linux-kernel - Re: [PATCH 05/27] mm, vmscan: Have kswapd only scan based on the highest requested zone

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Message-ID: <20160616074413.GE1868@techsingularity.net>
Date:	Thu, 16 Jun 2016 08:44:13 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...riel.com>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/27] mm, vmscan: Have kswapd only scan based on the
 highest requested zone

On Wed, Jun 15, 2016 at 03:13:13PM +0200, Vlastimil Babka wrote:
> On 06/09/2016 08:04 PM, Mel Gorman wrote:
> >kswapd checks all eligible zones to see if they need balancing even if it was
> >woken for a lower zone. This made sense when we reclaimed on a per-zone basis
> >because we wanted to shrink zones fairly so avoid age-inversion problems.
> 
> Now we reclaim a single lru, but still will skip over pages from the higher
> zones than reclaim_idx, so this is not much different from per-zone basis
> wrt age-inversion?
> 

Yes, but it only applies in the case where the allocation request is zone
restricted. Previously, even with fair zone allocation policy, we had
problems with a high zone with recently allocated pages being reclaimed
simply because the low watermark was reached. Think of bugs in the past
where the normal zone was a small percentage of memory.

-- 
Mel Gorman
SUSE Labs