Program run on CPU and GPU

If i run the program on CPU having say 2 cores,which i ran on gpu having many cores and in program all threads ran simultaneously,but .how it will be run on CPU? if same program is run on both GPU and CPU…so it will run only on single core of CPU or both cores of CPU simultaneously …

It depends on the particular OpenCL runtime, but yes, in general, when you run an OpenCL kernel on CPU, it will run on all cores, and in some cases will also take advantage of vector instructions (such as SSE/AVX) to run multiple work items per core.