Re: POC: make mxidoff 64 bits

On 07/03/2025 13:30, Maxim Orlov wrote:
> Here is a rebase, v14.

Thanks! I did some manual testing of this. I created a little helper
function to consume multixids, to test the autovacuum behavior, and
found one issue:

If you consume a lot of multixid members space, by creating lots of
multixids with huge number of members in each, you can end up with a
very bloated members SLRU, and autovacuum is in no hurry to clean it up.
Here's what I did:

1. Installed attached test module
2. Ran "select consume_multixids(10000, 100000);" many times
3. ran:

$ du -h data/pg_multixact/members/
26G data/pg_multixact/members/

When I run "vacuum freeze; select * from pg_database;", I can see that
'datminmxid' for the current database is advanced. However, autovacuum
is in no hurry to vacuum 'template0' and 'template1', so
pg_multixact/members/ does not get truncated. Eventually, when
autovacuum_multixact_freeze_max_age is reached, it presumably will, but
you will run out of disk space before that.

There is this check for members size at the end of SetOffsetVacuumLimit():

>
> /*
> * Do we need autovacuum? If we're not sure, assume yes.
> */
> return !oldestOffsetKnown ||
> (nextOffset - oldestOffset > MULTIXACT_MEMBER_AUTOVAC_THRESHOLD);

And the caller (SetMultiXactIdLimit()) will in fact signal the
autovacuum launcher after "vacuum freeze" because of that. But
autovacuum launcher will look at the datminmxid / relminmxid values, see
that they are well within autovacuum_multixact_freeze_max_age, and do
nothing.

This is a very extreme case, but clearly the code to signal autovacuum
launcher, and the freeze age cutoff that autovacuum then uses, are not
in sync.

This patch removed MultiXactMemberFreezeThreshold(), per my suggestion,
but we threw this baby with the bathwater. We discussed that in this
thread, but didn't come up with any solution. But ISTM we still need
something like MultiXactMemberFreezeThreshold() to trigger autovacuum
freezing if the members have grown too large.

--
Heikki Linnakangas
Neon (https://neon.tech)