document the enviromental variables controling multithreading

Author: Han Wang

README.md

@@ -437,8 +437,6 @@ positional arguments:
 
 optional arguments:
   -h, --help            show this help message and exit
-  -t INTER_THREADS, --inter-threads INTER_THREADS
-                        With default value 0. Setting the "inter_op_parallelism_threads" key for the tensorflow, the "intra_op_parallelism_threads" will be set by the env variable OMP_NUM_THREADS
   --init-model INIT_MODEL
                         Initialize a model by the provided checkpoint
   --restart RESTART     Restart the training from the provided checkpoint
@@ -449,6 +447,15 @@ The keys `intra_op_parallelism_threads` and `inter_op_parallelism_threads` are T
 
 **`--restart model.ckpt`**, continues the training from the checkpoint `model.ckpt`.
 
+On some resources limited machines, one may want to control the number of threads used by DeePMD-kit. This is achieved by three environmental variables: `OMP_NUM_THREADS`, `TF_INTRA_OP_PARALLELISM_THREADS` and `TF_INTER_OP_PARALLELISM_THREADS`. `OMP_NUM_THREADS` controls the multithreading of DeePMD-kit implemented operations. `TF_INTRA_OP_PARALLELISM_THREADS` and `TF_INTER_OP_PARALLELISM_THREADS` controls `intra_op_parallelism_threads` and `inter_op_parallelism_threads`, which are  Tensorflow configurations for multithreading. An explanation is found [here](https://stackoverflow.com/questions/41233635/meaning-of-inter-op-parallelism-threads-and-intra-op-parallelism-threads).
+
+For example if you wish to use 3 cores of 2 CPUs on one node, you may set the environmental variables and run DeePMD-kit as follows:
+```bash
+export OMP_NUM_THREADS=6
+export TF_INTRA_OP_PARALLELISM_THREADS=3
+export TF_INTER_OP_PARALLELISM_THREADS=2
+dp train input.json
+```
 
 ## Freeze a model
 

source/train/main.py

@@ -18,11 +18,6 @@ def main () :
     parser_train = subparsers.add_parser('train', help='train a model')
     parser_train.add_argument('INPUT', 
                               help='the input parameter file in json format')
-    parser_train.add_argument('-t','--inter-threads', type = int, default = default_num_inter_threads,
-                              help=
-                              'With default value %d. ' % default_num_inter_threads + 
-                              'Setting the "inter_op_parallelism_threads" key for the tensorflow, '  +
-                              'the "intra_op_parallelism_threads" will be set by the env variable OMP_NUM_THREADS')
     parser_train.add_argument('--init-model', type = str, 
                               help=
                               'Initialize the model by the provided checkpoint.')