Re: Dual core Opterons beating quad core Xeons? - Mailing list pgsql-performance

On Wed, 19 Dec 2007, Stuart Bishop wrote:

> For pgbench (PG 8.2 running Ubuntu), the Opteron is getting about 6x TPS
> over the Xeon (3000+ TPS on Opteron vs ~500 on Xeon). Things get a little
> better for Xeon with PG 8.3 (570-540 TPS).

The 3000+ TPS figure is the correct one for a controller that can cache
writes.  Around 500TPS is normal for a setup without one.  I suspect all
you're testing is the difference between the I/O subsystem in the two
serves, and it's probaby the case that the Opteron disk subsystem caches
writes while the Xeon doesn't.  You haven't drawn any useful conclusions
comparing Xeons and Opterons yet.

Warning:  the system with the write caching can easily be doing that
incorrectly, in a way that can corrupt your database one day.  See
http://momjian.us/main/writings/pgsql/sgml/wal-reliability.html for an
intro and
http://www.westnet.com/~gsmith/content/postgresql/TuningPGWAL.htm for way
more detail.

If you don't have a real disk setup, you can't use the default pgbench
test and expect the results to be useful.  The main thing it does is write
heavily in a way that makes the disk controller and associated I/O the
bottleneck in most cases.

The only useful test you can do right now with pgbench is to pass it the
-S parameter so that it does only reads instead.  That will give you a
much better idea how the CPUs compare.  You still need to be careful about
the database scale relative to the amount of RAM; at some point even the
read test will be limited by disk parameters instead of CPU.  Take a look
at http://www.westnet.com/~gsmith/content/postgresql/pgbench-scaling.htm
for a tutorial on using pgbench to quantify read performance.  Note that
the way I compute the sizes of things in there is a little difficult, one
day I'm going to use some of the suggestions at
http://andreas.scherbaum.la/blog/archives/282-table-size,-database-size.html
to improve that and you should take a look there as well.

You'll also need to vary the number of clients a bit.  You should see the
largest difference between the two servers with around 16 of them (where
the Xeon system has a dedicated core for each while the Opteron has 2
clients/core) while a useful spot to compare the maximum throughput of the
servers will be around 64 clients.

> With PG 8.2 and 8.3, is it still pretty much limited to 8 cores making 2 of
> the quad core Xeons redundant or detrimental?

Where'd you get the idea 8 cores was a limit?  As cores go up eventually
you run out of disk or memory bandwidth, but how that plays out is very
application dependant and there's no hard line anywhere.

> I expect we will be running this hardware for 8.2, 8.3 and 8.4. Anyone aware
> of anything that might change the landscape for 8.4?

8.4 is only in the earliest of planning stages right now, nobody knows
what that will bring yet.

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