On Thu, Mar 22, 2012 at 10:57 AM, Scott Marlowe <scott.marlowe@xxxxxxxxx> wrote: > On Thu, Mar 22, 2012 at 9:29 AM, Merlin Moncure <mmoncure@xxxxxxxxx> wrote: >> On Thu, Mar 22, 2012 at 10:02 AM, Scott Marlowe <scott.marlowe@xxxxxxxxx> wrote: >>> There's other issues you run into with large shared_buffers as well. >>> If you've got a large shared_buffers setting, but only regularly hit a >>> small subset of your db (say 32GB shared_buffers but only hit 4G or so >>> regularly in your app) then it's quite possible that older >>> shared_buffer segments will get swapped out because they're not being >>> used. Then, when the db goes to hit a page in shared_buffers, the OS >>> will have to swap it back in. What was supposed to make your db much >>> faster has now made it much slower. >>> >>> With Linux, the OS tends to swap out unused memory to make room for >>> file buffers. While you can change the swappiness settings to 0 to >>> slow it down, the OS will eventually swap out the least used segments >>> anyway. The only solution on large memory servers is often to just >>> turn off swap. >> >> Right -- but my take on that is that hacking the o/s to disable swap >> is dealing with symptoms of problem related to server >> misconfiguration. > > You can configure a big memory linux server anyway you want. After a > while, they seem to go crazy anyway and start swapping even when > you've told them not to. > >> In particular it probably means shared_buffers is set too high...the >> o/s thinks it needs that memory more than you do and it may very well >> be right. > > I've had machines with 128GB RAM and a 4G shared_buffers start > swapping for no apparent reason and just fall over. There's no memory > pressure etc, just kswapd decides to go nuts and start swapping. > > This was on Ubuntu 10.04 and 12.04 and RHEL 5.2 through 5.latest with > all updates. These machines typically had ~90GB+ of kernel cache and > zero memory pressure. hm, that's interesting -- noted. I'll keep an eye out for that. merlin -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
