On Tue, Apr 12, 2011 at 8:23 AM, Merlin Moncure <mmoncure@xxxxxxxxx> wrote: > On Tue, Apr 12, 2011 at 3:54 AM, Glyn Astill <glynastill@xxxxxxxxxxx> wrote: >> --- On Tue, 12/4/11, Merlin Moncure <mmoncure@xxxxxxxxx> wrote: >> >>> >> The issue I'm seeing is that 8 real cores >>> outperform 16 real >>> >> cores, which outperform 32 real cores under high >>> concurrency. >>> > >>> > With every benchmark I've done of PostgreSQL, the >>> "knee" in the >>> > performance graph comes right around ((2 * cores) + >>> > effective_spindle_count). With the database fully >>> cached (as I >>> > believe you mentioned), effective_spindle_count is >>> zero. If you >>> > don't use a connection pool to limit active >>> transactions to the >>> > number from that formula, performance drops off. The >>> more CPUs you >>> > have, the sharper the drop after the knee. >>> >>> I was about to say something similar with some canned >>> advice to use a >>> connection pooler to control this. However, OP >>> scaling is more or >>> less topping out at cores / 4...yikes!. Here are my >>> suspicions in >>> rough order: >>> >>> 1. There is scaling problem in client/network/etc. >>> Trivially >>> disproved, convert the test to pgbench -f and post results >>> 2. The test is in fact i/o bound. Scaling is going to be >>> hardware/kernel determined. Can we see >>> iostat/vmstat/top snipped >>> during test run? Maybe no-op is burning you? >> >> This is during my 80 clients test, this is a point at which the performance is well below that of the same machine limited to 8 cores. >> >> http://www.privatepaste.com/dc131ff26e >> >>> 3. Locking/concurrency issue in heavy_seat_function() >>> (source for >>> that?) how much writing does it do? >>> >> >> No writing afaik - its a select with a few joins and subqueries - I'm pretty sure it's not writing out temp data either, but all clients are after the same data in the test - maybe theres some locks there? >> >>> Can we see some iobound and cpubound pgbench runs on both >>> servers? >>> >> >> Of course, I'll post when I've gotten to that. > > Ok, there's no writing going on -- so the i/o tets aren't necessary. > Context switches are also not too high -- the problem is likely in > postgres or on your end. > > However, I Would still like to see: > pgbench select only tests: > pgbench -i -s 1 > pgbench -S -c 8 -t 500 > pgbench -S -c 32 -t 500 > pgbench -S -c 80 -t 500 > > pgbench -i -s 500 > pgbench -S -c 8 -t 500 > pgbench -S -c 32 -t 500 > pgbench -S -c 80 -t 500 > > write out bench.sql with: > begin; > select * from heavy_seat_function(); > select * from heavy_seat_function(); > commit; > > pgbench -n bench.sql -c 8 -t 500 > pgbench -n bench.sql -c 8 -t 500 > pgbench -n bench.sql -c 8 -t 500 whoops: pgbench -n bench.sql -c 8 -t 500 pgbench -n bench.sql -c 32 -t 500 pgbench -n bench.sql -c 80 -t 500 merlin -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
