On 13 January 2017 at 19:15, Alvaro Herrera <alvhe...@2ndquadrant.com> wrote:
> I think this is the same problem as reported in
> https://www.postgresql.org/message-id/CAMkU=1yE4YyCC00W_GcNoOZ4X2qxF7x5DUAR_kMt-Ta=ypy...@mail.gmail.com

Ah yes, this is the same problem. Not sure why I didn't land on that
thread when I tried to search pghackers using relevant keywords.
>> === Fix ===
> [...]
>> Instead, the attached patch (prevent_useless_vacuums.patch) prevents
>> the repeated cycle by noting that there's no point in doing whatever
>> vac_update_datfrozenxid() does, if we didn't find anything to vacuum
>> and there's already another worker vacuuming the same database. Note
>> that it uses wi_tableoid field to check concurrency. It does not use
>> wi_dboid field to check for already-processing worker, because using
>> this field might cause each of the workers to think that there is some
>> other worker vacuuming, and eventually no one vacuums. We have to be
>> certain that the other worker has already taken a table to vacuum.
> Hmm, it seems reasonable to skip the end action if we didn't do any
> cleanup after all. This would normally give enough time between vacuum
> attempts for the first worker to make further progress and avoid causing
> a storm.  I'm not really sure that it fixes the problem completely, but
> perhaps it's enough.

I had thought about this : if we didn't clean up anything, skip the
end action unconditionally without checking if there was any
concurrent worker. But then thought it is better to skip only if we
know there is another worker doing the same job, because :
a) there might be some reason we are just calling
vac_update_datfrozenxid() without any condition. But I am not sure
whether it was intentionally kept like that. Didn't get any leads from
the history.
b) it's no harm in updating datfrozenxid() it if there was no other
worker. In this case, we *know* that there was indeed nothing to be
cleaned up. So the next time this database won't be chosen again, so
there's no harm just calling this function.

> --
> Álvaro Herrera                https://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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