Re: Weird problem that enormous locks - Mailing list pgsql-general

On Thu, Jul 14, 2011 at 8:28 PM, Tony Wang <wwwjfy@gmail.com> wrote:
> On Fri, Jul 15, 2011 at 10:05, Scott Marlowe <scott.marlowe@gmail.com>
> wrote:
>>
>> On Thu, Jul 14, 2011 at 7:51 PM, Tony Wang <wwwjfy@gmail.com> wrote:
>> > On Fri, Jul 15, 2011 at 08:22, Scott Marlowe <scott.marlowe@gmail.com>
>> > wrote:
>> >>
>> >> On Thu, Jul 14, 2011 at 6:01 PM, Tony Wang <wwwjfy@gmail.com> wrote:
>> >> > On Fri, Jul 15, 2011 at 01:13, Scott Marlowe
>> >> > <scott.marlowe@gmail.com>
>> >> > wrote:
>> >> >>
>> >> >> On Wed, Jul 13, 2011 at 9:47 PM, Tony Wang <wwwjfy@gmail.com> wrote:
>> >> >> > On Thu, Jul 14, 2011 at 10:35, John R Pierce <pierce@hogranch.com>
>> >> >> > wrote:
>> >> >> > It's a game server, and the queries are updating users' money, as
>> >> >> > normal.
>> >> >> > The sql is like "UPDATE player SET money = money + 100 where id =
>> >> >> > 12345".
>> >> >> > The locks were RowExclusiveLock for the table "player" and the
>> >> >> > indexes.
>> >> >> > The
>> >> >> > weird thing is there was another ExclusiveLock for the table
>> >> >> > "player",
>> >> >> > i.e.
>> >> >> > "player" got two locks, one RowExclusiveLock and one
>> >> >> > ExclusiveLock.
>> >> >> > In the postgresql documentation
>> >> >> > (http://www.postgresql.org/docs/8.4/static/explicit-locking.html),
>> >> >> > it's
>> >> >> > said
>> >> >> > about the  Exclusive "This lock mode is not automatically acquired
>> >> >> > on
>> >> >> > user
>> >> >> > tables by any PostgreSQL command."
>> >> >>
>> >> >> You need to figure out what part of your app, or maybe a rogue
>> >> >> developer etc is throwing an exclusive lock.
>> >> >
>> >> > Yeah, that's what I'm trying to do
>> >>
>> >> Cool.  In your first post you said:
>> >>
>> >> > select pg_class.relname, pg_locks.mode, pg_locks.granted,
>> >> > pg_stat_activity.current_query, pg_stat_activity.query_start,
>> >> > pg_stat_activity.xact_start as transaction_start,
>> >> > age(now(),pg_stat_activity.query_start) as query_age,
>> >> > age(now(),pg_stat_activity.xact_start) as transaction_age,
>> >> > pg_stat_activity.procpid from pg_stat_activity,pg_locks left
>> >> > outer join pg_class on (pg_locks.relation = pg_class.oid) where
>> >> > pg_locks.pid=pg_stat_activity.procpid and
>> >> > substr(pg_class.relname,1,3) != 'pg_' order by query_start;
>> >>
>> >> > The only special thing I can find is that there were a lot
>> >> > ExclusiveLock, while it's normal the locks are
>> >> > only AccessShareLock and RowExclusiveLock.
>> >>
>> >> So what did / does current_query say when it's happening?  If it says
>> >> you don't have access permission then run that query as root when it
>> >> happens again.
>> >
>> > As I said, it's normal update like "UPDATE player SET money = money +
>> > 100
>> > WHERE id=12345", but there are quite many
>>
>> A regular update like that can't get a full exclusive lock by itself,
>> there'd have to be a previous query in the same transaction that took
>> out an explicit lock.  Is it possible for you to set up query logging
>> such that you can track connections to see which one does that in the
>> future?
>
> Yeah, and I also wonder when will an ExclusiveLock acquired.
> I set up query logging after that, that'll be really big file.
>
>>
>> Were there more than 1 exclusive lock (now row exclusive, but just
>> plain exclusive)?
>
> There were many such locks (not row exclusive) updating different player id.

How many just plain exclusive locks were there?