Re: Removing more vacuumlazy.c special cases, relfrozenxid optimizations

On Mon, Jan 17, 2022 at 5:41 PM Peter Geoghegan <pg@bowt.ie> wrote:
> That just seems like semantics to me. The very next sentence after the
> one you quoted in your reply was "And so it's highly unlikely that any
> given VACUUM will ever *completely* fail to advance relfrozenxid".
> It's continuous *within* each VACUUM. As far as I can tell there is
> pretty much no way that the patch series will ever fail to advance
> relfrozenxid *by at least a little bit*, barring pathological cases
> with cursors and whatnot.

I mean this boils down to saying that VACUUM will advance relfrozenxid
except when it doesn't.

> Actually, I think that even the people in the first category might
> well have about the same improved experience. Not just because of this
> patch series, mind you. It would also have a lot to do with the
> autovacuum_vacuum_insert_scale_factor stuff in Postgres 13. Not to
> mention the freeze map. What version are these users on?

I think it varies. I expect the increase in the default cost limit to
have had a much more salutary effect than
autovacuum_vacuum_insert_scale_factor, but I don't know for sure. At
any rate, if you make the database big enough and generate dirty data
fast enough, it doesn't matter what the default limits are.

> I never said that anti-wraparound vacuums just won't happen anymore. I
> said that they'll be limited to cases like the stock table or
> customers table case. I was very clear on that point.

I don't know how I'm supposed to sensibly respond to a statement like
this. If you were very clear, then I'm being deliberately obtuse if I
fail to understand. If I say you weren't very clear, then we're just
contradicting each other.

> It isn't that hard to see that the cases where we continue to get any
> anti-wraparound VACUUMs with the patch seem to be limited to cases
> like the stock/customers table, or cases like the pathological idle
> cursor cases we've been discussing. Pretty narrow cases, overall.
> Don't take my word for it - see for yourself.

I don't think that's really possible. Words like "narrow" and
"pathological" are value judgments, not factual statements. If I do an
experiment where no wraparound autovacuums happen, as I'm sure I can,
then those are the normal cases where the patch helps. If I do an
experiment where they do happen, as I'm sure that I also can, you'll
probably say either that the case in question is like the
stock/customers table, or that it's pathological. What will any of
this prove?

I think we're reaching the point of diminishing returns in this
conversation. What I want to know is that users aren't going to be
harmed - even in cases where they have behavior that is like the
stock/customers table, or that you consider pathological, or whatever
other words we want to use to describe the weird things that happen to
people. And I think we've made perhaps a bit of modest progress in
exploring that issue, but certainly less than I'd like. I don't want
to spend the next several days going around in circles about it
though. That does not seem likely to make anyone happy.

-- 
Robert Haas
EDB: http://www.enterprisedb.com