Re: Problem in Page Cache Replacement

Jaegeuk Hanse
2012-11-23T01:32:14Z

On 11/22/2012 11:26 PM, Fengguang Wu wrote:
> Hi Jaegeuk,
>
> Sorry for the delay. I'm traveling these days..
>
> On Wed, Nov 21, 2012 at 05:42:33PM +0800, 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
>
> We don't need to care about the above two pages states actually.
> Page cache pages will never be in the special reserved or
> all-flags-cleared state.

Hi Fengguang,

Thanks for your response. But which kind of pages are in the special
reserved and which are all-flags-cleared?

Regards,
Jaegeuk

>
>> 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?
> The (lru && !anonymous) pages are page cache pages.
>
> 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