https://zhuanlan.zhihu.com/p/26031658
https://www.tensorflow.org/api_docs/cc/class/tensorflow/ops/barrier
https://www.tensorflow.org/api_docs/cc/class/tensorflow/ops/accumulator-apply-gradient
https://www.tensorflow.org/api_docs/cc/class/tensorflow/ops/queue-enqueue
https://www.tensorflow.org/api_docs/python/tf/queue/FIFOQueue
MR抽象过于简单