linux-kernel - Re: [PATCH 07/27] mm, vmscan: Have kswapd only scan based on the highest requested zone

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Message-ID: <20160225221746.GA12258@cmpxchg.org>
Date:	Thu, 25 Feb 2016 17:17:46 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...riel.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 07/27] mm, vmscan: Have kswapd only scan based on the
 highest requested zone

On Tue, Feb 23, 2016 at 03:04:30PM +0000, 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.
> Ideally this is completely unnecessary when reclaiming on a per-node basis.
> In theory, there may still be anomalies when all requests are for lower
> zones and very old pages are preserved in higher zones but this should be
> the exceptional case.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Acked-by: Johannes Weiner <hannes@...xchg.org>

As I mentioned in the other subthread, this should probably be fine,
although it would be good, generally, to have some sort of statistics
on how much of the age-distorting reclaim is happening, and then maybe
mention its existence in the changelog of this patch. Just an idea.