From: | Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com> |
---|---|
To: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Cc: | Andres Freund <andres(at)anarazel(dot)de> |
Subject: | Re: logical replication busy-waiting on a lock |
Date: | 2017-05-27 13:48:22 |
Message-ID: | 12fb5b00-43cd-04ad-33bb-9407cdb25cd4@2ndquadrant.com |
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Lists: | pgsql-hackers |
On 27/05/17 15:44, Petr Jelinek wrote:
> On 27/05/17 01:25, Jeff Janes wrote:
>> When I create a subscription in the disabled state, and then later doing
>> "alter subscription sub enable;", on the master I sometimes get a tight
>> loop of the deadlock detector:
>>
>> (log_lock_waits is on, of course)
>>
>> deadlock_timeout is set to 1s, so I don't know why it seems to be
>> running several times per millisecond.
>>
>> .....
>>
>> And so on out to "after 9616.814", when it finally acquires the lock.
>>
>> The other process, 47457, is doing the initial COPY of another table as
>> part of the same publisher/subscriber set.
>>
>
> We lock wait for running transactions in snapshot builder while the
> snapshot is being built so I guess that's what you are seeing. I am not
> quite sure why the snapshot builder would hold the xid lock for
> prolonged period of time though, the XactLockTableWait releases the lock
> immediately after acquiring it. In fact AFAICS everything that acquires
> ShareLock on xid releases it immediately after acquiring as it's only
> used for waits.
>
Actually, I guess it's the pid 47457 (COPY process) who is actually
running the xid 73322726. In that case that's the same thing Masahiko
Sawada reported [1]. Which basically is result of snapshot builder
waiting for transaction to finish, that's normal if there is a long
transaction running when the snapshot is being created (and the COPY is
a long transaction).
--
Petr Jelinek http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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