linux-kernel - Re: [PATCH] mm: check zone->all_unreclaimable in all

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Date:	Wed, 9 Mar 2011 14:37:04 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrew Vagin <avagin@...il.com>,
	Andrey Vagin <avagin@...nvz.org>, Mel Gorman <mel@....ul.ie>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: check zone->all_unreclaimable in
 all_unreclaimable()

On Tue, 8 Mar 2011 08:45:51 +0900
Minchan Kim <minchan.kim@...il.com> wrote:

> On Tue, Mar 8, 2011 at 6:58 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> > On Sun, 6 Mar 2011 02:07:59 +0900
> > Minchan Kim <minchan.kim@...il.com> wrote:
> > Any alternative proposals?  We should get the livelock fixed if possible..
> >
> 
> And we should avoid unnecessary OOM kill if possible.
> 
> I think the problem is caused by (zone->pages_scanned <
> zone_reclaimable_pages(zone) * 6). I am not sure (* 6) is a best. It
> would be rather big on recent big DRAM machines.
> 

It means 3 times full-scan from the highest priority to the lowest
and cannot freed any pages. I think big memory machine tend to have
more cpus, so don't think it's big.

> I think it is a trade-off between latency and OOM kill.
> If we decrease the magic value, maybe we should prevent the almost
> livelock but happens unnecessary OOM kill.
> 

Hmm, should I support a sacrifice feature 'some signal(SIGINT?) will be sent by
the kernel when it detects system memory is in short' in cgroup ?
(For example, if full LRU scan is done in a zone, notifier
 works and SIGINT will be sent.)

> And I think zone_reclaimable not fair.
> For example, too many scanning makes reclaimable state to
> unreclaimable state. Maybe it takes a very long time. But just some
> page free makes unreclaimable state to reclaimabe with very easy. So
> we need much painful reclaiming for changing reclaimable state with
> unreclaimabe state. it would affect latency very much.
> 
> Maybe we need more smart zone_reclaimabe which is adaptive with memory pressure.
> 
I agree.

Thanks,
-Kame

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