Add experimental tf.data sleep tuning for better performance (tensorflow#6634)

* Introduce a short sleep before ds.prefetch in tf.data.

* Further limit dataset threads to reduce CPU contention

* Tuned dataset sleep time

* Rename dataset sleep flag; enable it only for Keras Graph mode

Author: haoyuz

official/resnet/keras/keras_common.py

@@ -150,10 +150,11 @@ def set_gpu_thread_mode_and_count(flags_obj):
   # Limit data preprocessing threadpool to CPU cores minus number of total GPU
   # private threads and memory copy threads.
   total_gpu_thread_count = per_gpu_thread_count * flags_obj.num_gpus
-  num_mem_copy_threads = flags_obj.num_gpus
+  num_runtime_threads = flags_obj.num_gpus
   if not flags_obj.datasets_num_private_threads:
-    flags_obj.datasets_num_private_threads = (cpu_count - total_gpu_thread_count
-                                              - num_mem_copy_threads)
+    flags_obj.datasets_num_private_threads = min(
+        cpu_count - total_gpu_thread_count - num_runtime_threads,
+        flags_obj.num_gpus * 8)
     tf.compat.v1.logging.info('Set datasets_num_private_threads to %s',
                               flags_obj.datasets_num_private_threads)
 
@@ -283,6 +284,13 @@ def define_keras_flags():
       'triggers the profiler to process 3 steps, starting from the 2nd step. '
       'Note that profiler has a non-trivial performance overhead, and the '
       'output file can be gigantic if profiling many steps.')
+  flags.DEFINE_boolean(
+      name='data_prefetch_with_slack', default=False,
+      help='Add a small delay in tf.data prefetch to prioritize memory copy of '
+      'other tensors over the data minibatch for the (T+1)th step. It should '
+      'help improve performance using EagerIterator and function. The codepath '
+      'when enabling this feature is experimental and will be removed once the '
+      'corresponding performance features are fully supported in TensorFlow.')
 
 
 def get_synth_input_fn(height, width, num_channels, num_classes,
@@ -341,6 +349,12 @@ def is_v2_0():
   return tf.__version__.startswith('2')
 
 
+def data_prefetch_with_slack():
+  """Use unstable code for perf tuning purposes."""
+  if not FLAGS.use_synthetic_data:
+    _monkey_patch_org_create_device_dataset()
+
+
 def _monkey_patch_org_assert_broadcastable():
   """Monkey-patch `assert_broadcast` op to avoid OOM when enabling XLA."""
   def no_op_assert_broadcastable(weights, values):
@@ -362,3 +376,29 @@ def _undo_monkey_patch_org_assert_broadcastable():
   if hasattr(weights_broadcast_ops, 'org_assert_broadcastable'):
     weights_broadcast_ops.assert_broadcastable = (
         weights_broadcast_ops.org_assert_broadcastable)
+
+
+# TODO(haoyuzhang): remove this monkey patch when the "prefetch with slack"
+# feature is available in tf.data.
+def _monkey_patch_org_create_device_dataset():
+  """Monkey-patch `_create_device_dataset` method with delayed prefetch."""
+
+  import ast  # pylint: disable=g-import-not-at-top
+  import inspect  # pylint: disable=g-import-not-at-top
+  from tensorflow.python.data.ops import multi_device_iterator_ops  # pylint: disable=g-import-not-at-top
+
+  tf.compat.v1.logging.info(
+      'Using monkey-patched version of MultiDeviceIterator. It should be '
+      'removed when the prefetch with slack feature is implemented in tf.data.')
+  cls_multi_device_iterator = ast.parse(
+      inspect.getsource(multi_device_iterator_ops.MultiDeviceIterator))
+  org_create_device_dataset_code = inspect.getsource(
+      multi_device_iterator_ops.MultiDeviceIterator._create_device_dataset)  # pylint: disable=protected-access
+  code_lines = org_create_device_dataset_code.split('\n')
+  # Insert in reverse order to avoid line number shift by previous insertions
+  code_lines.insert(5, '      ds = ds.apply(sleep_ops.sleep(11000))')  # 11ms
+  code_lines.insert(2, '    from tensorflow.python.data.experimental.ops import sleep as sleep_ops')  # pylint: disable=line-too-long
+  patched_code = '\n'.join(line[2:] for line in code_lines)
+  cls_multi_device_iterator.body[0].body[2] = ast.parse(patched_code).body[0]
+  exec(compile(cls_multi_device_iterator, '<string>', 'exec'),  # pylint: disable=exec-used
+       multi_device_iterator_ops.__dict__)

official/resnet/keras/keras_imagenet_benchmark.py

@@ -251,6 +251,21 @@ def benchmark_xla_1_gpu_fp16(self):
     FLAGS.batch_size = 256
     self._run_and_report_benchmark()
 
+  def benchmark_xla_1_gpu_fp16_tweaked(self):
+    """Test Keras model with XLA, 1 GPU, fp16, and manual config tuning."""
+    self._setup()
+
+    FLAGS.num_gpus = 1
+    FLAGS.enable_eager = True
+    FLAGS.enable_xla = True
+    FLAGS.distribution_strategy = 'default'
+    FLAGS.model_dir = self._get_model_dir('benchmark_xla_1_gpu_fp16_tweaked')
+    FLAGS.dtype = 'fp16'
+    FLAGS.batch_size = 256
+    FLAGS.tf_gpu_thread_mode = 'gpu_private'
+    FLAGS.data_prefetch_with_slack = True
+    self._run_and_report_benchmark()
+
   def benchmark_xla_1_gpu_fp16_dynamic(self):
     """Test Keras model with XLA, 1 GPU, fp16, and dynamic loss scaling."""
     self._setup()
@@ -313,6 +328,23 @@ def benchmark_graph_xla_1_gpu_fp16(self):
     FLAGS.batch_size = 256
     self._run_and_report_benchmark()
 
+  def benchmark_graph_xla_1_gpu_fp16_tweaked(self):
+    """Test Keras model in legacy graph mode with 1 GPU, fp16, XLA, and manual
+       config tuning.
+    """
+    self._setup()
+
+    FLAGS.num_gpus = 1
+    FLAGS.enable_eager = False
+    FLAGS.enable_xla = True
+    FLAGS.distribution_strategy = 'default'
+    FLAGS.model_dir = self._get_model_dir(
+        'benchmark_graph_xla_1_gpu_fp16_tweaked')
+    FLAGS.dtype = 'fp16'
+    FLAGS.batch_size = 256
+    FLAGS.tf_gpu_thread_mode = 'gpu_private'
+    self._run_and_report_benchmark()
+
   def benchmark_8_gpu(self):
     """Test Keras model with 8 GPUs."""
     self._setup()
@@ -334,6 +366,7 @@ def benchmark_8_gpu_tweaked(self):
     FLAGS.model_dir = self._get_model_dir('benchmark_8_gpu_tweaked')
     FLAGS.batch_size = 128 * 8  # 8 GPUs
     FLAGS.datasets_num_private_threads = 14
+    FLAGS.data_prefetch_with_slack = True
     self._run_and_report_benchmark()
 
   def benchmark_xla_8_gpu(self):
@@ -371,6 +404,7 @@ def benchmark_8_gpu_fp16_tweaked(self):
     FLAGS.model_dir = self._get_model_dir('benchmark_8_gpu_fp16_tweaked')
     FLAGS.batch_size = 256 * 8  # 8 GPUs
     FLAGS.tf_gpu_thread_mode = 'gpu_private'
+    FLAGS.data_prefetch_with_slack = True
     self._run_and_report_benchmark()
 
   def benchmark_8_gpu_fp16_dynamic_tweaked(self):
@@ -386,6 +420,7 @@ def benchmark_8_gpu_fp16_dynamic_tweaked(self):
     FLAGS.batch_size = 256 * 8  # 8 GPUs
     FLAGS.loss_scale = 'dynamic'
     FLAGS.tf_gpu_thread_mode = 'gpu_private'
+    FLAGS.data_prefetch_with_slack = True
     self._run_and_report_benchmark()
 
   def benchmark_xla_8_gpu_fp16(self):
@@ -412,7 +447,8 @@ def benchmark_xla_8_gpu_fp16_tweaked(self):
     FLAGS.distribution_strategy = 'default'
     FLAGS.model_dir = self._get_model_dir('benchmark_xla_8_gpu_fp16_tweaked')
     FLAGS.batch_size = 256 * 8  # 8 GPUs
-    FLAGS.tf_gpu_thread_mode = 'gpu_private'
+    # FLAGS.tf_gpu_thread_mode = 'gpu_private'
+    FLAGS.data_prefetch_with_slack = True
     self._run_and_report_benchmark()
 
   def benchmark_xla_8_gpu_fp16_dynamic_tweaked(self):
@@ -429,6 +465,7 @@ def benchmark_xla_8_gpu_fp16_dynamic_tweaked(self):
     FLAGS.batch_size = 256 * 8  # 8 GPUs
     FLAGS.loss_scale = 'dynamic'
     FLAGS.tf_gpu_thread_mode = 'gpu_private'
+    FLAGS.data_prefetch_with_slack = True
     self._run_and_report_benchmark()
 
   def benchmark_xla_8_gpu_fp16_tensorboard_tweaked(self):
@@ -444,6 +481,7 @@ def benchmark_xla_8_gpu_fp16_tensorboard_tweaked(self):
         'benchmark_xla_8_gpu_fp16_tensorboard_tweaked')
     FLAGS.batch_size = 256 * 8  # 8 GPUs
     FLAGS.tf_gpu_thread_mode = 'gpu_private'
+    FLAGS.data_prefetch_with_slack = True
     FLAGS.enable_tensorboard = True
     self._run_and_report_benchmark()
 
@@ -636,6 +674,7 @@ def benchmark_8_gpu_tweaked(self):
     FLAGS.model_dir = self._get_model_dir('benchmark_8_gpu_tweaked')
     FLAGS.batch_size = 256 * 8
     FLAGS.tf_gpu_thread_mode = 'gpu_private'
+    FLAGS.data_prefetch_with_slack = True
     self._run_and_report_benchmark()
 
   def benchmark_graph_8_gpu(self):

official/resnet/keras/keras_imagenet_main.py

@@ -107,6 +107,8 @@ def run(flags_obj):
   # Execute flag override logic for better model performance
   if flags_obj.tf_gpu_thread_mode:
     keras_common.set_gpu_thread_mode_and_count(flags_obj)
+  if flags_obj.data_prefetch_with_slack:
+    keras_common.data_prefetch_with_slack()
 
   dtype = flags_core.get_tf_dtype(flags_obj)
   if dtype == 'float16':