Re: Why do we let autovacuum give up?

Mark Kirkwood <mark.kirkwood@catalyst.net.nz> writes:
> On 24/01/14 09:49, Tom Lane wrote:
>> 2. What have you got that is requesting exclusive lock on 
>> pg_attribute? That seems like a pretty unfriendly behavior in itself. 

> I've seen this sort of problem where every db session was busily 
> creating temporary tables. I never got to the find *why* they needed to 
> make so many, but it seemed like a bad idea.

That shouldn't result in any table-level exclusive locks on system
catalogs, though.

[ thinks... ]  It's possible that what you saw is not the
kick-out-autovacuum-entirely behavior, but the behavior added in commit
bbb6e559c, whereby vacuum (auto or regular) will skip over pages that it
can't immediately get an exclusive buffer lock on.  On a heavily used
table, we might skip the same page repeatedly, so that dead tuples don't
get cleaned for a long time.

To add insult to injury, despite having done that, vacuum would reset the
pgstats dead-tuple count to zero, thus postponing the next autovacuum.
I think commit 115f41412 may have improved the situation, but I'd want
to see some testing of this theory before I'd propose back-patching it.
        regards, tom lane