linux-kernel - Re: Problem in Page Cache Replacement

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Date:	Thu, 22 Nov 2012 23:53:18 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Metin Döşlü <metindoslu@...il.com>
Cc:	Jaegeuk Hanse <jaegeuk.hanse@...il.com>, Jan Kara <jack@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: Problem in Page Cache Replacement

On Thu, Nov 22, 2012 at 11:41:07PM +0800, Fengguang Wu wrote:
> On Wed, Nov 21, 2012 at 12:07:22PM +0200, Metin Döşlü wrote:
> > On Wed, Nov 21, 2012 at 12:00 PM, Jaegeuk Hanse <jaegeuk.hanse@...il.com> wrote:
> > >
> > > On 11/21/2012 05:58 PM, metin d wrote:
> > >
> > > Hi Fengguang,
> > >
> > > I run tests and attached the results. The line below I guess shows the data-1 page caches.
> > >
> > > 0x000000080000006c       6584051    25718  __RU_lA___________________P________    referenced,uptodate,lru,active,private
> > >
> > >
> > > I thinks this is just one state of page cache pages.
> > 
> > But why these page caches are in this state as opposed to other page
> > caches. From the results I conclude that:
> > 
> > data-1 pages are in state : referenced,uptodate,lru,active,private
> 
> I wonder if it's this code that stops data-1 pages from being
> reclaimed:
> 
> shrink_page_list():
> 
>                 if (page_has_private(page)) {
>                         if (!try_to_release_page(page, sc->gfp_mask))
>                                 goto activate_locked;
> 
> What's the filesystem used?

Ah it's more likely caused by this logic:

        if (is_active_lru(lru)) {
                if (inactive_list_is_low(mz, file))
                        shrink_active_list(nr_to_scan, mz, sc, priority, file);

The active file list won't be scanned at all if it's smaller than the
active list. In this case, it's inactive=33586MB > active=25719MB. So
the data-1 pages in the active list will never be scanned and reclaimed.

> > data-2 pages are in state : referenced,uptodate,lru,mappedtodisk
> 
> Thanks,
> Fengguang
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