Message-ID: <716ad55e559db5d9802de179677d2fb0.squirrel@shrek.krogh.cc>
In-Reply-To: <20140731134025.GB5475@eldon.alvh.no-ip.org>
References: <26e211945432a41a56dd900440f27e6d.squirrel@shrek.krogh.cc>
 <20140731134025.GB5475@eldon.alvh.no-ip.org>
Date: Fri, 1 Aug 2014 10:59:36 +0200
Subject: Re: Autovacuum not keeping up. (PG 9.2.9)
From: jesper@krogh.cc
To: "Alvaro Herrera" <alvherre@2ndquadrant.com>
Cc: pgsql-admin@postgresql.org
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> jesper@krogh.cc wrote:
>> Hi.
>>
>> I have a large  database with a message queue table, that has high
>> activity. The database supports 1-300 client connection concurrently,
>>  having transactions open in up to 30 minutes each.
>>
>> Recently I am seeing autuvacuum being issued, but it takes
>> ages to get through the message queue table, with strace showing waiting
>> for semop's for 10's to 100's of seconds.
>
> Do you have data on how relfrozenxid advances for that table?

Not really, how would you normally pick that out?
2014-08-01 10:49:39.171 db=# select relname, age(relfrozenxid),
relfrozenxid  FROM pg_class WHERE relkind = 'r' and relname = 'job';
 relname |    age    | relfrozenxid
---------+-----------+--------------
 job     | 111893622 |    796259097
(1 row)

Time: 1.913 ms

This shouldn't qualify for a freeze vacuum, should it?

> Vacuuming needs to grab a "cleanup lock" on each page it's going to
> vacuum, which is a special kind of lock that requires that no other
> process is even looking at the page at that moment (we call this "to
> have the page pinned"), which is even weaker than having a shared lock
> on the page.  If traffic to some pages is high, it might be difficult
> for vacuum to acquire this.

Based on strace output this looks excactly like whats going on, there are
some activity, then it pauses for some time then continues.

> Normally, vacuum doesn't break much sweat about this: if it cannot
> acquire the cleanup lock, it ignores the page, keeps calm and carries
> on.  But if it's a for-wraparound vacuuming, it will need to wait until
> it is able to acquire cleanup lock.

Can I force it to tell me if it does the for-wraparound cleanup or normal?

> I think one idea might be to try to manually vacuum the table once in a
> while with a reduced value of min_freeze_table_age.  This will cause a
> full table scan (i.e. cleanup lock for all pages is waited for), which
> decreases the "frozen xid age", which moves the need to do this again
> further in the future; so the autovacuum-invoked vacuums will be able to
> skip the pages on which it cannot get cleanup lock.
>
> Another idea is to increase min_freeze_table_age for the queue table
> through ALTER TABLE, the idea being that you can delay forced vacuuming
> of hot pages for long enough that they can wait until they have cooled
> off.  Default value is 150 million transactions, which you can raise
> tenfold and even higher.
>
> See
> http://www.postgresql.org/docs/9.2/static/runtime-config-client.html#GUC-VACUUM-FREEZE-TABLE-AGE

I tried to read through that, but that is a GUC not a storage level
parameter. So I (hopefully correct) figured out that the storage level
parameter that need to be set were autovacuum_freeze_min_age and set that
one to 1.000.000.000 for the table. But apparently it didnt cause any
changes. It is still waiting for the lock. Is a database restart required
after setting storage parameters, or will autovacuum pick up the new one
when it starts over with the table.


> The other idea is that heap truncation is what's causing the problem,
> but AFAICS that uses conditional lock acquisition so you shouldn't be
> seeing stalls in semop().

That should only be once per vacuuming .. right? That doesn't fit the
pattern either.

-- 
Jesper


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