Re: Controlling CPU Usage in PostgreSQL

On Tue, 2006-08-01 at 14:22 -0500, Scott Marlowe wrote:
> On Tue, 2006-08-01 at 13:59, Aaron Bono wrote:
> > On 7/31/06, Thomas Pundt <mlists(at)rp-online(dot)de> wrote:
> > On Monday 31 July 2006 10:06, Aaron Bono wrote:
> > | Is there a way I can tell PostgreSQL to give session a low
> > priority so even
> > | if it does take the full CPU, it only does so if it would
> > otherwise be
> > | idle?
> >
> > the "nice" command might do what you want...
> >
> >
> > OK, so I tried:
> >
> > su - postgres -c "nice -n 19 psql my_db"
> >
> > The problem is, the psql command has a nice value of 19 but the
> > PostgreSQL server process that psql has connected to is running with a
> > nice value of 0. My assumption is that, if I then run my functions,
> > psql will get low priority but it is the server process that is
> > running with normal priority that will still use up all the CPU.
> >
> > Bottom line, I am skeptical if this will really achieve my goal - to
> > have the functions run with low priority.
> >
> > Is there a way to tell PostgreSQL to change the nice value of a
> > particular connection? I guess I could use renice but that means a
> > lot of shell scripting to determine what PID to change - I am not a
> > shell script expert and would rather not pursue that option.
>
> This is one of those things that only seems like a good idea at the
> time.
>
> While nice-ing the whole of a postgresql server is doable, nice-ing
> individual connections is no advisable.
>
> You can reach a bad state caused by "priority inversion". Search the
> lists (not just admin) for that phrase and you should find out what I
> mean. This should probably be a FAQ, as it shows up every few months.

I started going down that same road in May... here's the thread from
then explaining why it's a bad idea after all:
http://archives.postgresql.org/pgsql-performance/2006-05/msg00462.php

Bye, Chris.