Re: Problem in Page Cache Replacement

Jan Kara
2012-11-20T18:25:22Z

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

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR