Re: Problem in Page Cache Replacement

Jaegeuk Hanse
2012-11-22T20:41:04Z

On 11/22/2012 09:09 AM, Johannes Weiner wrote:
> On Thu, Nov 22, 2012 at 08:48:07AM +0800, Jaegeuk Hanse wrote:
>> On 11/22/2012 05:34 AM, Johannes Weiner wrote:
>>> Hi,
>>>
>>> On Tue, Nov 20, 2012 at 07:25:00PM +0100, 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.
>>>>>
>>>>> 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.
>>> This might be because we do not deactive pages as long as there is
>>> cache on the inactive list. I'm guessing that the inter-reference
>>> distance of data-2 is bigger than half of memory, so it's never
>>> getting activated and data-1 is never challenged.
>> Hi Johannes,
>>
>> What's the meaning of "inter-reference distance"
> It's the number of memory accesses between two accesses to the same
> page:
>
> A B C D A B C E ...
> |_______|
> | |
>
>> and why compare it with half of memoy, what's the trick?
> If B gets accessed twice, it gets activated. If it gets evicted in
> between, the second access will be a fresh page fault and B will not
> be recognized as frequently used.
>
> Our cutoff for scanning the active list is cache size / 2 right now
> (inactive_file_is_low), leaving 50% of memory to the inactive list.
> If the inter-reference distance for pages on the inactive list is
> bigger than that, they get evicted before their second access.

Hi Johannes,

Thanks for your explanation. But could you give a short description of
how you resolve this inactive list thrashing issues?

Regards,
Jaegeuk