Non blocking kernel launches in OpenCL intel implementation - opencl

Question

I have the following skeleton code
ret = clEnqueueNDRangeKernel(command_queue, kernel, 1, NULL,
&global_item_size,NULL,0, NULL, NULL);
printf("print immediately\n ");
I thought and read somewhere that clEnqueueNDRangeKernel is non blocking call and cpu continues its execution immediately after enqueuing the kernel.
But I see a different behaviour. printf statement executes after kernel completes execution. Why am I seeing this behaviour?. How to make any kernel calls non blocking?.

Answer 1

Yes, clEnqueueNDRangeKernel() is supposed to be non-blocking. However, the code you show does not allow to definitively conclude that the kernel finishes before the printf statement. There's several possibilities:
The kernel is not enqueued properly or fails to run. You need to check if the return value ret is CL_SUCCESS, and if not, fix whatever needs to be fixed and try again.
The kernel runs fast and the thread on which the kernel runs is likely to be given priority, such that the printf statement ends up being executed after the kernel finishes.
The kernel is actually running during the printf statement, since nothing in your code allows you to conclude otherwise. To check if the kernel is running or finished, you need to use an event. For example:
cl_event evt = NULL;
cl_int ret, evt_status;
// ...
ret = clEnqueueNDRangeKernel(command_queue, kernel, 1, NULL,
&global_item_size, NULL, 0, NULL, &evt);
// Check if it's finished or not
if (ret == CL_SUCCESS)
{
clGetEventInfo(evt, CL_EVENT_COMMAND_EXECUTION_STATUS,
sizeof(cl_int), (void*) &evt_status, NULL);
if (evt_status == CL_COMPLETE)
printf("Kernel is finished\n");
else
printf("Kernel is NOT finished\n");
}
else
{
printf("Something's wrong: %d\n", ret);
}