Re: Problem in Page Cache Replacement

Johannes Weiner
2012-11-22T23:03:52Z

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.