Subject: Re: Setting "nice" values
From: Scott Marlowe <smarlowe@g2switchworks.com>
To: Madison Kelly <linux@alteeve.com>
Cc: pgsql-performance@postgresql.org
In-Reply-To: <454A0DD3.2050104@alteeve.com>
References: <454A0B69.9070000@alteeve.com>
	<1162480836.28047.20.camel@state.g2switchworks.com>
	<454A0DD3.2050104@alteeve.com>
Content-Type: text/plain
Content-Transfer-Encoding: 7bit
Message-Id: <1162482071.28047.39.camel@state.g2switchworks.com>
Mime-Version: 1.0
Date: Thu, 02 Nov 2006 09:41:11 -0600

On Thu, 2006-11-02 at 09:25, Madison Kelly wrote:
> Scott Marlowe wrote:
> > On Thu, 2006-11-02 at 09:14, Madison Kelly wrote:
> >> Hi all,
> >>
> >>    I've got a script (perl, in case it matters) that I need to run once
> >> a month to prepare statements. This script queries and updates the
> >> database a *lot*. I am not concerned with the performance of the SQL
> >> calls so much as I am about the impact it has on the server's load.
> >>
> >>    Is there a way to limit queries speed (ie: set a low 'nice' value on
> >> a query)? This might be an odd question, or I could be asking the
> >> question the wrong way, but hopefully you the idea. :)
> > 
> > While you can safely set the priority lower on the calling perl script,
> > setting db backend priorities lower can result in problems caused by
> > "priority inversion"  Look up that phrase on the pgsql admin, perform,
> > general, or hackers lists for an explanation, or go here:
> > 
> > http://en.wikipedia.org/wiki/Priority_inversion
> > 
> > I have a simple script that grabs raw data from an oracle db and shoves
> > it into a postgresql database for reporting purposes.  Every 100 rows I
> > put into postgresql, I usleep 10 or so and the load caused by that
> > script on both systems is minimal.  You might try something like that.
> 
> Will the priority of the script pass down to the pgsql queries it calls? 
> I figured (likely incorrectly) that because the queries were executed by 
> the psql server the queries ran with the server's priority. If this 
> isn't the case, then perfect. :)

nope, the priorities don't pass down.  you connect via a client lib to
the server, which spawns a backend process that does the work for you. 
The backend process inherits its priority from the postmaster that
spawns it, and they all run at the same priority.

> Thanks for the tip, too, it's something I will try.

Sometimes it's the simple solutions that work best.  :)  Welcome to the
world of pgsql, btw...