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Re: Re: RC2 and open issues

From: <simon(at)2ndquadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Re: RC2 and open issues
Date: 2004-12-21 09:48:01
Message-ID: 28292295$ (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote on 21.12.2004, 05:05:36:
> Bruce Momjian  writes:
> > I am confused.  If we change the percentage to be X% of the entire
> > buffer cache, and we set it to 1%, and we exit when either the dirty
> > pages or % are reached, don't we end up just scanning the first 1% of
> > the cache over and over again?
> Exactly.  But 1% would be uselessly small with this definition.  Offhand
> I'd think something like 50% might be a starting point; maybe even more.
> What that says is that a page isn't a candidate to be written out by the
> bgwriter until it's fallen halfway down the LRU list.

I see the buffer list as a conveyor belt that carries unneeded blocks
away from the MRU. Cleaning near the LRU (I agree: How near?) should be
all that is sufficient to keep the list clean.

Cleaning the first 1% "over and over again" makes it sound like it is
the same list of blocks that are being cleaned. It may be the same
linked list data structure, but that is dynamically changing to contain
completely different blocks from the last time you looked.

Best Regards, Simon Riggs


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