linux-kernel - Re: [PATCH] mm: wait for congestion to clear on all zones

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Date:	Sat, 12 Jan 2013 18:46:44 -0600
From:	Simon Jeons <simon.jeons@...il.com>
To:	Zlatko Calusic <zlatko.calusic@...on.hr>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: wait for congestion to clear on all zones

On Fri, 2013-01-11 at 12:25 +0100, Zlatko Calusic wrote:
> On 11.01.2013 02:25, Simon Jeons wrote:
> > On Wed, 2013-01-09 at 22:41 +0100, Zlatko Calusic wrote:
> >> From: Zlatko Calusic <zlatko.calusic@...on.hr>
> >>
> >> Currently we take a short nap (HZ/10) and wait for congestion to clear
> >> before taking another pass with lower priority in balance_pgdat(). But
> >> we do that only for the highest zone that we encounter is unbalanced
> >> and congested.
> >>
> >> This patch changes that to wait on all congested zones in a single
> >> pass in the hope that it will save us some scanning that way. Also we
> >> take a nap as soon as congested zone is encountered and sc.priority <
> >> DEF_PRIORITY - 2 (aka kswapd in trouble).
> > 
> > But you still didn't explain what's the problem you meat and what
> > scenario can get benefit from your change.
> > 
> 
> I did in my reply to Andrew. Here's the relevant part:
> 
> > I have an observation that without it, under some circumstances that 
> > are VERY HARD to repeat (many days need to pass and some stars to align
> > to see the effect), the page cache gets hit hard, 2/3 of it evicted in
> > a split second. And it's not even under high load! So, I'm still
> > monitoring it, but so far the memory utilization really seems better
> > with the patch applied (no more mysterious page cache shootdowns). 
> 
> The scenario that should get benefit is everyday. I observed problems during
> light but constant reading from disk (< 10MB/s). And sending that data
> over the network at the same time. Think backup that compresses data on the
> fly before pushing it over the network (so it's not very fast).
> 
> The trouble is that you can't just fix up a quick benchmark and measure the
> impact, because many days need to pass for the bug to show up in all it's beauty.
> 
> Is there anybody out there who'd like to comment on the patch logic? I.e. do
> you think that waiting on every congested zone is the more correct solution
> than waiting on only one (only the highest one, and ignoring the fact that
> there may be other even more congested zones)?

What's the benefit of waiting on every congested zone than waiting on
only one against your scenario?

> 
> Regards,

-- 
Simon Jeons <simon.jeons@...il.com>

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