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
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