Re: WIP: 2nd-generation buffer ring patch

On Tue, 29 May 2007, Tom Lane wrote:

> Do we have any decent way of measuring the effectiveness of the
> clock-sweep allocation algorithm?

I put a view on top of the current pg_buffercache (now that it include
usage_count) that shows what the high usage_count buffers consist of.
Since they were basically what I hoped for (like plenty of index blocks on
popular tables) that seemed a reasonable enough measure of effectiveness
for my purposes. I briefly looked into adding some internal measurements
in this area, like how many buffers are scanned on average to satisfy an
allocation request; that would actually be easy to add to the buffer
allocation stats part of the auto bgwriter_max_pages patch I submitted
recently.

Based on my observations of buffer cache statistics, the number of pinned
buffers at any time is small enough that in a reasonably sized buffer
cache, I wouldn't expect a change in the pinned usage_count behavior to
have any serious impact. With what you're adjusting, the only time I can
think of that there would be a noticable shift in fairness would be if
ones buffer cache was very small relative to the number of clients, which
is kind of an unreasonable situation to go out of your way to accommodate.

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD