linux-kernel - Re: [RFC] respect the referenced bit of KVM guest pages?

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Message-Id: <20090807172425.8554da30.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 7 Aug 2009 17:24:25 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, Avi Kivity <avi@...hat.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Dike, Jeffrey G" <jeffrey.g.dike@...el.com>,
	"Yu, Wilfred" <wilfred.yu@...el.com>,
	"Kleen, Andi" <andi.kleen@...el.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC] respect the referenced bit of KVM guest pages?

On Fri, 7 Aug 2009 13:24:34 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> On Fri, Aug 7, 2009 at 8:41 AM, KOSAKI
> Motohiro<kosaki.motohiro@...fujitsu.com> wrote:
> > Current memcgroup logic also use recent_scan/recent_rotate statistics.
> > Isn't it enought?
> 
> I don't understand the context, I'll look at the problem when I am
> back (I am away from work for the next few days).
> 
Brief summary: (please point out if not correct)

prepare a memcg with
	memory.limit_in_bytex=128MB

Run kvm on it, and use apps, those working-set is near to 256MB (then, heavy swap)
In this case,
  - Anon memory are swapped-out even while there are file caches.
    Especially, a page for stack which is frequently accessed can be
    easily swapped out, again and again.

One of reasone is a recent change as:
"a page mapped with VM_EXEC is not pageout even if no reference"

Without memcg, a user can use Gigabytes of memory, above change
is very welcomed.

Then, current point is "how we can handle this case without bad effect".

One possibility I wonder is this is a problem of configuration mistake.
setting memory.memsw.limit_in_bytes to be proper value may change bahavior.
But it seems just a workaround.

Can't we find algorithmic/heuristic way to avoid too much swap-out ?
I think memcg can check # of swap-ins, but now, we don't have a tag
to see the sign of "recently swapped-out page is reused" case or
executable file pages are too much.

I wonder we can comapre
    # of pageouted file-caches v.s. # of swapout anon.
and keeping  "# of pageouted file-caches < # of swapout anon." (or use swappiness)
This state can be checked by recalim_stat. (per memcg)
Hmm?

I'm sorry I'll be on a trip Aug/11-Aug/17 and response will be delayed.

Thanks,
-Kame

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