Horovod以类似的方式支持Keras和常规TensorFlow。要使用Horovod，请在程序中添加以下内容。

  1. 运行hvd.init()。

 

  1. 使用固定服务器GPU，以供此过程使用config.gpu_options.visible_device_list。

    通过每个进程一个GPU的典型设置，您可以将其设置为local rank。在这种情况下，服务器上的第一个进程将被分配第一GPU，第二个进程将被分配第二GPU，依此类推。

 

  1. 通过工人人数来衡量学习率。

    同步分布式培训中的有效批处理规模是根据工人人数来衡量的。学习率的提高弥补了批量大小的增加。

 

  1. 将优化器包装在中hvd.DistributedOptimizer。

    分布式优化器将梯度计算委派给原始优化器，使用allreduce或allgather对梯度求平均，然后应用这些平均梯度。

 

  1. 添加hvd.callbacks.BroadcastGlobalVariablesCallback(0)到播放初始变量状态从0级到所有其他进程。

    当使用随机权重开始训练或从检查点恢复训练时，这是确保所有工人进行一致初始化的必要步骤。

 

  1. 修改您的代码以仅在工作程序0上保存检查点，以防止其他工作程序破坏它们。

    通过使用来保护模型检查点代码来实现此目的。hvd.rank() != 0

示例代码

 1 from __future__ import print_function
 2 import keras
 3 from keras.datasets import mnist
 4 from keras.models import Sequential
 5 from keras.layers import Dense, Dropout, Flatten
 6 from keras.layers import Conv2D, MaxPooling2D
 7 from keras import backend as K
 8 import math
 9 import tensorflow as tf
10 import horovod.keras as hvd
11 
12 # Horovod: initialize Horovod.
13 hvd.init()
14 
15 # Horovod: pin GPU to be used to process local rank (one GPU per process)
16 config = tf.ConfigProto()
17 config.gpu_options.allow_growth = True
18 config.gpu_options.visible_device_list = str(hvd.local_rank())
19 K.set_session(tf.Session(config=config))
20 
21 batch_size = 128
22 num_classes = 10
23 
24 # Horovod: adjust number of epochs based on number of GPUs.
25 epochs = int(math.ceil(12.0 / hvd.size()))
26 
27 # Input image dimensions
28 img_rows, img_cols = 28, 28
29 
30 # The data, shuffled and split between train and test sets
31 (x_train, y_train), (x_test, y_test) = mnist.load_data()
32 
33 if K.image_data_format() == 'channels_first':
34     x_train = x_train.reshape(x_train.shape[0], 1, img_rows, img_cols)
35     x_test = x_test.reshape(x_test.shape[0], 1, img_rows, img_cols)
36     input_shape = (1, img_rows, img_cols)
37 else:
38     x_train = x_train.reshape(x_train.shape[0], img_rows, img_cols, 1)
39     x_test = x_test.reshape(x_test.shape[0], img_rows, img_cols, 1)
40     input_shape = (img_rows, img_cols, 1)
41 
42 x_train = x_train.astype('float32')
43 x_test = x_test.astype('float32')
44 x_train /= 255
45 x_test /= 255
46 print('x_train shape:', x_train.shape)
47 print(x_train.shape[0], 'train samples')
48 print(x_test.shape[0], 'test samples')
49 
50 # Convert class vectors to binary class matrices
51 y_train = keras.utils.to_categorical(y_train, num_classes)
52 y_test = keras.utils.to_categorical(y_test, num_classes)
53 
54 model = Sequential()
55 model.add(Conv2D(32, kernel_size=(3, 3),
56                 activation='relu',
57                 input_shape=input_shape))
58 model.add(Conv2D(64, (3, 3), activation='relu'))
59 model.add(MaxPooling2D(pool_size=(2, 2)))
60 model.add(Dropout(0.25))
61 model.add(Flatten())
62 model.add(Dense(128, activation='relu'))
63 model.add(Dropout(0.5))
64 model.add(Dense(num_classes, activation='softmax'))
65 
66 # Horovod: adjust learning rate based on number of GPUs.
67 opt = keras.optimizers.Adadelta(1.0 * hvd.size())
68 
69 # Horovod: add Horovod Distributed Optimizer.
70 opt = hvd.DistributedOptimizer(opt)
71 
72 model.compile(loss=keras.losses.categorical_crossentropy,
73               optimizer=opt,
74               metrics=['accuracy'])
75 
76 callbacks = [
77     # Horovod: broadcast initial variable states from rank 0 to all other processes.
78     # This is necessary to ensure consistent initialization of all workers when
79     # training is started with random weights or restored from a checkpoint.
80     hvd.callbacks.BroadcastGlobalVariablesCallback(0),
81 ]
82 
83 # Horovod: save checkpoints only on worker 0 to prevent other workers from corrupting them.
84 if hvd.rank() == 0:
85     callbacks.append(keras.callbacks.ModelCheckpoint('./checkpoint-{epoch}.h5'))
86 
87 model.fit(x_train, y_train,
88           batch_size=batch_size,
89           callbacks=callbacks,
90           epochs=epochs,
91           verbose=1,
92           validation_data=(x_test, y_test))
93 score = model.evaluate(x_test, y_test, verbose=0)
94 print('Test loss:', score[0])
95 print('Test accuracy:', score[1])