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Re: SSI non-serializable UPDATE performance

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Dan Ports <drkp(at)csail(dot)mit(dot)edu>
Cc: Simon Riggs <simon(at)2ndQuadrant(dot)com>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: SSI non-serializable UPDATE performance
Date: 2011-04-28 16:24:15
Message-ID: 11386E7B-ACA8-4F3C-AA59-2DC30186A948@gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Apr 28, 2011, at 9:55 AM, Dan Ports <drkp(at)csail(dot)mit(dot)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

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