On Wed, 2010-08-04 at 14:00 -0400, Tom Lane wrote: > Hannu Krosing <hannu@xxxxxxxxxxxxxxx> writes: > > Of course there are more variables than just *_page_cost, so if you nail > > down any other one, you may end with less than 1 for both page costs. > > > I have always used seq_page_cost = 1 in my thinking and adjusted others > > relative to it. > > Right, seq_page_cost = 1 is sort of the traditional reference point, > but you don't have to do it that way. The main point here is that for > an all-in-RAM database, the standard page access costs are too high > relative to the CPU effort costs: > > regression=# select name, setting from pg_settings where name like '%cost'; > name | setting > ----------------------+--------- > cpu_index_tuple_cost | 0.005 > cpu_operator_cost | 0.0025 > cpu_tuple_cost | 0.01 > random_page_cost | 4 > seq_page_cost | 1 > (5 rows) > > To model an all-in-RAM database, you can either dial down both > random_page_cost and seq_page_cost to 0.1 or so, or set random_page_cost > to 1 and increase all the CPU costs. The former is less effort ;-) > > It should be noted also that there's not all that much evidence backing > up the default values of the cpu_xxx_cost variables. In the past those > didn't matter much because I/O costs always swamped CPU costs anyway. > But I can foresee us having to twiddle those defaults and maybe refine > the CPU cost model more, as all-in-RAM cases get more common. Especially the context switch + copy between shared buffers and system disk cache will become noticeable at these speeds. An easy way to test it is loading a table with a few indexes, once with a shared_buffers value, which is senough for only the main table and once with one that fits both table and indexes, > regards, tom lane -- Hannu Krosing http://www.2ndQuadrant.com PostgreSQL Scalability and Availability Services, Consulting and Training -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
