Are four cores required to surpass one ?

My recent post about running unit tests in parallel has led to some interesting food for thought:

When running regular unit tests, running with 2 cores seems to be no faster than one core. 4 cores (and arguably a newer CPU architecture) had a significant impact, where performance nearly doubled. The hyperthreaded quad-core i7 920 was *slower* than a 3Ghz core 2 duo when running single threaded, but nearly twice as fast in a multithreaded scenario. What's happening ?

I looked into the issue with jprofiler, but there is no obvious lock contention or other clearly visible bottleneck. The only explanation I can think of so far is this:

The test I was running was extremely simplistic, and there was no real cpu requirement in running the test methods. So the overhead of creating thread pools and all that actually amounts to just as much computation as running the tests. Likely ?

After my summer vacation, I will try to find out. I probably need to run with more realistic test cases. I figured I'd build surefire with threads, because thats a fairly heavy build. In the mean time, your thoughts are welcome.

From legoland,

Kristian