Date:	Wed, 21 Nov 2012 17:02:04 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Jaegeuk Hanse <jaegeuk.hanse@...il.com>
Cc:	metin d <metdos@...oo.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 Wed, Nov 21, 2012 at 04:34:40PM +0800, Jaegeuk Hanse wrote:
> Cc Fengguang Wu.
> 
> On 11/21/2012 04:13 PM, metin d wrote:
> >>   Curious. Added linux-mm list to CC to catch more attention. If you run
> >>echo 1 >/proc/sys/vm/drop_caches does it evict data-1 pages from memory?
> >I'm guessing it'd evict the entries, but am wondering if we could run any more diagnostics before trying this.
> >
> >We regularly use a setup where we have two databases; one gets used frequently and the other one about once a month. It seems like the memory manager keeps unused pages in memory at the expense of frequently used database's performance.

> >My understanding was that under memory pressure from heavily
> >accessed pages, unused pages would eventually get evicted. Is there
> >anything else we can try on this host to understand why this is
> >happening?

We may debug it this way.

1) run 'fadvise data-2 0 0 dontneed' to drop data-2 cached pages
   (please double check via /proc/vmstat whether it does the expected work)

2) run 'page-types -r' with root, to view the page status for the
   remaining pages of data-1

The fadvise tool comes from Andrew Morton's ext3-tools. (source code attached)
Please compile them with options "-Dlinux -I. -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE64_SOURCE"

page-types can be found in the kernel source tree tools/vm/page-types.c

Sorry that sounds a bit twisted.. I do have a patch to directly dump
page cache status of a user specified file, however it's not
upstreamed yet.

Thanks,
Fengguang

> >On Tue 20-11-12 09:42:42, metin d wrote:
> >>I have two PostgreSQL databases named data-1 and data-2 that sit on the
> >>same machine. Both databases keep 40 GB of data, and the total memory
> >>available on the machine is 68GB.
> >>
> >>I started data-1 and data-2, and ran several queries to go over all their
> >>data. Then, I shut down data-1 and kept issuing queries against data-2.
> >>For some reason, the OS still holds on to large parts of data-1's pages
> >>in its page cache, and reserves about 35 GB of RAM to data-2's files. As
> >>a result, my queries on data-2 keep hitting disk.
> >>
> >>I'm checking page cache usage with fincore. When I run a table scan query
> >>against data-2, I see that data-2's pages get evicted and put back into
> >>the cache in a round-robin manner. Nothing happens to data-1's pages,
> >>although they haven't been touched for days.
> >>
> >>Does anybody know why data-1's pages aren't evicted from the page cache?
> >>I'm open to all kind of suggestions you think it might relate to problem.
> >   Curious. Added linux-mm list to CC to catch more attention. If you run
> >echo 1 >/proc/sys/vm/drop_caches
> >   does it evict data-1 pages from memory?
> >
> >>This is an EC2 m2.4xlarge instance on Amazon with 68 GB of RAM and no
> >>swap space. The kernel version is:
> >>
> >>$ uname -r
> >>3.2.28-45.62.amzn1.x86_64
> >>Edit:
> >>
> >>and it seems that I use one NUMA instance, if  you think that it can a problem.
> >>
> >>$ numactl --hardware
> >>available: 1 nodes (0)
> >>node 0 cpus: 0 1 2 3 4 5 6 7
> >>node 0 size: 70007 MB
> >>node 0 free: 360 MB
> >>node distances:
> >>node   0
> >>    0:  10

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