2011/1/19 Bruce Momjian <bruce@xxxxxxxxxx>: > Tom Lane wrote: >> Robert Haas <robertmhaas@xxxxxxxxx> writes: >> > On Fri, Nov 12, 2010 at 4:15 AM, Cédric Villemain >> > <cedric.villemain.debian@xxxxxxxxx> wrote: >> >>> I wondering if we could do something with a formula like 3 * >> >>> amount_of_data_to_read / (3 * amount_of_data_to_read + >> >>> effective_cache_size) = percentage NOT cached. That is, if we're >> >>> reading an amount of data equal to effective_cache_size, we assume 25% >> >>> caching, and plot a smooth curve through that point. In the examples >> >>> above, we would assume that a 150MB read is 87% cached, a 1GB read is >> >>> 50% cached, and a 3GB read is 25% cached. >> >> >> But isn't it already the behavior of effective_cache_size usage ? >> >> > No. >> >> I think his point is that we already have a proven formula >> (Mackert-Lohmann) and shouldn't be inventing a new one out of thin air. >> The problem is to figure out what numbers to apply the M-L formula to. >> >> I've been thinking that we ought to try to use it in the context of the >> query as a whole rather than for individual table scans; the current >> usage already has some of that flavor but we haven't taken it to the >> logical conclusion. > > Is there a TODO here? it looks like, yes. > > -- > Bruce Momjian <bruce@xxxxxxxxxx> http://momjian.us > EnterpriseDB http://enterprisedb.com > > + It's impossible for everything to be true. + > -- Cédric Villemain 2ndQuadrant http://2ndQuadrant.fr/ ; PostgreSQL : Expertise, Formation et Support -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
