Horton

Re: SSI non-serializable UPDATE performance

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Dan Ports <drkp@csail.mit.edu>
Cc: Simon Riggs <simon@2ndQuadrant.com>, Kevin Grittner <Kevin.Grittner@wicourts.gov>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2011-04-28T16:24:15Z
Lists: pgsql-hackers

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  1. Add fast paths for cases when no serializable transactions are running.

On Apr 28, 2011, at 9:55 AM, Dan Ports <drkp@csail.mit.edu> wrote:
> On Thu, Apr 28, 2011 at 08:43:30AM +0100, Simon Riggs wrote:
>>> We added a quick return which didn't need to check any locks at the
>>> front of this routine which is taken if there are no active
>>> serializable transactions on the cluster at the moment of update.
>> 
>> Surprised to hear nobody mentioning memory reordering issues about
>> that, but I'm not running Itaniums anywhere.
> 
> I did spend a while thinking about it. There aren't any memory
> reordering issues with that optimization (even on the Alpha, where just
> about anything goes).
> 
> The memory barrier when acquiring the buffer page lwlock acts as the
> synchronization point we need. When we see that no serializable
> transactions are running, that could have been reordered, but that read
> still had to come after the lock was taken. That's all we need: even if
> another backend starts a serializable transaction after that, we know
> it can't take any SIREAD locks on the same target while we're holding
> the buffer page lock.

Sounds like that might be worth a comment.

...Robert