Add DistSamplerSeedHook for when runner is EpochBasedRunner (open-mml…

…ab#1449)

* Add DistSamplerSeedHook for when runner is EpochBasedRunner

* add comment

mmseg/apis/train.py

@@ -7,7 +7,8 @@
 import torch
 import torch.distributed as dist
 from mmcv.parallel import MMDataParallel, MMDistributedDataParallel
-from mmcv.runner import HOOKS, build_optimizer, build_runner, get_dist_info
+from mmcv.runner import (HOOKS, DistSamplerSeedHook, EpochBasedRunner,
+                         build_optimizer, build_runner, get_dist_info)
 from mmcv.utils import build_from_cfg
 
 from mmseg import digit_version
@@ -128,6 +129,12 @@ def train_segmentor(model,
     runner.register_training_hooks(cfg.lr_config, cfg.optimizer_config,
                                    cfg.checkpoint_config, cfg.log_config,
                                    cfg.get('momentum_config', None))
+    if distributed:
+        # when distributed training by epoch, using`DistSamplerSeedHook` to set
+        # the different seed to distributed sampler for each epoch, it will
+        # shuffle dataset at each epoch and avoid overfitting.
+        if isinstance(runner, EpochBasedRunner):
+            runner.register_hook(DistSamplerSeedHook())
 
     # an ugly walkaround to make the .log and .log.json filenames the same
     runner.timestamp = timestamp