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Re: foreign key locks, 2nd attempt

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: foreign key locks, 2nd attempt
Date: 2012-03-15 22:13:45
Message-ID: 1331849597-sup-8328@alvh.no-ip.org (view raw or flat)
Thread:
Lists: pgsql-hackers
Excerpts from Simon Riggs's message of jue mar 15 19:04:41 -0300 2012:
> 
> On Thu, Mar 15, 2012 at 9:54 PM, Alvaro Herrera
> <alvherre(at)commandprompt(dot)com> wrote:
> >
> > Excerpts from Simon Riggs's message of jue mar 15 18:38:53 -0300 2012:
> >> On Thu, Mar 15, 2012 at 2:26 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> >
> >> > But that would only make sense if
> >> > we thought that getting rid of the fsyncs would be more valuable than
> >> > avoiding the blocking here, and I don't.
> >>
> >> You're right that the existing code could use some optimisation.
> >>
> >> I'm a little tired, but I can't see a reason to fsync this except at checkpoint.
> >
> > Hang on.  What fsyncs are we talking about?  I don't see that the
> > multixact code calls any fsync except that checkpoint and shutdown.
> 
> If a dirty page is evicted it will fsync.

Ah, right.

> >> Also seeing that we issue 2 WAL records for each RI check. We issue
> >> one during MultiXactIdCreate/MultiXactIdExpand and then immediately
> >> afterwards issue a XLOG_HEAP_LOCK record. The comments on both show
> >> that each thinks it is doing it for the same reason and is the only
> >> place its being done. Alvaro, any ideas why that is.
> >
> > AFAIR the XLOG_HEAP_LOCK log entry only records the fact that the row is
> > being locked by a multixact -- it doesn't record the contents (member
> > xids) of said multixact, which is what the other log entry records.
> 
> Agreed. But issuing two records when we could issue just one seems a
> little strange, especially when the two record types follow one
> another so closely - so we end up queuing for the lock twice while
> holding the lock on the data block.

Hmm, that seems optimization that could be done separately.

-- 
Álvaro Herrera <alvherre(at)commandprompt(dot)com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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