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Re: testing HS/SR - 1 vs 2 performance

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Erik Rijkers <er(at)xs4all(dot)nl>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: testing HS/SR - 1 vs 2 performance
Date: 2010-04-25 17:00:58
Message-ID: 1272214858.4161.1819.camel@ebony (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Sun, 2010-04-25 at 12:51 -0400, Tom Lane wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> > On Sun, 2010-04-25 at 11:50 -0400, Tom Lane wrote:
> >> This needs a redesign before it can be considered committable.  I don't
> >> really care whether it makes things faster; it's too complicated and too
> >> poorly documented to be maintainable.
> > There are more than 60 lines of header comment explaining in detail how
> > this works, with a full algorithmic analysis. The remaining code is
> > commented to project standards, with easily more than 100 lines of
> > comments. 
> If the comments were correct, I wouldn't be complaining.  They're
> misleading or outright wrong on many points.  In particular, I don't
> think you actually understand the weak-memory-ordering issue, because
> the comments about that are entirely wrong.  

The comments says "on CPUs with
+ * weak-memory ordering we can't reliably move pointers atomically, so
+ * rule is that updates of head and tail of the array require
+ * in exclusive mode or (shared mode and known_assigned_xids_lck

I will reword this, so it is clear that I'm talking about the head and
tail of the array, not pointers in general.

 Simon Riggs 

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