Re: Problem in Page Cache Replacement

Jaegeuk Hanse
2012-11-23T02:14:14Z

On 11/23/2012 12:17 AM, Johannes Weiner wrote:
> On Thu, Nov 22, 2012 at 09:16:27PM +0800, Jaegeuk Hanse wrote:
>> 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?
> I remember a time stamp of evicted file pages in the page cache radix
> tree that let me reconstruct the inter-reference distance even after a
> page has been evicted from cache when it's faulted back in. This way
> I can tell a one-time sequence from thrashing, no matter how small the
> inactive list.
>
> When thrashing is detected, I start deactivating protected pages and
> put them next to the refaulted cache on the head of the inactive list
> and let them fight it out as usual. In this reported case, the old
> data will be challenged and since it's no longer used, it will just
> drop off the inactive list eventually. If the guess is wrong and the
> deactivated memory is used more heavily than the refaulting pages,
> they will just get activated again without incurring any disruption
> like a major fault.

Hi Johannes,

If you also add the time stamp to the protected pages which you deactive
when incur thrashing?

Regards,
Jaegeuk