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Re: Tuning shared_buffers with ipcs ?

From: Doug Y <dylists(at)ptd(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Tuning shared_buffers with ipcs ?
Date: 2004-10-15 18:51:33
Message-ID: 41701C35.4060808@ptd.net (view raw or flat)
Thread:
Lists: pgsql-performance
Tom Lane wrote:

>Doug Y <dylists(at)ptd(dot)net> writes:
>  
>
>>I've seen a couple references to using ipcs to help properly size 
>>shared_buffers.
>>    
>>
>
>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."

I've seen references in the admin

>>I tried all of the dash commands in the ipcs man page, and the only one 
>>that might give a clue is ipcs -t which shows the time the semaphores 
>>were last used. If you look at the example I give below, it appears as 
>>if I'm only using 4 of the 17 semaphores (PG was started on Oct 8).
>>    
>>
>
>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). 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.)

>			regards, tom lane
>
>
>  
>
I'm not saying you're wrong, because I don't know how the nitty gritty 
stuff works, I'm just trying to find something to work with, since 
presently there isn't anything other than anecdotal evidence. From what 
I've inferred, there seems to be some circumstantial evidence supporting 
my theory.

Thanks.

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