Fix race condition in DDP.

apex/parallel/distributed.py

@@ -48,14 +48,15 @@ class DistributedDataParallel(Module):
     Args:
         module: Network definition to be run in multi-gpu/distributed mode.
         message_size (Default = 100e6): Minimum number of elements in a communication bucket.
-
+        shared_param (Default = False): If your model uses shared parameters this must be true,
+        it will disable bucketing of parameters which is necessary to avoid race conditions.
 
     """
 
-    def __init__(self, module, message_size=100000000):
+    def __init__(self, module, message_size=100000000, shared_param=False):
         super(DistributedDataParallel, self).__init__()
         self.warn_on_half = True if dist._backend == dist.dist_backend.GLOO else False
-        
+        self.shared_param = shared_param
         self.message_size = message_size
         
         #reference to last iterations parameters to see if anything has changed
@@ -119,8 +120,7 @@ class DistributedDataParallel(Module):
                         Variable._execution_engine.queue_callback(allreduce_params)
                     else:
                         Variable._execution_engine.queue_callback(flush_buckets)
-                        self.param_state[self.record.index(param_i)] = 1
-                        self.comm_ready_buckets()
+                        self.comm_ready_buckets(self.record.index(param_i))
                     
                     
                 if param.requires_grad:
@@ -128,9 +128,14 @@ class DistributedDataParallel(Module):
             wrapper(param_i)
 
 
-    def comm_ready_buckets(self):
+    def comm_ready_buckets(self, param_ind):
+
+        if self.param_state[param_ind] != 0:
+            raise RuntimeError("Error: Your model uses shared parameters, DDP flag shared_params must be set to True in initialization.")
+            
         
         if self.param_state[self.ready_end] == 0:
+            self.param_state[param_ind] = 1
             return
 
 
@@ -141,6 +146,7 @@ class DistributedDataParallel(Module):
 
 
         if self.ready_numel < self.message_size:
+            self.param_state[param_ind] = 1
             return
             
         grads = [param.grad.data for param in self.ready_params]
@@ -168,11 +174,18 @@ class DistributedDataParallel(Module):
                 self.param_state[i] = 2
                 self.ready_params.pop(0)
 
+        self.param_state[param_ind] = 1
+        
     def forward(self, *inputs, **kwargs):
 
         param_list = [param for param in list(self.module.parameters()) if param.requires_grad]
 
 
+        #Force needs_refresh to True if there are shared params
+        #this will force it to always, only call flush_buckets which is safe
+        #for shared parameters in the model.
+        if self.shared_param:
+            self.param_refs = []
             
         self.needs_refresh = True if not self.param_refs else any(
             [param1 is not param2 for param1, param2 in zip(param_list, self.param_refs)]

tests/distributed/ddp_race_condition_test.py

+import torch
+import torch.distributed as dist
+from torch.nn import Parameter
+from torch.nn import Module
+from apex.parallel import DistributedDataParallel as DDP
+import argparse
+
+
+parser = argparse.ArgumentParser(description='allreduce hook example')
+parser.add_argument('--dist-url', default='tcp://224.66.41.62:23456', type=str,
+                    help='url used to set up distributed training')
+parser.add_argument('--dist-backend', default='nccl', type=str,
+                    help='distributed backend')
+parser.add_argument('--world-size', default=1, type=int,
+                    help='Number of GPUs to use. Can either be manually set ' +
+                    'or automatically set by using \'python -m multiproc\'.')
+parser.add_argument('--rank', default=0, type=int,
+                    help='Used for multi-process training. Can either be manually set ' +
+                    'or automatically set by using \'python -m multiproc\'.')
+
+args = parser.parse_args()
+
+args.distributed = args.world_size > 1
+
+if args.distributed:
+    torch.cuda.set_device(args.rank % torch.cuda.device_count())
+    dist.init_process_group(args.dist_backend, init_method=args.dist_url,
+                            world_size=args.world_size)
+    rank = torch.distributed.get_rank()
+torch.set_printoptions(precision=10)
+
+class Model(Module):
+    def __init__(self):
+        super(Model, self).__init__()
+        self.x = Parameter(torch.cuda.FloatTensor(1,4096*4096).fill_(1.0))
+    def forward(self, input):
+        return self.x*input
+model = DDP(Model(), message_size=1)
+
+z = torch.cuda.FloatTensor(4096*4096)
+
+for i in range(10):
+    z.fill_(i + rank) # fill z with new values every iteration for sanity
+    model.zero_grad()
+    out = model(z)
+    loss = out.sum()
+    torch.cuda.nvtx.range_push("backward")
+    loss.backward()
+    torch.cuda.nvtx.range_pop()
+    
+    torch.cuda.nvtx.range_push("synchronize() + sum")
+    torch.cuda.synchronize()
+    for param in model.parameters():
+        print("i = {},\n"
+              "param.grad.data_ptr() = {}\n"
+              "expected {},\n" 
+              "     got {}\n"
+              .format(i,
+                      param.grad.data_ptr(),
+                      4096*4096*(2.*i+1)/2.,
+                      param.grad.data.sum().item()))
+    torch.cuda.nvtx.range_pop()
+

tests/distributed/run_race_test.sh

+#!/bin/bash
+
+CUDA_VISIBLE_DEVICES=0,1 python -m apex.parallel.multiproc ddp_race_condition.py