can single work items kernels run in parallel on same device

Try putting "start_time = getCurrentTimestamp();" before the kernel invocation loop. I have a feeling your kernel run time is so short that total time is being dominated by the clCreateBuffer() call. Note that on most hardware, using host pointer will result in the OpenCL runtime actually allocating and transferring the whole buffer to device memory. Also considering increasing your input size so that your total run time is at least a few seconds.