linux-kernel - Re: [PATCH 6/9] kstaled: rate limit pages scanned per second.

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Message-Id: <20110928175947.d3af52f0.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 28 Sep 2011 17:59:47 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Balbir Singh <bsingharora@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <jweiner@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Michael Wolf <mjwolf@...ibm.com>
Subject: Re: [PATCH 6/9] kstaled: rate limit pages scanned per second.

On Wed, 28 Sep 2011 01:19:50 -0700
Michel Lespinasse <walken@...gle.com> wrote:

> On Wed, Sep 28, 2011 at 1:13 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
> > On Tue, 27 Sep 2011 17:49:04 -0700
> > Michel Lespinasse <walken@...gle.com> wrote:
> >
> >> Scan some number of pages from each node every second, instead of trying to
> >> scan the entime memory at once and being idle for the rest of the configured
> >> interval.
> >>
> >> In addition to spreading the CPU usage over the entire scanning interval,
> >> this also reduces the jitter between two consecutive scans of the same page.
> >>
> >>
> >> Signed-off-by: Michel Lespinasse <walken@...gle.com>
> >
> > Does this scan thread need to be signle thread ?
> 
> It tends to perform worse if we try making it multithreaded. What
> happens is that the scanning threads call page_referenced() a lot, and
> if they both try scanning pages that belong to the same file that
> causes the mapping's i_mmap_mutex lock to bounce. Same things happens
> if they try scanning pages that belong to the same anon VMA too.
> 

Hmm. with brief thinking, if you can scan list of page tables,
you can set young flags without any locks. 
For inode pages, you can hook page lookup, I think.

You only need to clear Young flag by scanning [pfn, end_pfn].
Then, multi-threaded. ?


Thanks,
-Kame



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