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Re: autovacuum truncate exclusive lock round two

From: Dimitri Fontaine <dimitri(at)2ndQuadrant(dot)fr>
To: Jan Wieck <JanWieck(at)Yahoo(dot)com>
Cc: PostgreSQL Development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: autovacuum truncate exclusive lock round two
Date: 2012-11-15 22:31:47
Message-ID: m24nkq8pm4.fsf@2ndQuadrant.fr (view raw or flat)
Thread:
Lists: pgsql-hackers
Jan Wieck <JanWieck(at)Yahoo(dot)com> writes:
> Use this lmgr feature inside count_nondeletable_pages() of vacuumlazy.c to
> periodically check, if there is a conflicting lock request waiting. If not,
> keep going. If there is a waiter, truncate the relation to the point checked
> thus far, release the AccessExclusiveLock, then loop back to where we
> acquire this lock in the first place and continue checking/truncating.

I think that maybe we could just bail out after releasing the
AccessExclusiveLock and trust autovacuum to get back to truncating that
relation later. That would allow removing 2 of the 3 GUCs below:

>     autovacuum_truncate_lock_check = 100ms # how frequent to check
>                                            # for conflicting locks

This is the one remaining. Could we maybe check for lock conflict after
every move backward a page, or some multiple thereof? The goal would be
to ensure that progress is made, while also being aware of concurrent
activity, ala CHECK_FOR_INTERRUPTS().

Regards,
-- 
Dimitri Fontaine
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support



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