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Re: [HACKERS] Bgwriter behavior

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>,Gavin Sherry <swm(at)linuxworld(dot)com(dot)au>,PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>,pgsql-patches(at)postgresql(dot)org
Subject: Re: [HACKERS] Bgwriter behavior
Date: 2004-12-31 11:46:49
Message-ID: 1104493609.3978.892.camel@localhost.localdomain (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
On Fri, 2004-12-31 at 01:14, Bruce Momjian wrote:
> Simon Riggs wrote:
> > On Mon, 2004-12-27 at 22:21, Bruce Momjian wrote:
> > > Should we consider at least adjusting the meaning of bgwriter_percent?
> > 
> > Yes. As things stand, this is the only change that seems safe.
> > 
> > Here's a very short patch that implements this change within BufferSync
> > in bufmgr.c 
> > 
> > - No algorithm changes
> > - No error message changes
> > - Only change is the call to StrategyDirtyBufferList is made using the
> > maximum number of buffers that will be cleaned, rather than uselessly
> > trawling through all of shared_buffers
> > 
> > This changes the meaning of bgwriter_percent from "percent of dirty
> > buffers" to "percent of shared_buffers". The default settings of 1% of
> > 1000 buffers gives up to 10 dirty block writes every 250ms
> > 
> > Benefit: allows performance tuning by increases options for setting
> > bgwriter_delay which would otherwise have an ineffectually high minimum
> > setting
> > 
> > Risk: low
> > 
> > 1-line doc patch to follow, if this is approved.
> 
> I am not objecting to the patch, but what value is there in having both
> bgwriter_percent and bgwriter_maxpages?  Seems both are redundant and
> that one would be enough.

In brief:
i) for now: as little change as possible is good
ii) the two parameters are OK
iii) trying to decide an alternative takes time, which we do not have
iv) what is presented here is simply a performance bug fix, not the best
long term alternative...

I'd like to move quickly: if we do this (or an alternative), it has to
be done soon and it would be easy to discuss this until we run out of
time. Could we vote: in RC3, or not?

In more detail... 

The value of having both is:
i) as little change as possible at this stage of RC - the main one
...which gives us stability
...and also avoids having to re-discuss what they *should* be

ii) Having two isn't that bad. bgwriter_percent auto adjusts the length
of the to-be-cleaned-list, so it is roughly useful anywhere between 500
and 10000 shared_buffers. That is IMHO slightly more useful than a hard
definition set via bgwriter_maxpages, since that is likely to be set
wrong anyway - but has some value as an outside limit on the number of
pages. [You may wish to set shared_buffers > 10000 even on smaller
servers, since many now have 2GB RAM and yet a relatively poor I/O
subsystem. Having maxpages set separately allows the majority of people
to set shared_buffers higher without swamping their I/O subsystems
because they didn't know about the r8.0 bgwriter feature/parameters]

iii) changing the parameters might tempt us towards changing the
algorithm, which is not a topic we have reached agreement on

iv) I see it as a goal to remove all of those parameters anyway, as well
as explore some of the many options and ideas everybody has presented,
so further change is likely at the next release whatever is done now.

The patch is as simple as I can make it and yet remove the unnecessary
performance effect in the existing code. Thanks to Neil and others for
showing that this was possible...I see this patch as a team effort.

I've already spoken against larger change and would do so again now: if
we don't agree this change, then I would vote for no-change.... simply
because this patch is minimal change. We *suspect* further change is
beneficial but we have no evidence to support what that change should
be, amongst the large range of possible solutions proposed.

-- 
Best Regards, Simon Riggs


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