avoid divide-by-zero in GPUUtilizationTracker (#999)

docs/tutorial/summary.md

@@ -61,3 +61,13 @@ As long as the type of data is supported, the data will be dispatched to and log
 As a result, tensorboard will show not only summaries in the graph, but also your custom data.
 For example, a precise validation error often needs to be computed manually, outside the TensorFlow graph.
 With a uniform monitor backend, this number will show up in tensorboard as well.
+
+### Textual Logs
+
+```python
+from tensorpack.utils import logger
+# logger has the methods of Python's logging.Logger
+logger.info("Hello World!")
+```
+
+See [APIs of utils.logger](../modules/utils.html#module-tensorpack.utils.logger)

tensorpack/callbacks/prof.py

@@ -105,9 +105,10 @@ class GPUUtilizationTracker(Callback):
                         if stop_evt.is_set():   # or on exit
                             return
                         evt.clear()
-                        # Ignore the last datapoint. Usually is zero, makes us underestimate the util.
-                        stats -= data
-                        cnt -= 1
+                        if cnt > 1:
+                            # Ignore the last datapoint. Usually is zero, makes us underestimate the util.
+                            stats -= data
+                            cnt -= 1
                         rst_queue.put(stats / cnt)
                         break