Sync BatchNorm statistics with nccl or horovod

tensorpack/models/_old_batch_norm.py

@@ -36,7 +36,6 @@ def get_bn_variables(n_out, use_scale, use_bias, gamma_init):
 
 def update_bn_ema(xn, batch_mean, batch_var,
                   moving_mean, moving_var, decay, internal_update):
-    # TODO is there a way to use zero_debias in multi-GPU?
     update_op1 = moving_averages.assign_moving_average(
         moving_mean, batch_mean, decay, zero_debias=False,
         name='mean_ema_op')
@@ -147,7 +146,6 @@ def BatchNorm(inputs, training=None, momentum=0.9, epsilon=1e-5,
                     mean=moving_mean, variance=moving_var, epsilon=epsilon,
                     data_format=data_format, is_training=False)
             else:
-                # avoid the reshape if possible (when channel is the last dimension)
                 xn = tf.nn.batch_normalization(
                     inputs, moving_mean, moving_var, beta, gamma, epsilon)
 

tensorpack/models/batch_norm.py

@@ -4,6 +4,9 @@
 
 import tensorflow as tf
 from tensorflow.contrib.framework import add_model_variable
+from tensorflow.python.training import moving_averages
+import re
+import six
 
 from ..utils import logger
 from ..utils.argtools import get_data_format
@@ -19,6 +22,42 @@ __all__ = ['BatchNorm', 'BatchRenorm']
 # eps: torch: 1e-5. Lasagne: 1e-4
 
 
+def get_bn_variables(n_out, use_scale, use_bias, beta_init, gamma_init):
+    if use_bias:
+        beta = tf.get_variable('beta', [n_out], initializer=beta_init)
+    else:
+        beta = tf.zeros([n_out], name='beta')
+    if use_scale:
+        gamma = tf.get_variable('gamma', [n_out], initializer=gamma_init)
+    else:
+        gamma = tf.ones([n_out], name='gamma')
+    # x * gamma + beta
+
+    moving_mean = tf.get_variable('mean/EMA', [n_out],
+                                  initializer=tf.constant_initializer(), trainable=False)
+    moving_var = tf.get_variable('variance/EMA', [n_out],
+                                 initializer=tf.constant_initializer(1.0), trainable=False)
+    return beta, gamma, moving_mean, moving_var
+
+
+def update_bn_ema(xn, batch_mean, batch_var,
+                  moving_mean, moving_var, decay, internal_update):
+    update_op1 = moving_averages.assign_moving_average(
+        moving_mean, batch_mean, decay, zero_debias=False,
+        name='mean_ema_op')
+    update_op2 = moving_averages.assign_moving_average(
+        moving_var, batch_var, decay, zero_debias=False,
+        name='var_ema_op')
+
+    if internal_update:
+        with tf.control_dependencies([update_op1, update_op2]):
+            return tf.identity(xn, name='output')
+    else:
+        tf.add_to_collection(tf.GraphKeys.UPDATE_OPS, update_op1)
+        tf.add_to_collection(tf.GraphKeys.UPDATE_OPS, update_op2)
+        return tf.identity(xn, name='output')
+
+
 @layer_register()
 @convert_to_tflayer_args(
     args_names=[],
@@ -35,20 +74,30 @@ def BatchNorm(inputs, axis=None, training=None, momentum=0.9, epsilon=1e-5,
               gamma_initializer=tf.ones_initializer(),
               virtual_batch_size=None,
               data_format='channels_last',
-              internal_update=False):
+              internal_update=False,
+              sync_statistics=None):
     """
-    Mostly equivalent to `tf.layers.batch_normalization`, but different in
-    the following due to historical reasons:
+    Almost equivalent to `tf.layers.batch_normalization`, but different (and more powerful)
+    in the following:
 
-    1. Accepts `data_format` when `axis` is None. For 2D input, this argument will be ignored.
+    1. Accepts an alternative `data_format` option when `axis` is None. For 2D input, this argument will be ignored.
     2. Default value for `momentum` and `epsilon` is different.
-    3. Default value for `training` is automatically obtained from `TowerContext`.
-    4. Support the `internal_update` option, which can be very useful in certain models.
+    3. Default value for `training` is automatically obtained from tensorpack's `TowerContext`, but can be overwritten.
+    4. Support the `internal_update` option, which enables the use of BatchNorm layer inside conditionals.
+    5. Support the `sync_statistics` option, which is very useful in small-batch models.
 
     Args:
         internal_update (bool): if False, add EMA update ops to
             `tf.GraphKeys.UPDATE_OPS`. If True, update EMA inside the layer
             by control dependencies.
+            They are very similar in speed, but `internal_update=True` can be used
+            when you have conditionals in your model, or when you have multiple networks to train.
+        sync_statistics: either None or "nccl". By default (None), it uses statistics of the input tensor to normalize.
+            When set to "nccl", this layer must be used under tensorpack multi-gpu trainers,
+            and it then uses per-machine (multiple GPU) statistics to normalize.
+
+            This option has no effect when not training.
+            The option is also known as "Cross-GPU BatchNorm" as mentioned in https://arxiv.org/abs/1711.07240.
 
     Variable Names:
 
@@ -58,9 +107,7 @@ def BatchNorm(inputs, axis=None, training=None, momentum=0.9, epsilon=1e-5,
     * ``variance/EMA``: the moving average of variance.
 
     Note:
-        1. About multi-GPU training: moving averages across GPUs are not aggregated.
-           Batch statistics are computed independently.  This is consistent with most frameworks.
-        2. Combinations of ``training`` and ``ctx.is_training``:
+        1. Combinations of ``training`` and ``ctx.is_training``:
             * ``training == ctx.is_training``: standard BN, EMA are
                 maintained during training and used during inference. This is
                 the default.
@@ -75,6 +122,9 @@ def BatchNorm(inputs, axis=None, training=None, momentum=0.9, epsilon=1e-5,
     shape = inputs.get_shape().as_list()
     ndims = len(shape)
     assert ndims in [2, 4], ndims
+    if sync_statistics is not None:
+        sync_statistics = sync_statistics.lower()
+    assert sync_statistics in [None, 'nccl', 'horovod'], sync_statistics
 
     if axis is None:
         if ndims == 2:
@@ -82,6 +132,9 @@ def BatchNorm(inputs, axis=None, training=None, momentum=0.9, epsilon=1e-5,
             axis = 1
         else:
             axis = 1 if data_format == 'NCHW' else 3
+    else:
+        data_format = 'NCHW' if axis == 1 else 'NHWC'
+    num_chan = shape[axis]
 
     # parse training/ctx
     ctx = get_current_tower_context()
@@ -98,36 +151,28 @@ def BatchNorm(inputs, axis=None, training=None, momentum=0.9, epsilon=1e-5,
         # Using moving_mean/moving_variance in training, which means we
         # loaded a pre-trained BN and only fine-tuning the affine part.
 
+    if sync_statistics is None or not (training and ctx.is_training):
         coll_bk = backup_collection([tf.GraphKeys.UPDATE_OPS])
         with rename_get_variable(
                 {'moving_mean': 'mean/EMA',
                  'moving_variance': 'variance/EMA'}):
-        if TF_version >= 1.5:
-            layer = tf.layers.BatchNormalization(
+            tf_args = dict(
                 axis=axis,
                 momentum=momentum, epsilon=epsilon,
                 center=center, scale=scale,
                 beta_initializer=beta_initializer,
                 gamma_initializer=gamma_initializer,
-                virtual_batch_size=virtual_batch_size,
                 fused=True,
-                _reuse=tf.get_variable_scope().reuse
-            )
+                _reuse=tf.get_variable_scope().reuse)
+            if TF_version >= 1.5:
+                tf_args['virtual_batch_size'] = virtual_batch_size
             else:
                 assert virtual_batch_size is None, "Feature not supported in this version of TF!"
-            layer = tf.layers.BatchNormalization(
-                axis=axis,
-                momentum=momentum, epsilon=epsilon,
-                center=center, scale=scale,
-                beta_initializer=beta_initializer,
-                gamma_initializer=gamma_initializer,
-                fused=True,
-                _reuse=tf.get_variable_scope().reuse
-            )
+            layer = tf.layers.BatchNormalization(**tf_args)
             xn = layer.apply(inputs, training=training, scope=tf.get_variable_scope())
 
         # maintain EMA only on one GPU is OK, even in replicated mode.
-    # because training time doesn't use EMA
+        # because during training, EMA isn't used
         if ctx.is_main_training_tower:
             for v in layer.non_trainable_variables:
                 add_model_variable(v)
@@ -150,6 +195,72 @@ def BatchNorm(inputs, axis=None, training=None, momentum=0.9, epsilon=1e-5,
             vh.gamma = layer.gamma
         if center:
             vh.beta = layer.beta
+    else:
+        red_axis = [0] if ndims == 2 else ([0, 2, 3] if axis == 1 else [0, 1, 2])
+
+        new_shape = None  # don't need to reshape unless ...
+        if ndims == 4 and axis == 1:
+            new_shape = [1, num_chan, 1, 1]
+
+        batch_mean = tf.reduce_mean(inputs, axis=red_axis)
+        batch_mean_square = tf.reduce_mean(tf.square(inputs), axis=red_axis)
+
+        if sync_statistics == 'nccl':
+            if six.PY3 and TF_version <= 1.8 and ctx.is_main_training_tower:
+                logger.warn("A TensorFlow bug will cause cross-GPU BatchNorm to fail. "
+                            "Apply this patch: https://github.com/tensorflow/tensorflow/pull/20360")
+
+            from tensorflow.contrib.nccl.ops import gen_nccl_ops
+            shared_name = re.sub('tower[0-9]+/', '', tf.get_variable_scope().name)
+            num_dev = ctx.total
+            batch_mean = gen_nccl_ops.nccl_all_reduce(
+                input=batch_mean,
+                reduction='sum',
+                num_devices=num_dev,
+                shared_name=shared_name + '_NCCL_mean') * (1.0 / num_dev)
+            batch_mean_square = gen_nccl_ops.nccl_all_reduce(
+                input=batch_mean_square,
+                reduction='sum',
+                num_devices=num_dev,
+                shared_name=shared_name + '_NCCL_mean_square') * (1.0 / num_dev)
+        elif sync_statistics == 'horovod':
+            # Require https://github.com/uber/horovod/pull/331
+            # Proof-of-concept, not ready yet.
+            import horovod.tensorflow as hvd
+            batch_mean = hvd.allreduce(batch_mean, average=True)
+            batch_mean_square = hvd.allreduce(batch_mean_square, average=True)
+        batch_var = batch_mean_square - tf.square(batch_mean)
+        batch_mean_vec = batch_mean
+        batch_var_vec = batch_var
+
+        beta, gamma, moving_mean, moving_var = get_bn_variables(
+            num_chan, scale, center, beta_initializer, gamma_initializer)
+        if new_shape is not None:
+            batch_mean = tf.reshape(batch_mean, new_shape)
+            batch_var = tf.reshape(batch_var, new_shape)
+            r_gamma = tf.reshape(gamma, new_shape)
+            r_beta = tf.reshape(beta, new_shape)
+        else:
+            r_gamma, r_beta = gamma, beta
+        xn = tf.nn.batch_normalization(
+            inputs, batch_mean, batch_var, r_beta, r_gamma, epsilon)
+
+        if ctx.is_main_training_tower:
+            ret = update_bn_ema(
+                xn, batch_mean_vec, batch_var_vec, moving_mean, moving_var,
+                momentum, internal_update)
+        else:
+            ret = tf.identity(xn, name='output')
+
+        vh = ret.variables = VariableHolder(
+            moving_mean=moving_mean,
+            mean=moving_mean,  # for backward-compatibility
+            moving_variance=moving_var,
+            variance=moving_var)  # for backward-compatibility
+        if scale:
+            vh.gamma = gamma
+        if center:
+            vh.beta = beta
     return ret