Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1441378 > unrolled thread

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

Back to article view | Back to linux.kernel

This discussion starts older than the indexed window; earlier articles aren't shown. The article labeled Started by below is the oldest one visible, not the original post.

Contents

  Re: [PATCH 06/34] mm, vmscan: have kswapd only scan based on the  highest requested zone Johannes Weiner <hannes@cmpxchg.org> - 2016-07-12 16:10 +0200
    Re: [PATCH 06/34] mm, vmscan: have kswapd only scan based on the  highest requested zone Mel Gorman <mgorman@techsingularity.net> - 2016-07-13 10:40 +0200

#1441378 — Re: [PATCH 06/34] mm, vmscan: have kswapd only scan based on the highest requested zone

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@techsingularity.net>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>

Acked-by: Johannes Weiner <hannes@cmpxchg.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. 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.

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.

[toc] | [next] | [standalone]

#1442176

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@techsingularity.net>
> > Acked-by: Vlastimil Babka <vbabka@suse.cz>
> 
> Acked-by: Johannes Weiner <hannes@cmpxchg.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

[toc] | [prev] | [standalone]

Back to top | Article view | linux.kernel

csiph-web