Messages in this thread | | | Date | Tue, 9 Jun 2009 09:14:25 +0100 | From | Mel Gorman <> | 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 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.
-- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab
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