Windows multi-threading performance issues on multi-core systems only

[Dan]

I have a question regarding performance issue I am seeing on multicore Windows systems.  I am creating many threads to do parallel tasks, and on multicore Windows systems the performance is abysmal.  If I use task manager to set the processor affinity to a single CPU, the program runs as I would expect.  Without that, it takes about 10 times as long to complete.

Am I doing something wrong?  I have tried DMD 2.0.37 and DMD 1.0.53 with the same results, running the binary on both a dual-core P4 and a newer Core2 duo.

Any help is greatly appreciated!