Re: Problem in Page Cache Replacement

[Jaegeuk Hanse <jaegeuk.hanse@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 3920 bytes --]

On 11/21/2012 05:39 PM, Jaegeuk Hanse wrote:
> 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 page-type

Hi Fengguang,

Could you explain confusion mentioned above? thanks in advance.

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
>


[-- Attachment #2: Type: text/html, Size: 5654 bytes --]