Re: Performance on Sun Fire X4150 x64 (dd, bonnie++, pgbench) - Mailing list pgsql-performance

On Sat, 19 Jul 2008, Stephane Bailliez wrote:

> OS is Ubuntu 7.10 x86_64 running  2.6.22-14

Note that I've had some issues with the desktop Ubuntu giving slower
results in tests like this than the same kernel release using the stock
kernel parameters.  Haven't had a chance yet to see how the server Ubuntu
kernel fits into that or exactly what the desktop one is doing wrong yet.
Could be worse--if you were running any 8.04 I expect your pgbench results
would be downright awful.

> data is on xfs noatime

While XFS has some interesting characteristics, make sure you're
comfortable with the potential issues the journal approach used by that
filesystem has.  With ext3, you can choose the somewhat risky writeback
behavior or not, you're stuck with it in XFS as far as I know.  A somewhat
one-sided intro here is at
http://zork.net/~nick/mail/why-reiserfs-is-teh-sukc

> postgresql 8.2.9 with data and xlog as mentioned above

There are so many known performance issues in 8.2 that are improved in 8.3
that I'd suggest you really should be considering it for a new install at
this point.

> Script running over scaling factor 1 to 1000 and running 3 times pgbench with
> "pgbench -t 2000 -c 8 -S pgbench"

In general, you'll want to use a couple of clients per CPU core for
pgbench tests to get a true look at the scalability.  Unfortunately, the
way the pgbench client runs means that it tends to top out at 20 or 30
thousand TPS on read-only tests no matter how many cores you have around.
But you may find operations where peak throughput comes at closer to 32
clients here rather than just 8.

> It's a bit limited and will try to do a much much longer run and increase the
> # of tests and calculate mean and stddev as I have a pretty large variation
> for the 3 runs sometimes (typically for the scaling factor at 1000, the runs
> are respectively 1952, 940, 3162)  so the graph is pretty ugly.

This is kind of a futile exercise and I wouldn't go crazy trying to
analyze here.  Having been through that many times, I predict you'll
discover no real value to a more statistically intense analysis.  It's not
like sampling at more points makes the variation go away, or that the
variation itself has some meaning worth analyzing.  Really the goal of
pgbench tests should be look at a general trend.  Looking at your data for
example, I'd say the main useful observation to draw from your tests is
that performance is steady then drops off sharply once the database itself
exceeds 10GB, which is a fairly positive statement that you're getting
something out of most of the the 16GB of RAM in the server during this
test.

As far as the rest of your results go, Luke's comment that you may need
more than one process to truly see the upper limit of your disk
performance is right on target.  More useful commentary on that issue I'd
recomend is near the end of

http://www.commandprompt.com/blogs/joshua_drake/2008/04/is_that_performance_i_smell_ext2_vs_ext3_on_50_spindles_testing_for_postgresql/

(man does that need to be a smaller URL)

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD