OpenCL, do I need to use events to synchronize kernel launches inside the same command queue?

Question

I am unsure if this is necessary. I have two kernels which I need to launch in serial. Do I need to create an event for the first one and then have the second kernel launch wait for that event or can I assume that everything is the queue executes in the order in which I placed it? Is my use of cl_event in the code bellow nessary?

cl_event acceleration_finished;
    cl_check(clEnqueueNDRangeKernel(cmdq, acceleration_kernel, 1, NULL, &acceleration_blocks, 
            &acceleration_threads, 0, NULL, &acceleration_finished));

    cl_event stepper_finished;
    cl_check(clEnqueueNDRangeKernel(cmdq, stepper_kernel, 1, NULL, &N, 
            NULL, 1, &acceleration_finished, &stepper_finished));

    cl_double3* positions_mapped = clEnqueueMapBuffer(cmdq, positions, CL_TRUE, CL_MAP_READ, 0, 
            sizeof(cl_double3) * N, 1, &stepper_finished, NULL, &error);
    cl_check(error);

Answer 1

For your case, you can just assume full in-order execution (if you did not manually enabled out-of-order).

There a re 2 types of queues in OpenCL:

• In-order (default):

  • Tasks are executed in the order they are queued. If any of them blocks for any reason, all the following task will not execute until that one finishes.
  • Events are still being used to check if a given task can start or not.

• Out-of-order (created with the flag CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE):

  • Tasks execute as soon as they are ready to be consumed (all the events they depend on are CL_COMPLETED).

  • This does not mean that N tasks can run in parallel in a single queue if they do not depend on each other. Some hardware does not support that behavior, and requires to create a separate queue to allow parallel execution of tasks. Some other hardware will only support 1 task at a time. That is implementation dependent.

https://www.khronos.org/registry/cl/sdk/1.0/docs/man/xhtml/clCreateCommandQueue.html