linux-kernel - Re: Over-eager swapping

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Message-ID: <20100804114933.GA13527@localhost>
Date:	Wed, 4 Aug 2010 19:49:33 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Chris Webb <chris@...chsys.com>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: Over-eager swapping

On Wed, Aug 04, 2010 at 05:58:12PM +0800, Chris Webb wrote:
> Wu Fengguang <fengguang.wu@...el.com> writes:
> 
> > This is interesting. Why is it waiting for 1m here? Are there high CPU
> > loads? Would you do a
> > 
> >         echo t > /proc/sysrq-trigger
> > 
> > and show us the dmesg?
> 
> Annoyingly, magic-sysrq isn't compiled in on these kernels. Is there another
> way I can get this info for you? Replacing the kernels on the machines is a
> painful job as I have to give the clients running on them quite a bit of
> notice of the reboot, and I haven't been able to reproduce the problem on a
> test machine.

Maybe turn off KSM? It helps to isolate problems. It's a relative new
and complex feature after all.

> I also think the swap use is much better following a reboot, and only starts
> to spiral out of control after the machines have been running for a week or
> so.

Something deteriorates over long time.. It may take time to catch this bug..

> However, your suggestion is right that the CPU loads on these machines are
> typically quite high. The large number of kvm virtual machines they run mean
> thatl oads of eight or even sixteen in /proc/loadavg are not unusual, and
> these are higher when there's swap than after it has been removed. I assume
> this is mostly because of increased IO wait, as this number increases
> significantly in top.

iowait = CPU (idle) waiting for disk IO

So iowait means not CPU load, but somehow disk load :)

Thanks,
Fengguang
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