linux-kernel - Re: [PATCH 06/34] mm, vmscan: have kswapd only scan based on the highest requested zone

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Date:	Wed, 13 Jul 2016 09:37:26 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...riel.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Minchan Kim <minchan@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/34] mm, vmscan: have kswapd only scan based on the
 highest requested zone

On Tue, Jul 12, 2016 at 10:05:04AM -0400, Johannes Weiner wrote:
> On Fri, Jul 08, 2016 at 10:34:42AM +0100, 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: Vlastimil Babka <vbabka@...e.cz>
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> 
> I wasn't quite sure at first what the rationale is for this patch,
> since it probably won't make much difference in pratice.

Possibly not, it depends on how much embedded 32-bit platforms use features
like zswap. What I wanted to avoid was a lowmem allocation for zswap
excessively reclaiming highmem putting even further pressure on zswap if
the pages are anonymous.

> But I do
> agree that the code is cleaner to have kswapd check exactly what it
> was asked to check, rather than some do-the-"right"-thing magic.
> 

But this is a justification on its own. I encountered an astonishing number
of magic number handling that just happened to mostly work. I wanted to
iron them out as much as possible.

> A hypothetical onslaught of low-zone allocations will wreak havoc to
> the page age in higher zones anyway, right? So I don't think that case
> matters all that much.

Possibly not, but it was straight-forward to mitigate the damage without
too many side-effects.

-- 
Mel Gorman
SUSE Labs