学习笔记TF041:分布式并行

TensorFlow分布式并行基于gRPC通信框架，一个master负责创建Session，多个worker负责执行计算图任务。

先创建TensorFlow Cluster对象，包含一组task(每个task一台独立机器)，分布式执行TensorFlow计算图。一个Cluster切分多个job，一个job是一类特定任务(parameter server ps,worker)，每个job可以包含多个task。每个task创建一个server，连接到Cluster，每个task执行在不同机器。也可以一台机器执行多个task(不同GPU)。tf.train.ClusterSpec初始化Cluster对象，初始化信息是Python dict，tf.train.ClusterSpec({"ps":["192.168.233.201:2222"],"worker":["192.168.233.202:2222","192.168.233.203:2222"]})，代表一个parameter server和两个worker，分别在三个不同机器上。每个task，定义自己身份，如server=tf.train.Server(cluster,job_name="ps",task_index=0)，机器job定义ps第0台机器。程序中with tf.device("/job:worker/task:7")，限定Variable存放在哪个task或机器。

TensorFlow分布式模式：In-graph replication模型并行，模型计算图不同部分放在不同机器执行；Between-graph replication数据并行，每台机器相同计算图，计算不同batch数据。异步并行，每台机器独立计算梯度，计算完更新到parameter server，不等其他机器。同步并行，等所有机器都完成梯度计算，多个梯度合成统一更新模型参数。同步并行训练，loss下降速度更快，可达到最大精度更高，同步并行速度取决最慢机器，设备速度一致，效率较高。

TensorFlow实现包含1个paramter server、2个worker分布式并行训练程序，MNIST手写数据识别任务示例。写一个完整Python文件，在不同机器不同task执行。载入依赖库。

tf.app.flags定义标记，命令行执行TensorFlow设置参数。命令行指定参数被TensorFlow读取，转flags。设定数据储存目录data_dir默认/tmp/mnist-data，隐藏节点数默认100,训练最大步数train_steps默认1000000,batch_size默认100,学习速率默认0.01。

设定是否使用同步并行标记sync_replicas默认False，命令行执行时可设True开户同步并行。设定需要累计梯度个数更新模型值默认None，代表同步并行积累多少个batch梯度再进行参数更新，设None 为worker数量，所有worker完成一个batch训练后再更新模型参数。

定义ps地址，默认192.168.233.201:2222,根据集群实际情况配置。worker地址设置192.168.233.202:2222和192.168.233.203:2222.设置job_name和task_index FLAG。

flags.FLAGS直接命名FLAGS，简化使用。设置图片尺寸IMAGE_PIXELS 28。

编写程序主函数main，input_data.read_data_sets下载读取MNIST数据集，设置one_hot编码格式。检测命令行输入参数，确保job_name和task_index两个必备参数。显示job_name和task_index，ps、worker所有地址解析成列表ps_spec、worker_spec。

计算总共worker数量，tf.train.ClusterSpec生成TensorFlow Cluster对象，传入参数ps地址信息和worker地址信息。tf.train.Server创建当前机器server，连接Cluster。如当前节点是parameter server，不进行后续操作，server.join等待worker工作。

判断当前机器是否主节点，task_index是否0。定义当前机器worker_device，格式"job:worker/task:0/gpu:0"。多台机器，每台机器1块GPU，总共需要机器数量worker。如一台机器多GPU，一个task管理多个GPU或多个task分别管理。tf.train.replica_device_setter()设置worker资源，worker_device计算资源，ps_device存储模型参数资源。replica_device_setter将模型参数部署在独立ps服务器"/job:ps/cpu:0"，训练操作部署在"/job:worker/task:0/gpu:0"，本机GPU。创建记录全局训练步数变量global_step。

定义神经网络模型，tf.truncated_normal初始化权重，tf.zeros初始化偏置，创建输入 placeholder，tf.nn.xw_plus_b输入矩阵乘法、偏置操作，ReLU激活函数处理，得到第一个隐层输出hid。tf.nn.xw_plus_b、tf.nn.softmax对第一层输出hid处理，得到网络最终输出y。最后计算损失cross_entropy，定义优化器Adam。

判断是否设置同步训练模式sync_replicas，如果同步模型，先获取同步更新模型参数需要副本数replicas_to_aggregate;如果没有单独设置，worker数作默认值。tf.train.SyncReplicasOptimizer创建同步训练优化器，对原有优化器扩展。传入原有优化器及其他参数(replicas_to_aggregate、total_num_replicas、replica_id)，原有优化器改造为同步分布式训练版本。用普通(异步)或同步优化器优化损失cross_entropy。

同步训练模式，主节点，opt.get_chief_queue_runner创建队列执行器，opt.get_init_tokens_op创建全局参数初始化器。

生成本地参数初始化操作init_op，创建临时训练目录，tf.train_Supervisor创建分布式训练监督器，传入参数is_chief、train_dir、init_op。Supervisor管理task参与到分布式训练。

设置Session参数，allow_soft_placement设True，代表操作在指定device不能执行时转到其他device执行。

如果主节点，显示初始化Session，其他节点显示等待主节点初始化操作。执行sv.prepate_or_wait_for_session()。

如果处于同步模型主节点，sv.start_queue_runners执行队列化执行器chief_queue_runner，执行全局参数初始化器init_tokens_op。

训练过程，记录worker执行训练启动时间，初始化本地训练步数local_step，进入训练循环。每步训练，从nnist.train.next_batch读取一个batch数据，生成feed_dict，调train_step执行训练。当全局训练步数达到预设最大值，停止训练。

训练结束，展示总训练时间，在验证数据上计算预测结果损失cross_entropy，展示。

在主程序执行tf.app.run()启动main函数，全部代码保存到distributed.py文件。3台不同机器分别执行distributed.py启动3个task，每次执行distributed.py，传入job_name、task_index指定worker身份。

分别在三台机器192.168.233.201､192.168.233.202、192.168.233.203执行python distributed.py。

同步模式，加上--sync_replicas=True。global_step，异步时，全局步数是所有worker训练步数和，同步时是多少轮并行训练。

    #from __future__ import absolute_import
    #from __future__ import division
    #from __future__ import print_function
    import math
    #import sys
    import tempfile
    import time
    import tensorflow as tf
    from tensorflow.examples.tutorials.mnist import input_data
    flags = tf.app.flags
    flags.DEFINE_string("data_dir", "/tmp/mnist-data",
                        "Directory for storing mnist data")
    #flags.DEFINE_boolean("download_only", False,
    #                     "Only perform downloading of data; Do not proceed to "
    #                     "session preparation, model definition or training")
    flags.DEFINE_integer("task_index", None,
                         "Worker task index, should be >= 0. task_index=0 is "
                         "the master worker task the performs the variable "
                         "initialization ")
    #flags.DEFINE_integer("num_gpus", 2,
    #                     "Total number of gpus for each machine."
    #                     "If you don't use GPU, please set it to '0'")
    flags.DEFINE_integer("replicas_to_aggregate", None,
                         "Number of replicas to aggregate before parameter update"
                         "is applied (For sync_replicas mode only; default: "
                         "num_workers)")
    flags.DEFINE_integer("hidden_units", 100,
                         "Number of units in the hidden layer of the NN")
    flags.DEFINE_integer("train_steps", 1000000,
                         "Number of (global) training steps to perform")
    flags.DEFINE_integer("batch_size", 100, "Training batch size")
    flags.DEFINE_float("learning_rate", 0.01, "Learning rate")
    flags.DEFINE_boolean("sync_replicas", False,
                         "Use the sync_replicas (synchronized replicas) mode, "
                         "wherein the parameter updates from workers are aggregated "
                         "before applied to avoid stale gradients")
    #flags.DEFINE_boolean(
    #    "existing_servers", False, "Whether servers already exists. If True, "
    #    "will use the worker hosts via their GRPC URLs (one client process "
    #    "per worker host). Otherwise, will create an in-process TensorFlow "
    #    "server.")
    flags.DEFINE_string("ps_hosts","192.168.233.201:2222",
                        "Comma-separated list of hostname:port pairs")
    flags.DEFINE_string("worker_hosts", "192.168.233.202:2223,192.168.233.203:2224",
                        "Comma-separated list of hostname:port pairs")
    flags.DEFINE_string("job_name", None,"job name: worker or ps")
    FLAGS = flags.FLAGS
    IMAGE_PIXELS = 28
    def main(unused_argv):
      mnist = input_data.read_data_sets(FLAGS.data_dir, one_hot=True)
    #  if FLAGS.download_only:
    #    sys.exit(0)
      if FLAGS.job_name is None or FLAGS.job_name == "":
        raise ValueError("Must specify an explicit `job_name`")
      if FLAGS.task_index is None or FLAGS.task_index =="":
        raise ValueError("Must specify an explicit `task_index`")
      print("job name = %s" % FLAGS.job_name)
      print("task index = %d" % FLAGS.task_index)
      #Construct the cluster and start the server
      ps_spec = FLAGS.ps_hosts.split(",")
      worker_spec = FLAGS.worker_hosts.split(",")
      # Get the number of workers.
      num_workers = len(worker_spec)
      cluster = tf.train.ClusterSpec({
          "ps": ps_spec,
          "worker": worker_spec})
      #if not FLAGS.existing_servers:
        # Not using existing servers. Create an in-process server.
      server = tf.train.Server(
          cluster, job_name=FLAGS.job_name, task_index=FLAGS.task_index)
      if FLAGS.job_name == "ps":
        server.join()
      is_chief = (FLAGS.task_index == 0)

    #  if FLAGS.num_gpus > 0:
    #    if FLAGS.num_gpus < num_workers:
    #      raise ValueError("number of gpus is less than number of workers")
    #    # Avoid gpu allocation conflict: now allocate task_num -> #gpu
    #    # for each worker in the corresponding machine
    #    gpu = (FLAGS.task_index % FLAGS.num_gpus)
    #    worker_device = "/job:worker/task:%d/gpu:%d" % (FLAGS.task_index, gpu)
    #  elif FLAGS.num_gpus == 0:
    #    # Just allocate the CPU to worker server
    #    cpu = 0
    #    worker_device = "/job:worker/task:%d/cpu:%d" % (FLAGS.task_index, cpu)
    #  # The device setter will automatically place Variables ops on separate
    #  # parameter servers (ps). The non-Variable ops will be placed on the workers.
    #  # The ps use CPU and workers use corresponding GPU

      worker_device = "/job:worker/task:%d/gpu:0" % FLAGS.task_index
      with tf.device(
          tf.train.replica_device_setter(
              worker_device=worker_device,
              ps_device="/job:ps/cpu:0",
              cluster=cluster)):
        global_step = tf.Variable(0, name="global_step", trainable=False)
        # Variables of the hidden layer
        hid_w = tf.Variable(
            tf.truncated_normal(
                [IMAGE_PIXELS * IMAGE_PIXELS, FLAGS.hidden_units],
                stddev=1.0 / IMAGE_PIXELS),
            name="hid_w")
        hid_b = tf.Variable(tf.zeros([FLAGS.hidden_units]), name="hid_b")
        # Variables of the softmax layer
        sm_w = tf.Variable(
            tf.truncated_normal(
                [FLAGS.hidden_units, 10],
                stddev=1.0 / math.sqrt(FLAGS.hidden_units)),
            name="sm_w")
        sm_b = tf.Variable(tf.zeros([10]), name="sm_b")
        # Ops: located on the worker specified with FLAGS.task_index
        x = tf.placeholder(tf.float32, [None, IMAGE_PIXELS * IMAGE_PIXELS])
        y_ = tf.placeholder(tf.float32, [None, 10])
        hid_lin = tf.nn.xw_plus_b(x, hid_w, hid_b)
        hid = tf.nn.relu(hid_lin)
        y = tf.nn.softmax(tf.nn.xw_plus_b(hid, sm_w, sm_b))
        cross_entropy = -tf.reduce_sum(y_ * tf.log(tf.clip_by_value(y, 1e-10, 1.0)))
        opt = tf.train.AdamOptimizer(FLAGS.learning_rate)
        if FLAGS.sync_replicas:
          if FLAGS.replicas_to_aggregate is None:
            replicas_to_aggregate = num_workers
          else:
            replicas_to_aggregate = FLAGS.replicas_to_aggregate
          opt = tf.train.SyncReplicasOptimizer(
              opt,
              replicas_to_aggregate=replicas_to_aggregate,
              total_num_replicas=num_workers,
              replica_id=FLAGS.task_index,
              name="mnist_sync_replicas")
        train_step = opt.minimize(cross_entropy, global_step=global_step)
        if FLAGS.sync_replicas and is_chief:
          # Initial token and chief queue runners required by the sync_replicas mode
          chief_queue_runner = opt.get_chief_queue_runner()
          init_tokens_op = opt.get_init_tokens_op()
        init_op = tf.global_variables_initializer()
        train_dir = tempfile.mkdtemp()
        sv = tf.train.Supervisor(
            is_chief=is_chief,
            logdir=train_dir,
            init_op=init_op,
            recovery_wait_secs=1,
            global_step=global_step)
        sess_config = tf.ConfigProto(
            allow_soft_placement=True,
            log_device_placement=False,
            device_filters=["/job:ps", "/job:worker/task:%d" % FLAGS.task_index])
        # The chief worker (task_index==0) session will prepare the session,
        # while the remaining workers will wait for the preparation to complete.
        if is_chief:
          print("Worker %d: Initializing session..." % FLAGS.task_index)
        else:
          print("Worker %d: Waiting for session to be initialized..." %
            FLAGS.task_index)
    #    if FLAGS.existing_servers:
    #      server_grpc_url = "grpc://" + worker_spec[FLAGS.task_index]
    #      print("Using existing server at: %s" % server_grpc_url)
    #
    #      sess = sv.prepare_or_wait_for_session(server_grpc_url, config=sess_config)
    #    else:
        sess = sv.prepare_or_wait_for_session(server.target,
                                            config=sess_config)
        print("Worker %d: Session initialization complete." % FLAGS.task_index)
        if FLAGS.sync_replicas and is_chief:
          # Chief worker will start the chief queue runner and call the init op
          print("Starting chief queue runner and running init_tokens_op")
          sv.start_queue_runners(sess, [chief_queue_runner])
          sess.run(init_tokens_op)
        # Perform training
        time_begin = time.time()
        print("Training begins @ %f" % time_begin)
        local_step = 0
        while True:
          # Training feed
          batch_xs, batch_ys = mnist.train.next_batch(FLAGS.batch_size)
          train_feed = {x: batch_xs, y_: batch_ys}
          _, step = sess.run([train_step, global_step], feed_dict=train_feed)
          local_step += 1
          now = time.time()
          print("%f: Worker %d: training step %d done (global step: %d)" %
                (now, FLAGS.task_index, local_step, step))
          if step >= FLAGS.train_steps:
            break
        time_end = time.time()
        print("Training ends @ %f" % time_end)
        training_time = time_end - time_begin
        print("Training elapsed time: %f s" % training_time)
        # Validation feed
        val_feed = {x: mnist.validation.images, y_: mnist.validation.labels}
        val_xent = sess.run(cross_entropy, feed_dict=val_feed)
        print("After %d training step(s), validation cross entropy = %g" %
              (FLAGS.train_steps, val_xent))
    if __name__ == "__main__":
      tf.app.run()

参考资料：
《TensorFlow实战》

欢迎付费咨询(150元每小时)，我的微信：qingxingfengzi