Re: [HACKERS] Bgwriter behavior

On Sat, 2005-01-01 at 17:47, Simon Riggs wrote:
> On Sat, 2005-01-01 at 17:01, Bruce Momjian wrote:
> > Simon Riggs wrote:
> >
> > > Well, I think we're saying: its not in 8.0 now, and we take our time to
> > > consider patches for 8.1 and accept the situation that the parameter
> > > names/meaning will change in next release.
> >
> > I have no problem doing something for 8.0 if we can find something that
> > meets all the items I mentioned.
> >
> > One idea would be to just remove bgwriter_percent. Beta/RC users would
> > still have it in their postgresql.conf, but it is commented out so it
> > should be OK. If they uncomment it their server would not start but we
> > could just tell testers to remove it. I see that as better than having
> > conflicting parameters.
>
> Can't say I like that at first thought. I'll think some more though...
>
> > Another idea is to have bgwriter_percent be the percent of the buffer it
> > will scan.
>
> Hmmm....well that was my original suggestion (bg2.patch on 12 Dec)
> (...though with a bug, as Neil pointed out)
>
> > We could default that to 50% or 100%, but we then need to
> > make sure all beta/RC users update their postgresql.conf with the new
> > default because the commented-out default will not be correct.
>
> ...we just differ/ed on what the default should be...
>
> > At this point I see these as our only two viable options, aside from
> > doing nothing.
>
> > I realize our current behavior requires a full scan of the buffer cache,
> > but how often is the bgwriter_maxpages limit met? If it is not a full
> > scan is done anyway, right?
>
> Well, if you heavy a very heavy read workload then that would be a
> problem. I was more worried about concurrency in a heavy write
> situation, but I can see your point, and agree.
>
> (Idea #1 still suffers from this, so we should rule it out...)
>
> > It seems the only way to really add
> > functionality is to change bgwriter_precent to control how much of the
> > buffer is scanned.
>
> OK. I think you've persuaded me on idea #2, if I understand you right:
>
> bgwriter_percent = 50 (default)
> bgwriter_maxpages = 100 (default)
>
> percent is the number of shared_buffers we scan, limited by maxpages.
>
> (I'll code it up in a couple of hours when the kids are in bed)

Here's the basic patch - no changes to current default values or docs.

Not sure if this is still interesting or not...

--
Best Regards, Simon Riggs