Re: Impact of checkpoint_segments under continual load conditions

On 7/18/05, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Christopher Petrilli <petrilli(at)gmail(dot)com> writes:
> > http://blog.amber.org/diagrams/comparison_mysql_pgsql.png
>
> > Notice the VERY steep drop off.
>
> Hmm. Whatever that is, it's not checkpoint's fault. I would interpret
> the regular upticks in the Postgres times (every several hundred
> iterations) as being the effects of checkpoints. You could probably
> smooth out those peaks some with appropriate hacking on bgwriter
> parameters, but that's not the issue at hand (is it?).

I tried hacking that, turning it up to be more agressive, it got
worse. Turned it down, it got worse :-)

> I have no idea at all what's causing the sudden falloff in performance
> after about 10000 iterations. COPY per se ought to be about a
> constant-time operation, since APPEND is (or should be) constant-time.
> What indexes, foreign keys, etc do you have on this table? What else
> was going on at the time?

The table has 15 columns, 5 indexes (character, inet and timestamp).
No foreign keys. The only other thing running on the machine was the
application actually DOING the benchmarking, written in Python
(psycopg), but it was, according to top, using less than 1% of the
CPU. It was just talking through a pipe to a psql prompt to do the
COPY.

Chris
--
| Christopher Petrilli
| petrilli(at)gmail(dot)com