Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for whenzone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA

[Wu Fengguang]

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?

That means to introduce some heuristics for it. 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).

Thanks,
Fengguang
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