Re: Setting shared buffers

Carlos,

> After checking some docs on performance tuning, I'm trying to
> follow Bruce Momjian (sp??) advice to set the shared_buffers
> at 25% of the amount of physical memory (1GB in our server)
> and 4% for the sort_mem.

I tend to set my shared_buffers somewhat higher, but that's a good
place to start. Be cautious about sort_mem on a server with a lot of
users; sort_mem is not shared, so make sure that you have enough that
your server could handle 1-2 sorts per concurrent user without running
out of RAM.

> When I try that, I get an error message when starting postgres,
> complaining that the amount of shared memory requested exceeds
> the maximum allowed by the kernel (they talk about increasing
> the kernel parameter SHMMAX -- does this mean that I have to
> recompile the kernel? Or is it just a "runtime" configuration
> parameter that I set and on the next reboot will be taken?)

It's easy, on Linux don't even have to reboot. Other OS's are harder.
See this very helpful page:
http://www.us.postgresql.org/users-lounge/docs/7.2/postgres/kernel-resources.html#SYSVIPC

In fact, I tend to up my SHMMAX and SHMMALL and shared_buffers at night
on some databases, when they are doing automatic updates, and adjust
them back down during the day, when I want to prevent heavy user loads
from using up all system RAM.

> I have 1GB, so I want 256MB as shared buffers memory; each
> shared buffer is 8kbytes, so I take 256M / 8k, which is 32k --
> so, I uncomment the line shared_buffers in the configuration
> file, and put:

See the calculations on the page link above. They are more specific
than that, and I have found the numbers there to be good estimates,
maybe only 10-20% high.

-Josh Berkus