Re: High CPU Utilization

On Mar 17, 2009, at 12:19 AM, Greg Smith wrote:

> On Tue, 17 Mar 2009, Gregory Stark wrote:
>
>> Hm, well the tests I ran for posix_fadvise were actually on a Perc5
>> -- though
>> who knows if it was the same under the hood -- and I saw better
>> performance
>> than this. I saw about 4MB/s for a single drive and up to about
>> 35MB/s for 15
>> drives. However this was using linux md raid-0, not hardware raid.
>
> Right, it's the hardware RAID on the Perc5 I think people mainly
> complain about. If you use it in JBOD mode and let the higher
> performance CPU in your main system drive the RAID functions it's
> not so bad.
>
> --
> * Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com
> Baltimore, MD

I have not yet had a chance to try software raid on the standby server
(still planning to) but wanted to follow up to see if there was any
good way to figure out what the postgresql processes are spending
their CPU time on.

We are under peak load right now, and I have Zabbix plotting CPU
utilization and CPU wait (from vmstat output) along with all sorts of
other vitals on charts. CPU utilization is a sustained 90% - 95% and
CPU Wait is hanging below 10%. Since being pointed at vmstat by this
list I have been watching CPU Wait and it does get high at times
(hence still wanting to try Perc5 in JBOD) but then there are
sustained periods, right now included, where our CPUs are just getting
crushed while wait and IO (only doing about 1.5 MB/sec right now) are
very low.

This high CPU utilization only occurs when under peak load and when
our JDBC pools are fully loaded. We are moving more things into our
cache and constantly tuning indexes/tables but just want to see if
there is some underlying cause that is killing us.

Any recommendations for figuring out what our database is spending its
CPU time on?