message: [tf.data] Improving auto-tuning logging.
diff: @@ -439,7 +439,15 @@ void Model::Optimize(int64 cpu_budget) {
  continue;
  }
  pair.second->value++;
  - int64 delta = output_time - OutputTime(snapshot);
  + int64 new_output_time = OutputTime(snapshot);
  + int64 delta = output_time - new_output_time;
  + if (delta < 0) {
  + VLOG(3) << "Increasing the parallelism of tunable parameter "
  + << pair.first << " resulted in slowdown (before=" << output_time
  + << ", after=" << new_output_time
  + << "). This should never happen because the latency "
  + "should be monotonic w.r.t. to parallelism.";
  + }
  if (delta > best_delta) {
  best_delta = delta;
  best_parameter = pair.second.get();
  