Update cluster_howto_en.rst (#807)

doc/fluid/user_guides/howto/training/cluster_howto_en.rst

@@ -35,7 +35,9 @@ In the training of data parallelism mode, Fluid uses two communication modes to
 
   The pserver process can be on a compute node that is completely different from the trainer, or it can share the same node with a trainer. The number of pserver processes required for a distributed task usually needs to be adjusted according to the actual situation to achieve the best performance. However, usually pserver processes are no more than trainer processes.
 
-  When using GPU training, the pserver can choose to use the GPU or only use the CPU. If the pserver also uses the GPU, it will result in the extra overhead of copying the gradient data received from the CPU to the GPU. In some cases, the overall training performance will be degraded.
+  **Note:** When using GPU training, the pserver can choose to use the GPU or only use the CPU. If the pserver also uses the GPU, it will result in the extra overhead of copying the gradient data received from the CPU to the GPU. In some cases, the overall training performance will be degraded.
+
+  **Note:** When using GPU training, if there are multiple GPU cards in each trainer node, the gradient polymerization will execute in NCCL2 way among the cards in one node, and then in multiple nodes through pserver.
 
 - Structure of NCCL2 communication method:
 
@@ -178,8 +180,10 @@ Distributed training in NCCL2 mode, because there is no parameter server role, t
 
 * Configure :code:`mode="nccl2"` in :code:`fluid.DistributeTranspilerConfig` .
 * When calling :code:`transpile`, :code:`trainers` is fed with the endpoints of all trainer nodes, and passed with the argument :code:`current_endpoint` .
+  In this step, :code:`gen_nccl_id_op` will add in :code:`startup program` to synchronize NCCLID information during the multi-computer program initialization.
 * Initialize :code:`ParallelExecutor` with :code:`num_trainers` and :code:`trainer_id` .
-
+  In this step, :code:`ParallelExecutor` will initialize NCCL2 by the multi-computer way and do the operations :code:`allreduce` across the nodes for the gradient of every parameter to execute muti-computer training
+   
 For example:
 
 .. code-block:: python
@@ -198,9 +202,9 @@ For example:
 .. csv-table:: Description of the necessary parameters for NCCL2 mode
 	:header: "parameter", "description"
 
-	"trainer_id", "The unique ID of each trainer node in the task, starting at 0, there cannot be any duplication"
-	"trainers", "endpoints of all trainer nodes in the task, used to broadcast NCCL IDs when NCCL2 is initialized"
-	"current_endpoint", "endpoint of current node"
+	"trainer_id", "(int)The unique ID of each trainer node in the task, starting at 0, there cannot be any duplication"
+	"trainers", "(int)endpoints of all trainer nodes in the task, used to broadcast NCCL IDs when NCCL2 is initialized"
+	"current_endpoint", "(string)endpoint of current node"
 
 Currently, distributed training using NCCL2 only supports synchronous training. The distributed training using NCCL2 mode is more suitable for the model which is relatively large and needs \
 synchronous training and GPU training. If the hardware device supports RDMA and GPU Direct, this can achieve high distributed training performance.
@@ -227,7 +231,9 @@ Attention during usage:
 Important Notes on NCCL2 Distributed Training
 ++++++++++++++++++++++++++++++++++++++++++++++
 
-**Note** : Please ensure each node has the same amount of data to train in NCCL2 mode distributed training, which prevents
+**Note:** When using distributed training in NCCL2 mode, if you only want to use a part of cards in one node, you can appoint by configuring the environment variable :code:`export CUDA_VISIBLE_DEVICES=0,1,2,3` .
+
+**Note:** Please ensure each node has the same amount of data to train in NCCL2 mode distributed training, which prevents
 exit at the final iteration. There are two common ways:
 
 - Randomly sample some data to complement nodes where less data are distributed. (We recommend this method for sake of a complete dataset to be trained)