linux-kernel - Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for when zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA

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Message-Id: <20090609184422.DD8B.A69D9226@jp.fujitsu.com>
Date:	Tue,  9 Jun 2009 18:45:29 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	kosaki.motohiro@...fujitsu.com, Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	yanmin.zhang@...el.com, Wu Fengguang <fengguang.wu@...el.com>,
	linuxram@...ibm.com, linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for when zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA

> > > Here it is just recording the jiffies value. The real smarts with the counter
> > > use time_before() which I assumed could handle jiffie wrap-arounds. Even
> > > if it doesn't, the consequence is that one scan will occur that could have
> > > been avoided around the time of the jiffie wraparound. The value will then
> > > be reset and it will be fine.
> > 
> > time_before() assume two argument are enough nearly time.
> > if we use 32bit cpu and HZ=1000, about jiffies wraparound about one month.
> > 
> > Then, 
> > 
> > 1. zone reclaim failure occur
> > 2. system works fine for one month
> > 3. jiffies wrap and time_before() makes mis-calculation.
> > 
> 
> And the scan occurs uselessly and zone_reclaim_failure gets set again.
> I believe the one useless scan is not significant enough to warrent dealing
> with jiffie wraparound.

Thank you for kindful explanation.
I fully agreed.



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