Re: LISTEN / NOTIFY performance in 8.3

At 11:58 PM -0500 2/23/08, Tom Lane wrote:
> > Attached is a perl script that sort of simulates what's going on.
>
>Thanks for the script. It's not showing any particular problems here,
>though. With log_min_duration_statement = 10, the only statements that
>(slightly) exceed 10ms are the select count(*) from generate_series(1,
>15000) ones.

I tried the test script on another machine (similar but not identical
to the original machine) running 8.3 and although the notify
performance was *much* better than the original I still see
notifications taking longer than the select count(*) from
generate_series(1, 15000) queries, and also longer than some simple
updates to other tables that are also happening on the server.

duration: 10.030 ms statement: select count(*) from generate_series(1, 15000)
duration: 224.833 ms statement: NOTIFY to_producer

Perhaps this shouldn't be made much of as I'm sure there are many way
that this could quite naturally happen.

I've been thinking of LISTEN / NOTIFY as one of the least expensive
and therefore speedy ways to get the word out to participating
processes that something has changed (versus using a manually setup
signals table that interested parties updated and selected from).

Now that I see a little bit more of what goes on under the hood of
this function I see that it's still basically table-driven and I'll
adjust my expectations accordingly, but I'm still puzzled by the
hugely slow notifications showing up on the original server running
the producer / consumer setup.

With ps I can see some postgres backends with a 'notify interrupt
waiting' command line during the tests - could it be an issue with
signal handling on the original machine - something entirely outside
of PG's control?

Thx,
-Joel