Re: New strategies for freezing, advancing relfrozenxid early

On Wed, Jan 25, 2023 at 10:56 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> but that's only true because page level freezing neutered
> vacuum_freeze_min_age. Compared to <16, it's a *huge* change.

Do you think that page-level freezing
(1de58df4fec7325d91f5a8345757314be7ac05da) was improvidently
committed?

I have always been a bit skeptical of vacuum_freeze_min_age as a
mechanism. It's certainly true that it is a waste of energy to freeze
tuples that will soon be removed anyway, but on the other hand,
repeatedly dirtying the same page for various different freezing and
visibility related reasons *really sucks*, and even repeatedly reading
the page because we kept deciding not to do anything yet isn't great.
It seems possible that the page-level freezing mechanism could help
with that quite a bit, and I think that the heuristic that patch
proposes is basically reasonable: if there's at least one tuple on the
page that is old enough to justify freezing, it doesn't seem like a
bad bet to freeze all the others that can be frozen at the same time,
at least if it means that we can mark the page all-visible or
all-frozen. If it doesn't, then I'm not so sure; maybe we're best off
deferring as much work as possible to a time when we *can* mark the
page all-visible or all-frozen.

In short, I think that neutering vacuum_freeze_min_age at least to
some degree might be a good thing, but that's not to say that I'm
altogether confident in that patch, either.

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