Subject: Re: Proposal of tunable fix for scalability of 8.4
From: Simon Riggs <simon@2ndQuadrant.com>
To: Matthew Wakeling <matthew@flymine.org>
Cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
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Date: Wed, 18 Mar 2009 07:53:53 +0000
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On Mon, 2009-03-16 at 16:26 +0000, Matthew Wakeling wrote:
> One possibility would be for the locks to alternate between exclusive
> and 
> shared - that is:
> 
> 1. Take a snapshot of all shared waits, and grant them all -
> thundering
>      herd style.
> 2. Wait until ALL of them have finished, granting no more.
> 3. Take a snapshot of all exclusive waits, and grant them all, one by
> one.
> 4. Wait until all of them have been finished, granting no more.
> 5. Back to (1)

I agree with that, apart from the "granting no more" bit.

Currently we queue up exclusive locks, but there is no need to since for
ProcArrayLock commits are all changing different data.

The most useful behaviour is just to have two modes:
* exclusive-lock held - all other x locks welcome, s locks queue
* shared-lock held - all other s locks welcome, x locks queue

This *only* works for ProcArrayLock.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support