Messages in this thread | | | Date | Wed, 13 Jul 2011 12:02:46 +0100 | From | Mel Gorman <> | Subject | Re: [PATCH 2/3] mm: page allocator: Initialise ZLC for first zone eligible for zone_reclaim |
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On Wed, Jul 13, 2011 at 10:15:15AM +0900, KOSAKI Motohiro wrote: > (2011/07/11 22:01), Mel Gorman wrote: > > The zonelist cache (ZLC) is used among other things to record if > > zone_reclaim() failed for a particular zone recently. The intention > > is to avoid a high cost scanning extremely long zonelists or scanning > > within the zone uselessly. > > > > Currently the zonelist cache is setup only after the first zone has > > been considered and zone_reclaim() has been called. The objective was > > to avoid a costly setup but zone_reclaim is itself quite expensive. If > > it is failing regularly such as the first eligible zone having mostly > > mapped pages, the cost in scanning and allocation stalls is far higher > > than the ZLC initialisation step. > > > > This patch initialises ZLC before the first eligible zone calls > > zone_reclaim(). Once initialised, it is checked whether the zone > > failed zone_reclaim recently. If it has, the zone is skipped. As the > > first zone is now being checked, additional care has to be taken about > > zones marked full. A zone can be marked "full" because it should not > > have enough unmapped pages for zone_reclaim but this is excessive as > > direct reclaim or kswapd may succeed where zone_reclaim fails. Only > > mark zones "full" after zone_reclaim fails if it failed to reclaim > > enough pages after scanning. > > > > Signed-off-by: Mel Gorman <mgorman@suse.de> > > If I understand correctly this patch's procs/cons is, > > pros. > 1) faster when zone reclaim doesn't work effectively >
Yes.
> cons. > 2) slower when zone reclaim is off
How is it slower with zone_reclaim off?
Before
if (zone_reclaim_mode == 0) goto this_zone_full; ... this_zone_full: if (NUMA_BUILD) zlc_mark_zone_full(zonelist, z); if (NUMA_BUILD && !did_zlc_setup && nr_online_nodes > 1) { ... }
After if (NUMA_BUILD && !did_zlc_setup && nr_online_nodes > 1) { ... } if (zone_reclaim_mode == 0) goto this_zone_full; this_zone_full: if (NUMA_BUILD) zlc_mark_zone_full(zonelist, z);
Bear in mind that if the watermarks are met on the first zone, the zlc setup does not occur.
> 3) slower when zone recliam works effectively >
Marginally slower. It's now calling zlc setup so once a second it's zeroing a bitmap and calling zlc_zone_worth_trying() on the first zone testing a bit on a cache-hot structure.
As the ineffective case can be triggered by a simple cp, I think the cost is justified. Can you think of a better way of doing this?
> (2) and (3) are frequently happen than (1), correct?
Yes. I'd still expect zone_reclaim to be off on the majority of machines and even when enabled, I think it's relatively rare we hit the case where the workload is regularly falling over to the other node except in the case where it's a file server. Still, a cp is not to uncommon that the kernel should slow to a crawl as a result.
> At least, I think we need to keep zero impact when zone reclaim mode is off. >
I agree with this but I'm missing where we are taking the big hit with zone_reclaim==0.
-- Mel Gorman SUSE Labs
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