On 10/9/18 5:15 AM, Kyotaro HORIGUCHI wrote:
> At Fri, 5 Oct 2018 15:35:04 +0900, Michael Paquier <michael@paquier.xyz> wrote in
<20181005063504.GB14664@paquier.xyz>
>> On Fri, Oct 05, 2018 at 12:16:03PM +0900, Michael Paquier wrote:
>>> So, I have come back to this stuff, and finished with the attached
>>> instead, so as the assertion is in a single place. I find that
>>> clearer. The comments have also been improved. Thoughts?
>> And so... I have been looking at committing this thing, and while
>> testing in-depth I have been able to trigger a case where an autovacuum
>> has been able to be not aggressive but anti-wraparound, which is exactly
>> what should not be possible, no? I have simply created an instance with
>> autovacuum_freeze_max_age = 200000, then ran pgbench with
>> autovacuum_freeze_table_age=200000 set for each table, and also ran
>> installcheck-world in parallel. This has been able to trigger the
>> assertion pretty quickly.
> I investigated it and in short, it can happen.
>
> It is a kind of race consdition between two autovacuum
> processes. do_autovacuum() looks into pg_class (using a snapshot)
> and vacuum_set_xid_limits() looks into relcache. If concurrent
> vacuum happens and one has finished the relation, another gets
> relcache invalidation and relfrozenxid is updated. If this
> happens between do_autovacuum() and vacuum_set_xid_limits(), the
> latter sees newer relfrozenxid than the former. The problem
> happens when it moves by more than 5% of
> autovacuum_freeze_max_age.
>
> If lazy_vacuum_rel() sees the situation, the relation is already
> aggressively vacuumed by a cocurrent worker. We can just ingore
> the state safely but also we know that the vacuum is useless.
>
> 1. Just allow the case there (and add comment).
> Causes redundant anti-wraparound vacuum.
>
> 2. Skip the relation by the condition.
>
> I think that we can safely skip the relation in the
> case. (attached)
>
> 3. Ensure that do_autovacuum always sees the same relfrozenxid
> with vacuum_set_xid_limits().
>
> 4. Prevent concurrent acuuming of the same relation rigorously,
> somehow.
>
> Thoughts?
>
I notice that this seems never to have been acted on. I think we should
apply this and remove the (confusing) message setting for the case we'll
now be avoiding. If not we should at least comment there that this is a
case we only expect to see in pathological cases.
cheers
andrew
--
Andrew Dunstan https://www.2ndQuadrant.com
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