Re: [HACKERS] Adjust autovacuum naptime automatically

Alvaro Herrera wrote:
> ITAGAKI Takahiro wrote:
>
>> In the case of a heavily update workload, the default naptime (60 seconds)
>> is too long to keep the number of dead tuples low. With my patch, the naptime
>> will be adjusted around 3 seconds at the case of pgbench (scale=10, 80 tps)
>> with default other autovacuum parameters.

What is this based on? That is, based on what information is it
deciding to reduce the naptime?

> Interesting. To be frank I don't know what the sleep scale factor was
> supposed to do.

I'm not sure that sleep scale factor is a good idea or not at this
point, but what I was thinking back in the day when i originally wrote
the contrib autovacuum is that I didn't want the system to get bogged
down constantly vacuuming. So, if it just spent a long time working on
one database, it would sleep for long time.

Given that we can now specify the vacuum cost delay settings for
autovacuum and disable tables and everything else, I'm not sure we this
anymore, at least not as it was originally designed. It sounds like
Itagaki is doing things a little different with his patch, but I'm not
sure I understand it.

>> - I removed autovacuum_naptime guc variable, because it is adjusted
>> automatically now. Is it appropriate?
>
> I think we should provide the user with a way to stop the naptime from
> changing at all. Eventually we will have the promised "maintenance
> windows" feature which will mean the user will not have to worry at all
> about the naptime, but in the meantime I think we should keep it.

I'm not sure that's true. I believe we will want the naptime GUC option
even after we have the maintenance window. I think we might ignore the
naptime during the maintenance window, but even after we have the
maintenance window, we will still vacuum during the day as required.

My vision of the maintenance window has always been very simple, that
is, during the maintenance window the thresholds get reduced by some
factor (probably a GUC variable) so during the day it might take 10000
updates on a table to cause a vacuum but during the naptime it might be
10% of that, 1000. Is this in-line with what others were thinking?