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SubjectRe: [PATCH 1/3] Reintroduce zone_reclaim_interval for when zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA
On Tue, Jun 09, 2009 at 06:59:03PM +0900, KOSAKI Motohiro wrote:
> > > > > Here it is just recording the jiffies value. The real smarts with the counter
> > > > > use time_before() which I assumed could handle jiffie wrap-arounds. Even
> > > > > if it doesn't, the consequence is that one scan will occur that could have
> > > > > been avoided around the time of the jiffie wraparound. The value will then
> > > > > be reset and it will be fine.
> > > >
> > > > time_before() assume two argument are enough nearly time.
> > > > if we use 32bit cpu and HZ=1000, about jiffies wraparound about one month.
> > > >
> > > > Then,
> > > >
> > > > 1. zone reclaim failure occur
> > > > 2. system works fine for one month
> > > > 3. jiffies wrap and time_before() makes mis-calculation.
> > > >
> > >
> > > And the scan occurs uselessly and zone_reclaim_failure gets set again.
> > > I believe the one useless scan is not significant enough to warrent dealing
> > > with jiffie wraparound.
> >
> > Thank you for kindful explanation.
> > I fully agreed.
>
> Bah, no, not agreed.
> simple last failure recording makes following scenario.
>
>
> 1. zone reclaim failure occur. update zone_reclaim_failure.
> ^
> | time_before() return 1, and zone_reclaim() return immediately.
> v
> 2. after 32 second.
> ^
> | time_before() return 0, and zone_reclaim() works normally
> v
> 3. after one month
> ^
> | time_before() return 1, and zone_reclaim() return immediately.
> | although recent zone_reclaim() never failed.
> v
> 4. after more one month
>

Pants.

/me slaps self

+ /* Watch for jiffie wraparound */
+ if (unlikely(jiffies < zone->zone_reclaim_failure))
+ zone->zone_reclaim_failure = jiffies;
+
+ /* Do not attempt a scan if scanning failed recently */
+ if (time_before(jiffies,
+ zone->zone_reclaim_failure + zone_reclaim_interval))
+ return 0;
+

?

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab


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