----- Original Message -----
From: Jaegeuk Hanse
<jaegeuk.hanse@xxxxxxxxx>
To: Fengguang Wu
<fengguang.wu@xxxxxxxxx>
Cc: metin d
<metdos@xxxxxxxxx>; Jan Kara
<jack@xxxxxxx>;
"linux-kernel@xxxxxxxxxxxxxxx"
<linux-kernel@xxxxxxxxxxxxxxx>;
"linux-mm@xxxxxxxxx"
<linux-mm@xxxxxxxxx>
Sent: Wednesday, November 21, 2012 11:42 AM
Subject: Re: Problem in Page Cache Replacement
On 11/21/2012 05:02 PM, Fengguang Wu wrote:
> 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.
Hi Fengguang,
Thanks for you detail steps, I think metin can have a try.
flags page-count MB symbolic-flags
long-symbolic-flags
0x0000000000000000 607699 2373
___________________________________
0x0000000100000000 343227 1340
_______________________r___________ reserved
But I have some questions of the print of page-type:
Is 2373MB here mean total memory in used include page cache?
I don't
think so.
Which kind of pages will be marked reserved?
Which line of long-symbolic-flags is for page cache?
Regards,
Jaegeuk
>
> 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