本文目的

在介绍estimator分布式的时候，官方文档由于版本更新导致与接口不一致。具体是：在estimator分布式当中，使用dataset作为数据输入，在1.12版本中，数据训练只是dataset的数据，就是所有设备加起来，跑一遍数据。

而在2.0版本中，训练数据是dataset的数据乘以分
布式的设备数。也就是说，在每个设备当中都会完整地跑一遍dataset的所有数据。

1.12版本读取

1. 在主线程当中创建图

下面这段代码中，在client中调用了input function，得到迭代器。这是属于estimator distribute train调用的代码

with ops.Graph().as_default() as g:
      # We want to create the iterations variable outside the distribution scope
      # as that is just stored on the host and mainly used to drive the loop
      # and doesn\'t need to be a Mirrored/Device variable.
      if is_tpu_strategy:
        steps_per_run_variable = training.get_or_create_steps_per_run_variable()
      with self._train_distribution.scope():
        random_seed.set_random_seed(self._config.tf_random_seed)
        iterator, input_hooks = self._get_iterator_from_input_fn(
            input_fn, model_fn_lib.ModeKeys.TRAIN, self._train_distribution)

• _get_iterator_from_input_fn * 这个函数会生成迭代器供后续训练读取数据。

  def _get_iterator_from_input_fn(self, input_fn, mode, distribution=None):
    if distribution is not None:
      result = distribution.distribute_dataset(
          lambda: self._call_input_fn(input_fn, mode))
    else:
      result = self._call_input_fn(input_fn, mode)

    iterator = result.make_initializable_iterator()
    input_hooks = [estimator_util._DatasetInitializerHook(iterator)]  # pylint: disable=protected-access
    return iterator, input_hooks

这里会调用distribute_dataset生成dataset。
再点进去看以后可看到会创建这样一个PerDeviceDataset

class PerDeviceDataset(object):
  """Like `tf.data.Dataset` split devices, producing `PerDevice` data."""

  def __init__(self, dataset, devices, prefetch_on_device=None):
    self._devices = devices

    # Default to using prefetching in graph mode, unless specified.
    # TODO(priyag): Enable prefetching in eager mode.
    self._prefetch_on_device = prefetch_on_device
    if self._prefetch_on_device is None:
      self._prefetch_on_device = not context.executing_eagerly()
    assert not (self._prefetch_on_device and context.executing_eagerly()), (
        "Prefetching is only supported in graph mode currently")

    if self._prefetch_on_device:
      self._dataset = dataset.apply(
          prefetching_ops_v2.prefetch_to_devices(self._devices))
    else:
      # TODO(priyag): If dropping remainder is not appropriate, find another
      # approach to distributing the dataset when not possible to divide evenly.
      # Possibly not an issue when we start using PartitionedDataset.
      self._dataset = dataset.batch(len(devices), drop_remainder=True)

最后一行代码可以看到，在原dataset上又封装了一层batch。将数据根据设备数切分。
后面创建迭代器也是封装为PerDeviceDataIterator，形成一个字典映射，不同设备不同数据，根据batch 的index切分。

分布式训练

在1.12版本中的训练比较简单。对于MirroredStrategy来说，会给每个一个device创建一个线程，
有一个缺点就是，每一次run都会创建线程，在todo里看到，后续会优化掉应该。

下面是在client中从迭代器获取数据，传递给每个device去运算的代码，
self._train_distribution.call_for_each_tower

features, labels = estimator_util.parse_iterator_result(
              iterator.get_next())
          grouped_estimator_spec = self._train_distribution.call_for_each_tower(
              self._call_model_fn,
              features,
              labels,  # although this will be None it seems
              model_fn_lib.ModeKeys.TRAIN,
              self.config)
          loss = self._train_distribution.unwrap(
              self._train_distribution.reduce(
                  distribute_lib.get_loss_reduction(),
                  grouped_estimator_spec.loss,
                  destinations=\'/device:CPU:0\'))[0]
          distributed_train_op = grouped_estimator_spec.train_op

call_for_each_tower是每个设备训练的接口

def _call_for_each_tower(distribution, fn, *args, **kwargs):
  """Run `fn` in separate threads, once per tower/worker device.
  run_concurrently = kwargs.pop("run_concurrently", True)
  if not context.executing_eagerly():
    # Lots of TF library code isn\'t thread-safe in graph mode, and
    # there is little to be gained by turning on multithreading when
    # constructing a graph.
    run_concurrently = False
    # Needed for per-thread device, etc. contexts in graph mode.
    ops.get_default_graph().switch_to_thread_local()
  elif run_concurrently is None:
    run_concurrently = True

  coord = coordinator.Coordinator(clean_stop_exception_types=(_RequestedStop,))

  shared_variable_store = {}

  # TODO(isaprykin): Create these threads once instead of during every run()
  # call.
  threads = []
  for index, d in enumerate(distribution.worker_devices):
    variable_creator_fn = shared_variable_creator.make_fn(
        shared_variable_store, index)
    t = MirroredStrategy._MirroredTowerThread(  # pylint: disable=protected-access
        distribution, coord, d, variable_creator_fn, fn,
        *values.select_device(d, args), **values.select_device(d, kwargs))
    threads.append(t)

  for t in threads:
    t.start()

其中，select_device就是取对应设备key对应的值。完成整个分布式训练。