OpenMP线程在同一个cpu核心上执行

I'm currently parallelizing program using openmp on a 4-core phenom2. However I noticed that my parallelization does not do anything for the performance. Naturally I assumed I missed something (falsesharing, serialization through locks, ...), however I was unable to find anything like that. Furthermore from the CPU Utilization it seemed like the program was executed on only one core. From what I found I'm currently parallelizing program using openm