Re: Page replacement algorithm in buffer cache

On 2013-04-02 11:54:32 -0400, Robert Haas wrote:
> On Tue, Apr 2, 2013 at 11:32 AM, Merlin Moncure <mmoncure(at)gmail(dot)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/
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