- 控制依賴

with tf.control_dependencies([train_step, variables_averages_op]):train_op = tf.no_op(name='train') #train_op does nothin

或者

train_op = tf.group(train_step, variables_averages_op)

貼上stackflow上大神對train_op的回答：

As the documentation says, tf.no_op() does nothing. However, when you create a tf.no_op() inside a with tf.control_dependencies([x, y, z]): block, the op will gain control dependencies on ops x, y, and z. Therefore it can be used to group together a set of side effecting ops, and give you a single op to pass to sess.run() in order to run all of them in a single step.

- tf.control_dependencies

tf.control_dependencies(control_inputs) control_inputs: A list of Operation or Tensor objects, which must be executed or computed before running the operations defined in the context. returns：A context manager that specifies control dependencies for all operations constructed within the context

該函數的參數是一個包含Op或者tensor的列表，列表里的操作的執行順序先于context里面定義的Op和tensor；
該函數返回一個上下文管理器

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代碼示例

with g.control_dependencies([a, b, c]):# `d` and `e` will only run after `a`, `b`, and `c` have executed.d = ...e = ...#也可以嵌套使用 with g.control_dependencies([a, b]):# Ops constructed here run after `a` and `b`.with g.control_dependencies([c, d]):# Ops constructed here run after `a`, `b`, `c`, and `d`....#傳入None消除依賴關系 with g.control_dependencies([a, b]):# Ops constructed here run after `a` and `b`.with g.control_dependencies(None):# Ops constructed here run normally, not waiting for either `a` or `b`.with g.control_dependencies([c, d]):# Ops constructed here run after `c` and `d`, also not waiting# for either `a` or `b`.

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另外：控制依賴只對那些在上下文環境中建立的操作有效，僅僅在context中使用一個操作或張量是沒用的

# WRONG def my_func(pred, tensor):t = tf.matmul(tensor, tensor)with tf.control_dependencies([pred]):# The matmul op is created outside the context, so no control# dependency will be added.return t# RIGHT def my_func(pred, tensor):with tf.control_dependencies([pred]):# The matmul op is created in the context, so a control dependency# will be added.return tf.matmul(tensor, tensor)

參考

https://stackoverflow.com/questions/46801154/what-does-tf-no-op-do
https://blog.csdn.net/PKU_Jade/article/details/73498753
https://blog.csdn.net/u012436149/article/details/72084744

記錄時間

2018/9/13 12:07 第一次

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