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)