Re: Design notes for BufMgrLock rewrite

Tom Lane wrote:
> "Jim C. Nasby" <decibel(at)decibel(dot)org> writes:
> > The advantage of using a counter instead of a simple active
> > bit is that buffers that are (or have been) used heavily will be able to
> > go through several sweeps of the clock before being freed. Infrequently
> > used buffers (such as those from a vacuum or seq. scan), would get
> > marked as inactive the first time they were hit by the clock hand.
>
> Hmm. It would certainly be nearly as easy to adjust a counter as to
> manipulate the RECENTLY_USED flag bit that's in the patch now. (You
> could imagine the RECENTLY_USED flag bit as a counter with max value 1.)
>
> What I'm envisioning is that pinning (actually unpinning) a buffer
> increments the counter (up to some limit), and the clock sweep
> decrements it (down to zero), and only buffers with count zero are taken
> by the sweep for recycling. That could work well, but I think the limit
> needs to be relatively small, else we could have the clock sweep having
> to go around many times before it finally frees a buffer. Any thoughts
> about that? Anyone seen any papers about this sort of algorithm?

One idea would be for the clock to check X% of the buffer cache and just
recycle the page it saw with the lowest usage count.

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