Re: cache estimates, cache access cost

Cédric Villemain wrote:
> http://git.postgresql.org/gitweb?p=users/c2main/postgres.git;a=shortlog;h=refs/heads/analyze_cache
>

This rebases easily to make Cedric's changes move to the end; I just
pushed a version with that change to
https://github.com/greg2ndQuadrant/postgres/tree/analyze_cache if anyone
wants a cleaner one to browse. I've attached a patch too if that's more
your thing.

I'd recommend not getting too stuck on the particular hook Cédric has
added here to compute the cache estimate, which uses mmap and mincore to
figure it out. It's possible to compute similar numbers, albeit less
accurate, using an approach similar to how pg_buffercache inspects
things. And I even once wrote a background writer extension that
collected this sort of data as it was running the LRU scan anyway.
Discussions of this idea seem to focus on how the "what's in the cache?"
data is collected, which as far as I'm concerned is the least important
part. There are multiple options, some work better than others, and
there's no reason that can't be swapped out later. The more important
question is how to store the data collected and then use it for
optimizing queries.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us