Re: Problem in Page Cache Replacement

metin d
2012-11-23T08:08:46Z

----- Original Message -----

From: Jaegeuk Hanse <jaegeuk.hanse@gmail.com>
To: metin d <metdos@yahoo.com>
Cc: Jan Kara <jack@suse.cz>; "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>; linux-mm@kvack.org
Sent: Friday, November 23, 2012 3:58 AM
Subject: Re: Problem in Page Cache Replacement

On 11/21/2012 02:25 AM, Jan Kara wrote:
> 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.

> Hi metin d,

> fincore is a tool or ...? How could I get it?

> Regards,
> Jaegeuk

Hi Jaegeuk,

Yes, it is a tool, you get it from here :
http://code.google.com/p/linux-ftools/

Regards,
Metin
>>
>> 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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