On Wed, Feb 27, 2013 at 3:15 AM, Costin Oproiu <costin.oproiu@gmail.com> wrote:
> I took some time to figure out a reasonable tuning for my fresh 9.2.3
> installation when I've noticed the following:
>
> [costin@fsr costin]$ /home/pgsql/bin/pgbench -h 192.1.1.2 -p 5432 -U
> postgres -i -s 1
> ...
> 100000 tuples done.
> ...
> vacuum...done.
> [costin@fsr costin]$ /home/pgsql/bin/pgbench -h 192.1.1.2 -p 5432 -U
> postgres -c 32 -t 5000
> ...
> tps = 245.628075 (including connections establishing)
> tps = 245.697421 (excluding connections establishing)
> ...
> [costin@fsr costin]$ /home/pgsql/bin/pgbench -h 192.1.1.2 -p 5432 -U
> postgres -i -s 100
> ...
> 10000000 tuples done.
> ...
> vacuum...done.
> [costin@fsr costin]$ /home/pgsql/bin/pgbench -h 192.1.1.2 -p 5432 -U
> postgres -c 32 -t 5000
> ...
> tps = 1125.035567 (including connections establishing)
> tps = 1126.490634 (excluding connections establishing)
>
> 32 connections makes a comfortable load for the 8 core 4GB production
> server, a rather old machine. I kept testing for almost two days with
> various configuration parameters. In the beginning I was warned to
> increase the checkpoint_segments, which is now 32. The results were
> consistent and always showing small scale test (-s 1) at about 245-248
> tps while big scale test (-s 100) at least 4 and up to 7 times
> better.
>
> According to top, at small scale tests, server processes are doing a
> lot of UPDATE waiting. A "select relation::regclass, * from pg_locks
> where not granted" showed frequent contention on tellers rows.
>
> First, I've got no good explanation for this and it would be nice to
> have one. As far as I can understand this issue, the heaviest update
> traffic should be on the branches table and should affect all tests.
>
Its not very surprising. The smallest table in the test i.e.
pgbench_branches has the number of rows equal to the scale factor.
When you test with scale factor 1 and 32 clients, all those clients
are contending to update that single row in the table. Since a
transaction must wait for the other updating transaction before it can
update the same row, you would get a almost linear behaviour in this
test. You may actually want to test with just 1 or 5 or 10 clients and
my gut feel is you will still get the same or similar tps.
As the scale factor is increased, the contention on the smaller tables
reduces and you will start seeing an increase in the tps as you
increase the number of clients. Of course, beyond a point either it
will flatten out or even go down.
While testing with pgbench, its recommended that the scale factor
should be set larger than the number of clients.
Thanks,
Pavan
--
Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee
