Re: Weird problem that enormous locks

On Fri, Jul 15, 2011 at 18:50, Radosław Smogura <rsmogura(at)softperience(dot)eu>wrote:

> On Fri, 15 Jul 2011 18:36:19 +0800, Tony Wang wrote:
>
>> Weird that I receive your each message twice.
>>
>> On Fri, Jul 15, 2011 at 15:33, Radoslaw Smogura wrote:
>>
>>
>> Simple and obvious question right now do You call commit after
>>> transaction? If yes do you use any query or connection pooler?
>>>
>>
>> Yes. connection pool is used as application level, not db level.
>> no commit after transaction is possible (Im trying to check the
>>
>> logic), I just cannot imagine it happened for so many users at the
>> same time, and then calmed down for long time, and came again.
>>
>> I found the query I used to log locks would miss locks that relname is
>> null. will add that, though no idea why its null
>>
>>
>> ------------------------
>>> Regards,
>>> Radoslaw Smogura
>>> (mobile)
>>> -------------------------
>>> From: Tony Wang
>>> Sent: 15 lipca 2011 03:51
>>> To: Scott Marlowe
>>> Cc: PostgreSQL
>>>
>>> Subject: Re: [GENERAL] Weird problem that enormous locks
>>>
>>> On Fri, Jul 15, 2011 at 08:22, Scott Marlowe wrote:
>>>
>>> On Thu, Jul 14, 2011 at 6:01 PM, Tony Wang wrote:
>>>>
>>>>> On Fri, Jul 15, 2011 at 01:13, Scott Marlowe
>>>>>
>>>> > wrote:
>>>>
>>>>
>>>>>> On Wed, Jul 13, 2011 at 9:47 PM, Tony Wang wrote:
>>>>>>
>>>>>
>>>> > On Thu, Jul 14, 2011 at 10:35, John R Pierce
>>>>>>
>>>>> >> > wrote:
>>>>
>>>>> > Its 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<http://www.postgresql.org/docs/8.4/static/explicit-locking.html>
>>>> [5]), its
>>>>
>>>> >> > 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, thats what Im 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 its normal the locks are
>>>>
>>>>> only AccessShareLock and RowExclusiveLock.
>>>>>
>>>>
>>>> So what did / does current_query say when its happening? If it
>>>> says
>>>> you dont have access permission then run that query as root when
>>>> it
>>>> happens again.
>>>>
>>>
>>> As I said, its normal update like "UPDATE player SET money = money +
>>>
>>> 100 WHERE id=12345", but there are quite many
>>>
>>
>>
>>
>> Links:
>> ------
>> [1] mailto:wwwjfy(at)gmail(dot)com
>> [2] mailto:scott(dot)marlowe(at)gmail(dot)com
>> [3] mailto:wwwjfy(at)gmail(dot)com
>> [4] mailto:pierce(at)hogranch(dot)com
>> [5] http://www.postgresql.org/**docs/8.4/static/explicit-**locking.html<http://www.postgresql.org/docs/8.4/static/explicit-locking.html>
>> [6] mailto:scott(dot)marlowe(at)gmail(dot)com
>> [7] mailto:rsmogura(at)softperience(dot)**eu <rsmogura(at)softperience(dot)eu>
>>
> Actually I don't know what pool You use (I think PHP - I don't know much
> about this), but I imagine following, If You don't use auto commit or
> commit:
> 1. User A updates moneys, gets connections C1, locks his row, no commit
> 2. User A updates moneys again, gets connection C2, but C1 still holds
> lock.
> Regards,
> Radosław Smogura
>

Any connection pool behaves similarly. The connection C1 surely will be
committed and returned after the operation finished. Having said that, the
ONLY possible reason is some transactions hanged holding the locks, and
cause others cannot work any more, and the "ExclusiveLock" is not a problem,
right?
The interesting thing is, I didn't find any timeout/exception after the
"lock" period ended in postgresql log, only long query time.