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Re: cheaper snapshots

From: Hannu Krosing <hannu(at)krosing(dot)net>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: cheaper snapshots
Date: 2011-07-28 15:36:53
Message-ID: 1311867413.3117.1477.camel@hvost (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, 2011-07-28 at 11:15 -0400, Robert Haas wrote:
> On Thu, Jul 28, 2011 at 11:10 AM, Hannu Krosing <hannu(at)2ndquadrant(dot)com> wrote:
> > My main point was, that we already do synchronization when writing wal,
> > why not piggyback on this to also update latest snapshot .
> 
> Well, one problem is that it would break sync rep.

Can you elaborate, in what way it "breaks" sync rep ?

> Another problem is that pretty much the last thing I want to do is
> push more work under WALInsertLock.  Based on the testing I've done so
> far, it seems like WALInsertLock, ProcArrayLock, and CLogControlLock
> are the main bottlenecks here.  I'm focusing on ProcArrayLock and
> CLogControlLock right now, but I am pretty well convinced that
> WALInsertLock is going to be the hardest nut to crack, so putting
> anything more under there seems like it's going in the wrong
> direction. 

probably it is not just the WALInsertLock, but the fact that we have
just one WAL. It can become a bottleneck once we have significant number
of processors fighting to write in single WAL.

>  IMHO, anyway.
> 
> -- 
> Robert Haas
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
> 



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