Date: Tue, 2 Apr 2013 18:15:34 +0200
From: Andres Freund <andres@2ndquadrant.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Merlin Moncure <mmoncure@gmail.com>, Jim Nasby <jim@nasby.net>,
	Jeff Janes <jeff.janes@gmail.com>, Ants Aasma <ants@cybertec.at>,
	Tom Lane <tgl@sss.pgh.pa.us>, Atri Sharma <atri.jiit@gmail.com>,
	Greg Stark <stark@mit.edu>, Amit Kapila <amit.kapila@huawei.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Page replacement algorithm in buffer cache
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On 2013-04-02 11:54:32 -0400, Robert Haas wrote:
> On Tue, Apr 2, 2013 at 11:32 AM, Merlin Moncure <mmoncure@gmail.com> wrote:
> > That's a very fair point, although not being able to evict pinned
> > buffers is a highly mitigating aspect.  Also CLOG is a different beast
> > entirely -- it's much more dense (2 bits!) vs a tuple so a single page
> > can a lot of high priority things.  But you could be right anyways.
> >
> > Given that, I wouldn't feel very comfortable with forced eviction
> > without knowing for sure high priority buffers were immune from that.
> > Your nailing idea is maybe the ideal solution.   Messing around with
> > the usage_count mechanic is tempting (like raising the cap and making
> > the sweeper more aggressive as it iterates), but probably really
> > difficult to get right, and, hopefully, ultimately moot.
> 
> One thought I had for fiddling with usage_count is to make it grow
> additively (x = x + 1) and decay exponentially (x = x >> 1).  I'm not
> sure the idea is any good, but one problem with the current system is
> that it's pretty trivial for a buffer to accumulate five touches, and
> after that we lose all memory of what the frequency of access is, so a
> pages of varying different levels of "hotness" can all have usage
> count 5.  This might allow a little more refinement without letting
> the time to degrade the usage count get out of control.
> 
> But, having said that, I still think the best idea is what Andres
> proposed, which pretty much matches my own thoughts: the bgwriter
> needs to populate the free list, so that buffer allocations don't have
> to wait for linear scans of the buffer array.  That's just plain too
> slow.

That way the usagecount should go down far more slowly which essentially makes
it more granular. And I think fiddling on that level before we have a more
sensible buffer acquiration implementation is pretty premature since that will
change too much.

Greetings,

Andres Freund

-- 
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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