Re: New strategies for freezing, advancing relfrozenxid early

On Wed, 14 Dec 2022 at 00:07, Peter Geoghegan <pg@bowt.ie> wrote:
>
> On Tue, Dec 13, 2022 at 9:16 AM Peter Geoghegan <pg@bowt.ie> wrote:
> > That's not the only thing we care about, though. And to the extent we
> > care about it, we mostly care about the consequences of either
> > freezing or not freezing eagerly. Concentration of unfrozen pages in
> > one particular table is a lot more of a concern than the same number
> > of heap pages being spread out across multiple tables. Those tables
> > can all be independently vacuumed, and come with their own
> > relfrozenxid, that can be advanced independently, and are very likely
> > to be frozen as part of a vacuum that needed to happen anyway.
>
> At the suggestion of Jeff, I wrote a Wiki page that shows motivating
> examples for the patch series:
>
> https://wiki.postgresql.org/wiki/Freezing/skipping_strategies_patch:_motivating_examples
>
> These are all cases where VACUUM currently doesn't do the right thing
> around freezing, in a way that is greatly ameliorated by the patch.
> Perhaps this will help other hackers to understand the motivation
> behind some of these mechanisms. There are plenty of details that only
> make sense in the context of a certain kind of table, with certain
> performance characteristics that the design is sensitive to, and seeks
> to take advantage of in one way or another.

In this mentioned wiki page, section "Simple append-only", the
following is written:

> Our "transition from lazy to eager strategies" concludes with an autovacuum that actually advanced relfrozenxid eagerly:
>> automatic vacuum of table "regression.public.pgbench_history": index scans: 0
>> pages: 0 removed, 1078444 remain, 561143 scanned (52.03% of total)
>> [...]
>> frozen: 560841 pages from table (52.00% of total) had 88051825 tuples frozen
>> [...]
>> WAL usage: 1121683 records, 557662 full page images, 4632208091 bytes

I think that this 'transition from lazy to eager' could benefit from a
limit on how many all_visible blocks each autovacuum iteration can
freeze. This first run of (auto)vacuum after the 8GB threshold seems
to appear as a significant IO event (both in WAL and relation
read/write traffic) with 50% of the table updated and WAL-logged. I
think this should be limited to some degree, such as only freeze
all_visible blocks up to 10% of the table's blocks in eager vacuum, so
that the load is spread across a larger time frame and more VACUUM
runs.


Kind regards,

Matthias van de Meent.