Date:	Thu, 14 Jul 2011 07:11:38 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] mm: page allocator: Initialise ZLC for first zone
 eligible for zone_reclaim

On Thu, Jul 14, 2011 at 10:20:12AM +0900, KOSAKI Motohiro wrote:
> (2011/07/13 20:02), Mel Gorman wrote:
> > 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@...e.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.
> 
> Right you are. thank you correct me.
> 
> 
> >>  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?
> 
> So, now I'm revisit your number in [0/3]. and I've conclude your patch
> improve simple cp case too. then please forget my last mail. this patch
> looks nicer.
> 
> 	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> 

Thanks.

-- 
Mel Gorman
SUSE Labs
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