modify MMDistributedDataParallel, no longer inherited from DistributedDataParallel

mmdet/nn/parallel/distributed.py

-from torch.nn.parallel import DistributedDataParallel
+import torch
+import torch.distributed as dist
+import torch.nn as nn
+from torch._utils import (_flatten_dense_tensors, _unflatten_dense_tensors,
+                          _take_tensors)
 
 from .scatter_gather import scatter_kwargs
 
 
-class MMDistributedDataParallel(DistributedDataParallel):
+class MMDistributedDataParallel(nn.Module):
+
+    def __init__(self, module, dim=0, broadcast_buffers=True):
+        super(MMDistributedDataParallel, self).__init__()
+        self.module = module
+        self.dim = dim
+        self.broadcast_buffers = broadcast_buffers
+
+        self.first_synced = False
+        self.broadcast_bucket_size = 32 * 1024 * 1024
+
+    def _dist_broadcast_coalesced(self, tensors, buffer_size):
+        for tensors in _take_tensors(tensors, buffer_size):
+            flat_tensors = _flatten_dense_tensors(tensors)
+            dist.broadcast(flat_tensors, 0)
+            for tensor, synced in zip(
+                    tensors, _unflatten_dense_tensors(flat_tensors, tensors)):
+                tensor.copy_(synced)
+
+    def sync_params(self):
+        module_states = list(self.module.state_dict().values())
+        if len(module_states) > 0:
+            self._dist_broadcast_coalesced(module_states,
+                                           self.broadcast_bucket_size)
+        if self.broadcast_buffers:
+            buffers = [b.data for b in self.module._all_buffers()]
+            if len(buffers) > 0:
+                self._dist_broadcast_coalesced(buffers,
+                                               self.broadcast_bucket_size)
 
     def scatter(self, inputs, kwargs, device_ids):
         return scatter_kwargs(inputs, kwargs, device_ids, dim=self.dim)
+
+    def forward(self, *inputs, **kwargs):
+        if not self.first_synced:
+            self.sync_params()
+            self.first_synced = True
+        inputs, kwargs = self.scatter(inputs, kwargs,
+                                      [torch.cuda.current_device()])
+        return self.module(*inputs[0], **kwargs[0])

tools/dist_train.sh

@@ -2,8 +2,4 @@
 
 PYTHON=${PYTHON:-"python"}
 
-$PYTHON train.py $1 --dist --world-size $2 --rank 0 &
-let MAX_RANK=$2-1
-for i in `seq 1 $MAX_RANK`; do
-    $PYTHON train.py $1 --dist --world-size $2 --rank $i > /dev/null 2>&1 &
-done
+$PYTHON -m torch.distributed.launch --nproc_per_node=$2 train.py $1 --launcher pytorch
\ No newline at end of file

tools/train.py

@@ -95,10 +95,7 @@ def main():
     model = build_detector(
         cfg.model, train_cfg=cfg.train_cfg, test_cfg=cfg.test_cfg)
     if dist:
-        model = MMDistributedDataParallel(
-            model,
-            device_ids=[torch.cuda.current_device()],
-            broadcast_buffers=False).cuda()
+        model = MMDistributedDataParallel(model).cuda()
     else:
         model = MMDataParallel(model, device_ids=range(cfg.gpus)).cuda()