Subject: Re: Proposal of tunable fix for scalability of 8.4
From: Simon Riggs <simon@2ndQuadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Matthew Wakeling <matthew@flymine.org>,
	"pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
In-Reply-To: <15144.1237408005@sss.pgh.pa.us>
References: <alpine.GSO.2.01.0903121613270.1925@westnet.com>
	<C5DED7A7.33B2%scott@richrelevance.com>
	<603c8f070903121829r31cf1472ha9876e5c480a542e@mail.gmail.com>
	<12292.1236964604@sss.pgh.pa.us> <49BB699D.1070809@enterprisedb.com>
	<alpine.DEB.2.00.0903161543590.21772@aragorn.flymine.org>
	<1237362833.3953.186.camel@ebony.2ndQuadrant>
	<15144.1237408005@sss.pgh.pa.us>
Content-Type: text/plain
Date: Wed, 18 Mar 2009 23:06:56 +0000
Message-Id: <1237417616.3953.318.camel@ebony.2ndQuadrant>
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit


On Wed, 2009-03-18 at 16:26 -0400, Tom Lane wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
> > 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
> 
> My goodness, it seems people have forgotten about the "lightweight"
> part of the LWLock design.

"Lightweight" is only useful if it fits purpose. If the LWlock design
doesn't fit all cases, especially with critical lock types, then we can
have special cases. We have both spinlocks and LWlocks, plus we split
hash tables into multiple lock partitions. If we have 3 types of
lightweight locking, why not consider having 4?

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