Messages in this thread | | | Date | Tue, 9 Jun 2009 16:25:39 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for when zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA |
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On Tue, Jun 09, 2009 at 04:14:25PM +0800, Mel Gorman wrote: > On Tue, Jun 09, 2009 at 09:58:22AM +0800, Wu Fengguang wrote: > > On Mon, Jun 08, 2009 at 09:01:28PM +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, > > > zone_reclaim_mode defaults to 1 meaning that clean unmapped pages will be > > > reclaimed if the zone watermarks are not being met. The problem is that > > > zone_reclaim() can be in a situation where it scans excessively without > > > making progress. > > > > > > One such situation is where a large tmpfs mount is occupying a large > > > percentage of memory overall. The pages do not get cleaned or reclaimed by > > > zone_reclaim(), but the lists are uselessly scanned frequencly making the > > > CPU spin at 100%. The scanning occurs because zone_reclaim() cannot tell > > > in advance the scan is pointless because the counters do not distinguish > > > between pagecache pages backed by disk and by RAM. The observation in > > > the field is that malloc() stalls for a long time (minutes in some cases) > > > when this situation occurs. > > > > > > Accounting for ram-backed file pages was considered but not implemented on > > > the grounds it would be introducing new branches and expensive checks into > > > the page cache add/remove patches and increase the number of statistics > > > needed in the zone. As zone_reclaim() failing is currently considered a > > > corner case, this seemed like overkill. Note, if there are a large number > > > of reports about CPU spinning at 100% on NUMA that is fixed by disabling > > > zone_reclaim, then this assumption is false and zone_reclaim() scanning > > > and failing is not a corner case but a common occurance > > > > > > This patch reintroduces zone_reclaim_interval which was removed by commit > > > 34aa1330f9b3c5783d269851d467326525207422 [zoned vm counters: zone_reclaim: > > > remove /proc/sys/vm/zone_reclaim_interval] because the zone counters were > > > considered sufficient to determine in advance if the scan would succeed. > > > As unsuccessful scans can still occur, zone_reclaim_interval is still > > > required. > > > > Can we avoid the user visible parameter zone_reclaim_interval? > > > > You could, but then there is no way of disabling it by setting it to 0 > either. I can't imagine why but the desired behaviour might really be to > spin and never go off-node unless there is no other option. They might > want to set it to 0 for example when determining what the right value for > zone_reclaim_mode is for their workloads. > > > That means to introduce some heuristics for it. > > I suspect the vast majority of users will ignore it unless they are runing > zone_reclaim_mode at the same time and even then will probably just leave > it as 30 as a LRU scan every 30 seconds worst case is not going to show up > on many profiles. > > > Since the whole point > > is to avoid 100% CPU usage, we can take down the time used for this > > failed zone reclaim (T) and forbid zone reclaim until (NOW + 100*T). > > > > i.e. just fix it internally at 100 seconds? How is that better than > having an obscure tunable? I think if this heuristic exists at all, it's > important that an administrator be able to turn it off if absolutly > necessary and so something must be user-visible.
That 100*T don't mean 100 seconds. It means to keep CPU usage under 1%: after busy scanning for time T, let's go relax for 100*T.
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