Re: Monitoring multixact members growth

On Fri, Aug 19, 2022 at 8:40 AM Vido Vlahinic
<Vido(dot)Vlahinic(at)milestonegroup(dot)com> wrote:
> However it is not quite clear to me how I can interpret results from above and e.g. conclude: my_table accumulates x more multixact members since I measured last.

You can't. And not just because nobody got around to implementing it
yet -- it's quite a fundamental restriction. VACUUM must always make
sure of that, and must always scan all unfrozen pages to safely
determine that much (it may or may not have to freeze *any* MultiXacts
as part of that process, but it must always be sure that no Multis <
its final relminmxid remain).

> My goal here is to predict where multixact members are growing the fastest so I can perform manual VACUUM FREEZE only on those tables

The problem with that strategy is that you still have to do
anti-wraparound autovacuums when the mxid_age(relminmxid) of a table
crosses the usual threshold, even when in reality there are *zero*
MultiXacts in the table (often the case with the largest tables).
That's just how it works, unfortunately.

There is one piece of good news, though: work in Postgres 15 taught
VACUUM to track the oldest extant XID and MXID in the table, and set
relfrozenxid and remind to those oldest values (rather than using the
cutoffs for freezing, which in general might be much older than the
oldest remaining unfrozen XID/MXID).

I expect that this will make it much less likely that anti-wraparound
autovacuums affecting many tables will all stampede, hurting
performance. This will be possible because VACUUM will now be able to
set relminmxid to a value that actually tells us something about
what's really going on in each table, MultiXact-wise (not just what
you set vacuum_multixact_freeze_min_age and
autovacuum_multixact_freeze_max_age to in postgresql.conf, which is
pretty far removed from what matters most of the time). Simply by
noticing that there are no remaining MultiXacts (and probably never
were any in the first place) with the larger tables.

The timeline for anti-wraparound autovacuums will tend to make a lot
more sense for *your* workload, where huge differences in the rate of
MultiXact consumption among tables is likely the norm. This still
isn't perfect (far from it), but it has the potential to make things
far better here.

--
Peter Geoghegan