title: OpenCL local_work_size NULL
body: <p>When enqueueing an OpenCL kernel, <code>local_work_size</code> can be set to <code>NULL</code>, in which case the OpenCL implementation will determine how to break the global work-items into appropriate work-group instances.  </p>
  
  <p>Automatically calculating the <code>local_work_size</code> seems like a great feature (better than guessing a multiple of 64).  </p>
  
  <p>Does OpenCL's work group size choice tend to be optimal?  Are there cases where it would be better to manually specify <code>local_work_size</code>?</p>