Re: Tuning shared_buffers with ipcs ?

Doug Y <dylists(at)ptd(dot)net> writes:
> Tom Lane wrote:
>> I have not seen any such claim, and I do not see any way offhand that
>> ipcs could help.
>>
> Directly from:
> http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html
> "As a rule of thumb, observe shared memory usage of PostgreSQL with
> tools like ipcs and determine the setting."

[ shrug ... ] So ask elein why she thinks that will help.

>> This might tell you something about how many concurrent backends you've
>> used, but nothing about how many shared buffers you need.
>>
> Thats strange, I know I've had more than 4 concurrent connections on
> that box... (I just checked and there were at least a dozen).

There is more than one per-backend semaphore per semaphore set, 16 per
set if memory serves; so the ipcs evidence points to a maximum of
between 49 and 64 concurrently active backends. It's not telling you a
darn thing about appropriate shared_buffers settings, however.

> A mirror DB with the same config also has the same basic output from
> ipcs, except that it has times for 11 of the 17 arrays slots and most
> of them are the time when we do our backup dump (which makes sense
> that it would require more memory at that time.)

That doesn't follow either. I think you may have some bottleneck that
causes client requests to pile up during a backup dump.

regards, tom lane