PyTorch

PyTorch is a Python package that provides two high-level features:

• Tensor computation (like NumPy) with strong GPU acceleration
• Deep neural networks built on a tape-based autograd system

If you are porting a PyTorch program to a Compute Canada cluster, you should follow our tutorial on the subject.

Disambiguation

PyTorch has a distant connection with Torch, but for all practical purposes you can treat them as separate projects.

PyTorch developers also offer LibTorch, which allows one to implement extensions to PyTorch using C++, and to implement pure C++ machine learning applications. Models written in Python using PyTorch can be converted and used in pure C++ through TorchScript.

Installation

Latest available wheels

To see the latest version of PyTorch that we have built:

[name@server ~]$ avail_wheels "torch*"

For more information on listing wheels, see listing available wheels.

Installing Compute Canada wheel

The preferred option is to install it using the Python wheel as follows:

1. Load a Python module, either python/2.7, python/3.5, python/3.6 or python/3.7

2. Create and start a virtual environment.

3. Install PyTorch in the virtual environment with pip install.

GPU and CPU

(venv) [name@server ~] pip install --no-index torch

Extra

In addition to torch, you can install torchvision, torchtext and torchaudio:

(venv) [name@server ~] pip install --no-index torch torchvision torchtext torchaudio

Job submission

Here is an example of a job submission script using the python wheel, with a virtual environment inside a job:

File : pytorch-test.sh

#!/bin/bash
#SBATCH --gres=gpu:1       # Request GPU "generic resources"
#SBATCH --cpus-per-task=6  # Cores proportional to GPUs: 6 on Cedar, 16 on Graham.
#SBATCH --mem=32000M       # Memory proportional to GPUs: 32000 Cedar, 64000 Graham.
#SBATCH --time=0-03:00
#SBATCH --output=%N-%j.out

module load python/3.6
virtualenv --no-download $SLURM_TMPDIR/env
source $SLURM_TMPDIR/env/bin/activate
pip install torch --no-index

python pytorch-test.py

The Python script pytorch-test.py has the form

File : pytorch-test.py

import torch
x = torch.Tensor(5, 3)
print(x)
y = torch.rand(5, 3)
print(y)
# let us run the following only if CUDA is available
if torch.cuda.is_available():
    x = x.cuda()
    y = y.cuda()
    print(x + y)

You can then submit a PyTorch job with:

[name@server ~]$ sbatch pytorch-test.sh

PyTorch with Multiple GPUs

There are several ways to use PyTorch with multiple GPUs. This section features tutorials on two of them: using the DistributedDataParallel class and using the PyTorch Lightning package.

import os import time import datetime

import torch import torch.nn as nn import torch.nn.functional as F import torch.optim as optim import torch.backends.cudnn as cudnn

import torchvision import torchvision.transforms as transforms from torchvision.datasets import CIFAR10 from torch.utils.data import DataLoader

import torch.distributed as dist import torch.utils.data.distributed

import argparse

parser = argparse.ArgumentParser(description='cifar10 classification models, distributed data parallel test') parser.add_argument('--lr', default=0.1, help=) parser.add_argument('--batch_size', type=int, default=768, help=) parser.add_argument('--max_epochs', type=int, default=4, help=) parser.add_argument('--num_workers', type=int, default=0, help=)

parser.add_argument('--init_method', default='tcp://127.0.0.1:3456', type=str, help=) parser.add_argument('--dist-backend', default='gloo', type=str, help=) parser.add_argument('--world_size', default=1, type=int, help=) parser.add_argument('--distributed', action='store_true', help=)

def main():

   print("Starting...")

   args = parser.parse_args()

   ngpus_per_node = torch.cuda.device_count()

   """ This next line is the key to getting DistributedDataParallel working on SLURM:

SLURM_NODEID is 0 or 1 in this example, SLURM_LOCALID is the id of the

		current process inside a node and is also 0 or 1 in this example."""

   local_rank = int(os.environ.get("SLURM_LOCALID")) 

rank = int(os.environ.get("SLURM_NODEID"))*ngpus_per_node + local_rank

""" This next block parses CUDA_VISIBLE_DEVICES to find out which GPUs have been allocated to the job, then sets torch.device to the GPU corresponding to the local rank (local rank 0 gets the first GPU, local rank 1 gets the second GPU etc.) """

available_gpus = list(os.environ.get('CUDA_VISIBLE_DEVICES').replace(',',""))

torch.cuda.set_device(local_rank)

   """ this block initializes a process group and initiate communications

between all processes running on all nodes """

   print('From Rank: {}, ==> Initializing Process Group...'.format(rank))
   #init the process group
   dist.init_process_group(backend=args.dist_backend, init_method=args.init_method, world_size=args.world_size, rank=rank)
   print("process group ready!")

   print('From Rank: {}, ==> Making model..'.format(rank))

   class Net(nn.Module):

      def __init__(self):
         super(Net, self).__init__()

         self.conv1 = nn.Conv2d(3, 6, 5)
         self.pool = nn.MaxPool2d(2, 2)
         self.conv2 = nn.Conv2d(6, 16, 5)
         self.fc1 = nn.Linear(16 * 5 * 5, 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 * 5 * 5)
         x = F.relu(self.fc1(x))
         x = F.relu(self.fc2(x))
         x = self.fc3(x)
         return x

   net = Net()

   net.cuda()

net = torch.nn.parallel.DistributedDataParallel(net,device_ids=[current_device])

   print('From Rank: {}, ==> Preparing data..'.format(rank))

   transform_train = transforms.Compose([transforms.ToTensor(),transforms.Normalize((0.5, 0.5, 0.5), (0.5, 0.5, 0.5))])

   dataset_train = CIFAR10(root='./data', train=True, download=False, transform=transform_train)

   train_sampler = torch.utils.data.distributed.DistributedSampler(dataset_train)
   train_loader = DataLoader(dataset_train, batch_size=args.batch_size, shuffle=(train_sampler is None), num_workers=args.num_workers, sampler=train_sampler)

   criterion = nn.CrossEntropyLoss().cuda()
   optimizer = optim.SGD(net.parameters(), lr=args.lr, momentum=0.9, weight_decay=1e-4)

   for epoch in range(args.max_epochs):

       train_sampler.set_epoch(epoch)

       train(epoch, net, criterion, optimizer, train_loader, rank)

def train(epoch, net, criterion, optimizer, train_loader, train_rank):

   train_loss = 0
   correct = 0
   total = 0

   epoch_start = time.time()

   for batch_idx, (inputs, targets) in enumerate(train_loader):

      start = time.time()

      inputs = inputs.cuda()
      targets = targets.cuda()
      outputs = net(inputs)
      loss = criterion(outputs, targets)

      optimizer.zero_grad()
      loss.backward()
      optimizer.step()

      train_loss += loss.item()
      _, predicted = outputs.max(1)
      total += targets.size(0)
      correct += predicted.eq(targets).sum().item()
      acc = 100 * correct / total

      batch_time = time.time() - start

      elapse_time = time.time() - epoch_start
      elapse_time = datetime.timedelta(seconds=elapse_time)
      print("From Rank: {}, Training time {}".format(train_rank, elapse_time))

if __name__=='__main__':

  main()

Using PyTorch Lightning

PyTorch Lightning is a Python package that provides interfaces to PyTorch to make many common, but otherwise code-heavy tasks, more straightforward. This includes training on multiple GPUs. The following is the same tutorial from the section above, but using PyTorch Lightning instead of explicitly leveraging the DistributedDataParallel class:

File : pytorch-ddp-test-pl.sh

#!/bin/bash
#SBATCH --nodes 2              # Request 2 node so all resources are in two nodes.
#SBATCH --gres=gpu:2          # Request 2 GPU "generic resources”. You will get 2 per node.

#SBATCH --tasks-per-node=2      # Request 1 process per GPU. You will get 1 CPU per process by default. Request more CPUs with the "cpus-per-task" parameter to enable multiple data-loader workers to load data in parallel.

#SBATCH --mem=8G      
#SBATCH --time=0-03:00
#SBATCH --output=%N-%j.out

module load python/3.6
virtualenv --no-download $SLURM_TMPDIR/env
source $SLURM_TMPDIR/env/bin/activate
pip install torchvision pytorch-lightning --no-index

export NCCL_BLOCKING_WAIT=1 #Pytorch Lightning uses the NCCL backend for inter-GPU communication by default. Set this variable to avoid timeout errors.

srun python pytorch-ddp-test-pl.py  --batch_size 256

File : pytorch-ddp-test-pl.py

import datetime

import torch
from torch import nn
import torch.nn.functional as F

import pytorch_lightning as pl

import torchvision
import torchvision.transforms as transforms
from torchvision.datasets import CIFAR10
from torch.utils.data import DataLoader


import argparse


parser = argparse.ArgumentParser(description='cifar10 classification models, pytorch-lightning parallel test')
parser.add_argument('--lr', default=0.1, help='')
parser.add_argument('--max_epochs', type=int, default=4, help='')
parser.add_argument('--batch_size', type=int, default=768, help='')
parser.add_argument('--num_workers', type=int, default=0, help='')

def main():
    print("Starting...")

    args = parser.parse_args()

    class Net(pl.LightningModule):

       def __init__(self):
          super(Net, self).__init__()

          self.conv1 = nn.Conv2d(3, 6, 5)
          self.pool = nn.MaxPool2d(2, 2)
          self.conv2 = nn.Conv2d(6, 16, 5)
          self.fc1 = nn.Linear(16 * 5 * 5, 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 * 5 * 5)
          x = F.relu(self.fc1(x))
          x = F.relu(self.fc2(x))
          x = self.fc3(x)
          return x

       def training_step(self, batch, batch_idx):
          x, y = batch
          y_hat = self(x)
          loss = F.cross_entropy(y_hat, y)
          return loss

       def configure_optimizers(self):
          return torch.optim.Adam(self.parameters(), lr=args.lr)

    net = Net()

    """ Here we initialize a Trainer() explicitly with 2 nodes and 2 GPUs per node.
        To make this script more generic, you can use torch.cuda.device_count() to set the number of GPUs
        and you can use int(os.environ.get("SLURM_JOB_NUM_NODES")) to set the number of nodes."""

    trainer = pl.Trainer(gpus=2, num_nodes=2,accelerator='ddp', max_epochs = args.max_epochs)

    transform_train = transforms.Compose([transforms.ToTensor(),transforms.Normalize((0.5, 0.5, 0.5), (0.5, 0.5, 0.5))])

    dataset_train = CIFAR10(root='./data', train=True, download=False, transform=transform_train)

    train_loader = DataLoader(dataset_train, batch_size=args.batch_size, num_workers=args.num_workers)

    trainer.fit(net,train_loader)


if __name__=='__main__':
   main()

Troubleshooting

Memory leak

On AVX512 hardware (Béluga, Skylake or V100 nodes), older versions of Pytorch (less than v1.0.1) using older libraries (cuDNN < v7.5 or MAGMA < v2.5) may considerably leak memory resulting in an out-of-memory exception and death of your tasks. Please upgrade to the latest torch version.

LibTorch

LibTorch allows one to implement both C++ extensions to PyTorch and pure C++ machine learning applications. It contains "all headers, libraries and CMake configuration files required to depend on PyTorch" (as mentioned in the docs).

How to use LibTorch

Get the library

wget https://download.pytorch.org/libtorch/cu100/libtorch-shared-with-deps-latest.zip
unzip libtorch-shared-with-deps-latest.zip
cd libtorch
export LIBTORCH_ROOT=$(pwd)  # this variable is used in the example below

Patch the library (this workaround is needed for compiling on Compute Canada clusters):

sed -i -e 's/\/usr\/local\/cuda\/lib64\/libculibos.a;dl;\/usr\/local\/cuda\/lib64\/libculibos.a;//g' share/cmake/Caffe2/Caffe2Targets.cmake

Compile a minimal example

Create the following two files:

File : example-app.cpp

#include <torch/torch.h>
#include <iostream>

int main() {
    torch::Device device(torch::kCPU);
    if (torch::cuda::is_available()) {
        std::cout << "CUDA is available! Using GPU." << std::endl;
        device = torch::Device(torch::kCUDA);
    }

    torch::Tensor tensor = torch::rand({2, 3}).to(device);
    std::cout << tensor << std::endl;
}

File : CMakeLists.txt

cmake_minimum_required(VERSION 3.0 FATAL_ERROR)
project(example-app)

find_package(Torch REQUIRED)

add_executable(example-app example-app.cpp)
target_link_libraries(example-app "${TORCH_LIBRARIES}")
set_property(TARGET example-app PROPERTY CXX_STANDARD 11)

Load the necessary modules:

module load cmake intel/2018.3 cuda/10 cudnn

Compile the program:

mkdir build
cd build
cmake -DCMAKE_PREFIX_PATH="$LIBTORCH_ROOT;$EBROOTCUDA;$EBROOTCUDNN" ..
make

Run the program:

./example-app

To test an application with CUDA, request an interactive job with a GPU.

Resources

https://pytorch.org/cppdocs/