Re: Problem in Page Cache Replacement

Johannes Weiner
2012-11-22T23:03:55Z

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.