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If I call model.cuda() in pytorch where model is a subclass of nn.Module, and say if I have four GPUs, how it will utilize the four GPUs and how do I know which GPUs that are using?

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model.cuda() by default will send your model to the "current device", which can be set with torch.cuda.set_device(device).

An alternative way to send the model to a specific device is model.to(torch.device('cuda:0')).

This, of course, is subject to the device visibility specified in the environment variable CUDA_VISIBLE_DEVICES.

You can check GPU usage with nvidia-smi. Also, nvtop is very nice for this.

The standard way in PyTorch to train a model in multiple GPUs is to use nn.DataParallel which copies the model to the GPUs and during training splits the batch among them and combines the individual outputs.

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