Robert Haas wrote:
On Mon, Aug 10, 2009 at 11:19 AM, Kevin Grittner<Kevin.Grittner@xxxxxxxxxxxx> wrote:
(2) Somehow use effective_cache_size in combination with some sort of
current activity metrics to dynamically adjust random access costs.
(I know, that one's total hand-waving, but it seems to have some
possibility of better modeling reality than what we currently do.)
I was disappointed when I learned that effective_cache_size doesn't get
generally used to predict the likelihood of a buffer fetch requiring
physical io.
Yeah, I gave a lightning talk on this at PGcon, but I haven't had time
to do anything with it. There are a couple of problems. One is that
you have to have a source for your current activity metrics. Since a
lot of the pages of interest will be in the OS buffer pool rather than
PG shared buffers, there's no easy way to handle this
While there are portability concerns, mmap + mincore works across BSD,
Linux, Solaris and will return a vector of file pages in the OS buffer
pool. So it's certainly possible that on supported systems, an activity
monitor can have direct knowledge of OS caching effectiveness on a per
relation/index basis.
--
-Devin
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