Re: [patch 1/3]vmscan: clear ZONE_CONGESTED for zone with good watermark

[Minchan Kim <minchan.kim@gmail.com>]

On Fri, Jul 29, 2011 at 08:35:25AM +0800, Shaohua Li wrote:
> On Thu, 2011-07-28 at 18:56 +0800, Mel Gorman wrote:
> > On Thu, Jul 28, 2011 at 04:13:01PM +0800, Shaohua Li wrote:
> > > correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
> > > should clear ZONE_CONGESTED regardless if this is a high order
> > > allocation, because pages can be reclaimed in other tasks but
> > > ZONE_CONGESTED is only cleared in kswapd.
> > > 
> > 
> > What problem does this solve?
> > 
> > As it is, for high order allocations it takes the following steps
> > 
> > If reclaiming at high order {
> > 	for each zone {
> > 		if all_unreclaimable
> > 			skip
> > 		if watermark is not met
> > 			order = 0
> > 			loop again
> > 		
> > 		/* watermark is met */
> > 		clear congested
> > 	}
> > }
> > 
> > If high orders are failing, kswapd balances for order-0 where there
> > is already a cleaning of ZONE_CONGESTED if the zone was shrunk and
> > became balanced. I see the case for hunk 1 of the patch because now
> > it'll clear ZONE_CONGESTED for zones that are already balanced which
> > might have a noticable effect on wait_iff_congested. Is this what
> > you see? Even if it is, it does not explain hunk 2 of the patch.
> I first looked at the hunk 2 place and thought we don't clear
> ZONE_CONGESTED there. I then figured out we need do the same thing for
> the hunk 1. But you are correct, with hunk 1, hunk 2 isn't required.
> updated patch.
> 
> 
> 
> correctly clear ZONE_CONGESTED. If a zone watermark is ok, we
> should clear ZONE_CONGESTED because pages can be reclaimed in
> other tasks but ZONE_CONGESTED is only cleared in kswapd.
> 
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

Even it will fix that when kswapd wakes up lately by order-0 and look at zones,
all zones would become okay so it jumps out with "if (i < 0) goto out" with missing
clearing ZONE_CONGESTED.

-- 
Kind regards,
Minchan Kim

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