom executed slow path that only > > > runs on NUMA. > > > > We are not talking about NR_TMPFS_PAGES, but NR_TMPFS_MAPPED :) > > > > We only need to account it in page_add_file_rmap() and page_remove_rmap(), > > I don't think they are too hot paths. And the relative cost is low enough. > > > > It will look like this. > > > > Ok, you're right, that is much simplier than what I had in mind. I was fixated > on accounting for TMPFS pages. I think this patch has definite possibilities > and would help us with the tmpfs problem. If the tests come back "failed", > I'll be adding taking this logic and seeing can it be made work
And the results look great! While constantly watching /proc/zoneinfo, I observe that unlike earlier there was no unnecessary attempt to scan for reclaimable pages, instead pages were allocated from the other node's zone normal.
RP
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