Why 4 threads run faster than 2 on my dual core?

I've built a ray tracer in Java. It generates images like this:



(this is not mine, it's just an example). I multithreaded it by cutting up the image horizontally and telling each thread to render a chunk of the final image. I did quite a few tests and these are the average times:

1 thread: 35 seconds
2 threads: 21 seconds
4 threads: 20 seconds
8 threads: 21 seconds

I own a Intel Wolfdale @ 3.0 GHz (Dual Core). The improvement when going to 2 threads is obvious, but I can't make sense of why going to 4 threads makes it go even faster. I thought Intel didn't have hyperthreading since Pentium IV, so my 2 cores mean that I can only run 2 threads simultaneously. The dip in 8 threads makes sense because of more context switching.

Seems like you didn't do enough runs of it, and you're within the % error of your test sample.

I just tested the same program 6 times. I think it's pretty conclusive that 4 threads is faster than 2.

Two threads:
59.203 s
59.468 s
60.468 s

Four threads:
53.204 s
56.109 s
52.422 s

It is possible that caching either of the disk or the CPU is providing a slight benefit for 4 threads over 2.

One thing to remember is that even with a single core, the Processor does not run a thread to completion before starting the next one. Instead, it runs part of one thread, then part of the next and so on until everything is done. Therefore, it may be that it happens to be more efficient to do the first part of multiple threads and then do the second part of them than to do parts one and two of the first and then part one and two of the second.