Messages in this thread | | | Date | Tue, 9 Jun 2009 11:44:04 +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 |
| |
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
| |