Re: cost delay brainstorming

On Tue, Jun 18, 2024 at 01:32:38PM -0700, Andres Freund wrote:
> On 2024-06-18 13:50:46 -0500, Nathan Bossart wrote:
>> Have we ruled out further adjustments to the cost parameters as a first
>> step?
>
> I'm not against that, but I it doesn't address the issue that with the current
> logic one set of values just isn't going to fit a 60MB that's allowed to burst
> to 100 iops and a 60TB database that has multiple 1M iops NVMe drives.

True.

>> If you are still recommending that folks raise it and never recommending
>> that folks lower it, ISTM that our defaults might still not be in the right
>> ballpark. The autovacuum_vacuum_cost_delay adjustment you reference (commit
>> cbccac3) is already 5 years old, so maybe it's worth another look.
>
> Adjusting cost delay much lower doesn't make much sense imo. It's already only
> 2ms on a 1ms granularity variable. We could increase the resolution, but
> sleeping for much shorter often isn't that cheap (you need to set up hardware
> timers all the time and due to the short time they can't be combined with
> other timers) and/or barely gives time to switch to other tasks.
>
>
> So we'd have to increase the cost limit.

Agreed.

--
nathan