Test Suite Performance

I was trying to gauge what the best setup for a PIFR node is, and I noticed that this page (http://qa.drupal.org/node/62) seemed way out of date, so I ran some tests of my own. Here I was trying to figure out a couple of things:
- Which concurrency setting is best?
- How does performance improve with number of cores

For testing, I used two separate PIFR nodes running in KVM virtual machines with the following specs:

CPUs are Opteron 6172, 2.1 Ghz
2 GB RAM
Local disk (10K SAS)
Running Debian Squeeze 64-bit, installed PIFR using DamZ's puppet setup

My results were as follows:

With 8 CPUs:
Concurrency = 9, enable test took 25:23

With 4 CPUs:
Concurrency = 3, enable test took 49:06 (retest took: 46:29)
Concurrency = 4, enable test took 37:20
Concurrency = 5, enable test took 38:20

With 2 CPUs:
Concurrency = 2, enable test took 1:08:01
Concurrency = 3, enable test took 1:10:13

Note that http://qa.drupal.org/performance-tuning-tips-for-D7 claims concurrency should be set to [number of CPU cores] + 1 (e.g. set to 5 for a 4-core system). From my tests with 2- and 4-core systems, it looks like it is actually better to set concurrency to be equal to the number of cores. Also, improvement isn't linear with number of cores -- rfay had mentioned this was the case, but I wanted to be sure. So doubling the cores doesn't quite double the speed (though it does get "close").