如何在 pytorch 中使用多个 GPU？

Question

I use this command to use a GPU.

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

But, I want to use two GPUs in jupyter , like this:

device = torch.device("cuda:0,1" if torch.cuda.is_available() else "cpu")

Answer 1

Using multi-GPUs is as simply as wrapping a model in DataParallel and increasing the batch size. Check these two tutorials for a quick start:

• Multi-GPU Examples
• Data Parallelism

Answer 2

Assuming that you want to distribute the data across the available GPUs (If you have batch size of 16, and 2 GPUs, you might be looking providing the 8 samples to each of the GPUs), and not really spread out the parts of models across difference GPU's. This can be done as follows:

If you want to use all the available GPUs:

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

model = CreateModel()

model= nn.DataParallel(model)
model.to(device)

If you want to use specific GPUs: (For example, using 2 out of 4 GPUs)

device = torch.device("cuda:1,3" if torch.cuda.is_available() else "cpu") ## specify the GPU id's, GPU id's start from 0.

model = CreateModel()

model= nn.DataParallel(model,device_ids = [1, 3])
model.to(device)

To use the specific GPU's by setting OS environment variable:

Before executing the program, set CUDA_VISIBLE_DEVICES variable as follows:

export CUDA_VISIBLE_DEVICES=1,3 (Assuming you want to select 2nd and 4th GPU)

Then, within program, you can just use DataParallel() as though you want to use all the GPUs. (similar to 1st case). Here the GPUs available for the program is restricted by the OS environment variable.

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

model = CreateModel()

model= nn.DataParallel(model)
model.to(device)

In all of these cases, the data has to be mapped to the device.

If X and y are the data:

X.to(device)
y.to(device)

Answer 3

Another option would be to use some helper libraries for PyTorch:

PyTorch Ignite library Distributed GPU training

In there there is a concept of context manager for distributed configuration on:

• nccl - torch native distributed configuration on multiple GPUs
• xla-tpu - TPUs distributed configuration

PyTorch Lightning Multi-GPU training

This is of possible the best option IMHO to train on CPU/GPU/TPU without changing your original PyTorch code.

Worth cheking Catalyst for similar distributed GPU options.

Answer 4

In 2022, PyTorch says:

It is recommended to use DistributedDataParallel, instead of this class, to do multi-GPU training, even if there is only a single node. See: Use nn.parallel.DistributedDataParallel instead of multiprocessing or nn.DataParallel and Distributed Data Parallel.

in https://pytorch.org/docs/stable/generated/torch.nn.DataParallel.html#torch.nn.DataParallel

Thus, seems that we should use DistributedDataParallel , not DataParallel .

Answer 5

When I ran naiveinception_googlenet, the above methods didn't work for me. The following method solved my problem.

import os

os.environ["CUDA_DEVICE_ORDER"]="PCI_BUS_ID"

os.environ["CUDA_VISIBLE_DEVICES"]="0,3" # specify which GPU(s) to be used

Answer 6

If you want to run your code only on specific GPUs (eg only on GPU id 2 and 3), then you can specify that using the CUDA_VISIBLE_DEVICES=2,3 variable when triggering the python code from terminal.

CUDA_VISIBLE_DEVICES=2,3 python lstm_demo_example.py --epochs=30 --lr=0.001

and inside the code, leave it as:

device = torch.device("cuda" if torch.cuda.is_available() else 'cpu')
model = LSTMModel()
model = nn.DataParallel(model)
model = model.to(device)

Source : https://glassboxmedicine.com/2020/03/04/multi-gpu-training-in-pytorch-data-and-model-parallelism/