Efficiency of DataParallel(DP), ModelParallel and Distributed DP

I was playing around with the PyTorch distributed methodologies, and am wondering whether there’s any comparison documentation/tutorial available?

It appears that using DP actually takes 2x time on FashionMNIST classification task, than not using it.

For e.g., in the reference code below, net_dp took 367.25 s to train compared to 149.9 s using a single NVIDIA RTX a6000 GPU.

Wondering why is this or am I doing it wrong?


  1. Sample code
class Net(nn.Module):
    def __init__(self):
        super(Net, self).__init__()
        self.conv1 = nn.Conv2d(1, 6, 5)
        self.pool = nn.MaxPool2d(2, 2)
        self.conv2 = nn.Conv2d(6, 16, 5)
        self.fc1 = nn.Linear(16 * 4 * 4, 120)
        self.fc2 = nn.Linear(120, 84)
        self.fc3 = nn.Linear(84, 10)

    def forward(self, x):
        x = self.pool(F.relu(self.conv1(x)))
        x = self.pool(F.relu(self.conv2(x)))
        x = x.view(-1, 16 * 4 * 4)
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        x = self.fc3(x)
        return x

def train(net: nn.Module, is_dp=False): 

device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")

net1 = nn.DataPrallel(Net())
train(net1, True)

  (module): Net(
    (conv1): Conv2d(1, 6, kernel_size=(5, 5), stride=(1, 1))
    (pool): MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=False)
    (conv2): Conv2d(6, 16, kernel_size=(5, 5), stride=(1, 1))
    (fc1): Linear(in_features=256, out_features=120, bias=True)
    (fc2): Linear(in_features=120, out_features=84, bias=True)
    (fc3): Linear(in_features=84, out_features=10, bias=True)
net2_single-gpu = Net().to(device)
train(net2_single-gpu, False)

Hey @rabot, DP broadcasts model to all devices in every iteration. So if the parallelism on compute does not lead to sufficient speed up, the overhead of broadcasting model and gathering gradients make it slower than local training.

Usually, DDP/DP is used when the global batch size large enough to make full use of the parallelism. For comparisons, please see this post: PyTorch Data Parallel Best Practices on Google Cloud | by PyTorch | PyTorch | Medium

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