Subject: Re: Hot Standby on git
From: Simon Riggs <simon@2ndQuadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
In-Reply-To: <6950.1254752345@sss.pgh.pa.us>
References: <1253958559.4449.833.camel@ebony.2ndQuadrant>
	<4ABF4522.6060601@enterprisedb.com>
	<1254743455.4691.153.camel@ebony.2ndQuadrant>
	<6950.1254752345@sss.pgh.pa.us>
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Date: Mon, 05 Oct 2009 19:49:49 +0100
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On Mon, 2009-10-05 at 10:19 -0400, Tom Lane wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
> > I don't see how that helps at all. The objective of lock counters was to
> > know if we can skip acquiring an LWlock on all lock partitions. This
> > change keeps the lock counters yet acquires the locks we were trying to
> > avoid. This change needs some justification since it is not a bug fix.
> 
> [ scratches head ... ]  Why is hot standby messing with this sort of
> thing at all?  It sounds like a performance optimization that should
> be considered separately, and *later*.

Possibly.

We have 3 suggested approaches:
* Avoid taking LockPartition locks while we get info for Hot Standby
during normal running, by means of a ref counting scheme (Simon)
* Take the locks and implement a ref counting scheme (Heikki)
* Take the locks, worry later (Tom)

The middle ground seems pointless to me.

I'm happy to go with simple lock-everything-for-now but it's pretty
clear its going to be a annoying performance hit. If we do that we
should put in a parameter to turn on/off so that those who will never
use Hot Standby can avoid this completely.

I'll wait for Heikki's thoughts before implementing anything.

-- 
 Simon Riggs           www.2ndQuadrant.com