On Mon, Nov 8, 2010 at 8:33 AM, umut orhan <umut_angelfire@xxxxxxxxx> wrote: > Hi all, > I've collected some interesting results during my experiments which I > couldn't figure out the reason behind them and need your assistance. > I'm running PostgreSQL 9.0 on a quad-core machine having two level on-chip > cache hierarchy. PostgreSQL has a large and warmed-up buffer > cache thus, no disk I/O is observed during experiments (i.e. for each query > buffer cache hit rate is 100%). I'm pinning each query/process to an > individual core. Queries are simple read-only queries (only selects). Nested > loop (without materialize) is used for the join operator. > When I pin a single query to an individual core, its execution time is > observed as 111 seconds. This result is my base case. Then, I fire two > instances of the same query concurrently and pin them to two different cores > separately. However, each execution time becomes 132 seconds in this case. > In a similar trend, execution times are increasing for three instances (164 > seconds) and four instances (201 seconds) cases too. What I was expecting is > a linear improvement in throughput (at least). I tried several different > queries and got the same trend at each time. > I wonder why execution times of individual queries are increasing when I > increase the number of their instances. > Btw, I don't think on-chip cache hit/miss rates make a difference since L2 > cache misses are decreasing as expected. I'm not an expert in PostgreSQL > internals. Maybe there is a lock-contention (spinlocks?) occurring even if > the queries are read-only. Anyways, all ideas are welcome. My guess would be it's memory contention. What architecture is your quad core cpu? -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
