Horton

Re: foreign key locks, 2nd attempt

Simon Riggs <simon@2ndquadrant.com>

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From: Simon Riggs <simon@2ndQuadrant.com>
To: Alvaro Herrera <alvherre@commandprompt.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Noah Misch <noah@leadboat.com>, Tom Lane <tgl@sss.pgh.pa.us>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2012-03-15T22:04:41Z
Lists: pgsql-hackers

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  1. Try to avoid running with a full fsync request queue.

On Thu, Mar 15, 2012 at 9:54 PM, Alvaro Herrera
<alvherre@commandprompt.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@gmail.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.

>> 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.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
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