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PyTorch Distributed Overview

Author: Shen Li

This is the overview page for the torch.distributed package. As there are more and more documents, examples and tutorials added at different locations, it becomes unclear which document or tutorial to consult for a specific problem or what is the best order to read these contents. The goal of this page is to address this problem by categorizing documents into different topics and briefly describe each of them. If this is your first time building distributed training applications using PyTorch, it is recommended to use this document to navigate to the technology that can best serve your use case.

Introduction

As of PyTorch v1.6.0, features in torch.distributed can be categorized into three main components:

  • Distributed Data-Parallel Training (DDP) is a widely adopted single-program multiple-data training paradigm. With DDP, the model is replicated on every process, and every model replica will be fed with a different set of input data samples. DDP takes care of gradient communications to keep model replicas synchronized and overlaps it with the gradient computations to speed up training.

  • RPC-Based Distributed Training (RPC) is developed to support general training structures that cannot fit into data-parallel training, such as distributed pipeline parallelism, parameter server paradigm, and combination of DDP with other training paradigms. It helps manage remote object lifetime and extend autograd engine to beyond machine boundaries.

  • Collective Communication (c10d) library support sending tensors across processes within a group. It offers both collective communication APIs (e.g., all_reduce and all_gather) and P2P communication APIs (e.g., send and isend). DDP and RPC (ProcessGroup Backend) are built on c10d as of v1.6.0, where the former uses collective communications and the latter uses P2P communications. Usually, developers do not need to directly use this raw communication API, as DDP and RPC features above can serve many distributed training scenarios. However, there are use cases where this API is still helpful. One example would be distributed parameter averaging, where applications would like to compute the average values of all model parameters after the backward pass instead of using DDP to communicate gradients. This can decouple communications from computations and allow finer-grain control over what to communicate, but on the other hand, it also gives up the performance optimizations offered by DDP. The Writing Distributed Applications with PyTorch shows examples of using c10d communication APIs.

Most of the existing documents are written for either DDP or RPC, the remainder of this page will elaborate materials for these two components.

Data Parallel Training

PyTorch provides several options for data-parallel training. For applications that gradually grow from simple to complex and from prototype to production, the common development trajectory would be:

  1. Use single-device training, if the data and model can fit in one GPU, and the training speed is not a concern.

  2. Use single-machine multi-GPU DataParallel, if there are multiple GPUs on the server, and you would like to speed up training with the minimum code change.

  3. Use single-machine multi-GPU DistributedDataParallel, if you would like to further speed up training and are willing to write a little more code to set it up.

  4. Use multi-machine DistributedDataParallel and the launching script, if the application needs to scale across machine boundaries.

  5. Use torchelastic to launch distributed training, if errors (e.g., OOM) are expected or if the resources can join and leave dynamically during the training.

Note

Data-parallel training also works with Automatic Mixed Precision (AMP).

torch.nn.DataParallel

The DataParallel package enables single-machine multi-GPU parallelism with the lowest coding hurdle. It only requires a one-line change to the application code. The tutorial Optional: Data Parallelism shows an example. The caveat is that, although DataParallel is very easy to use, it usually does not offer the best performance. This is because the implementation of DataParallel replicates the model in every forward pass, and its single-process multi-thread parallelism naturally suffers from GIL contentions. To get better performance, please consider using DistributedDataParallel.

torch.nn.parallel.DistributedDataParallel

Compared to DataParallel, DistributedDataParallel requires one more step to set up, i.e., calling init_process_group. DDP uses multi-process parallelism, and hence there is no GIL contention across model replicas. Moreover, the model is broadcast at DDP construction time instead of in every forward pass, which also helps to speed up training. DDP is shipped with several performance optimization technologies. For a more in-depth explanation, please refer to this DDP paper (VLDB‘20).

DDP materials are listed below:

  1. DDP notes offer a starter example and some brief descriptions of its design and implementation. If this is your first time using DDP, please start from this document.

  2. Getting Started with Distributed Data Parallel explains some common problems with DDP training, including unbalanced workload, checkpointing, and multi-device models. Note that, DDP can be easily combined with single-machine multi-device model parallelism which is described in the Single-Machine Model Parallel Best Practices tutorial.

  3. The Launching and configuring distributed data parallel applications document shows how to use the DDP launching script.

  4. PyTorch Distributed Trainer with Amazon AWS demonstrates how to use DDP on AWS.

TorchElastic

With the growth of the application complexity and scale, failure recovery becomes an imperative requirement. Sometimes, it is inevitable to hit errors like OOM when using DDP, but DDP itself cannot recover from those errors nor does basic try-except block work. This is because DDP requires all processes to operate in a closely synchronized manner and all AllReduce communications launched in different processes must match. If one of the processes in the group throws an OOM exception, it is likely to lead to desynchronization (mismatched AllReduce operations) which would then cause a crash or hang. If you expect failures to occur during training or if resources might leave and join dynamically, please launch distributed data-parallel training using torchelastic.

General Distributed Training

Many training paradigms do not fit into data parallelism, e.g., parameter server paradigm, distributed pipeline parallelism, reinforcement learning applications with multiple observers or agents, etc. The torch.distributed.rpc aims at supporting general distributed training scenarios.

The torch.distributed.rpc package has four main pillars:

  • RPC supports running a given function on a remote worker.

  • RRef helps to manage the lifetime of a remote object. The reference counting protocol is presented in the RRef notes.

  • Distributed Autograd extends the autograd engine beyond machine boundaries. Please refer to Distributed Autograd Design for more details.

  • Distributed Optimizer that automatically reaches out to all participating workers to update parameters using gradients computed by the distributed autograd engine.

RPC Tutorials are listed below:

  1. The Getting Started with Distributed RPC Framework tutorial first uses a simple Reinforcement Learning (RL) example to demonstrate RPC and RRef. Then, it applies a basic distributed model parallelism to an RNN example to show how to use distributed autograd and distributed optimizer.

  2. The Implementing a Parameter Server Using Distributed RPC Framework tutorial borrows the spirit of HogWild! training and applies it to an asynchronous parameter server (PS) training application.

  3. The Distributed Pipeline Parallelism Using RPC tutorial extends the single-machine pipeline parallel example (presented in Single-Machine Model Parallel Best Practices) to a distributed environment and shows how to implement it using RPC.

  4. The Implementing Batch RPC Processing Using Asynchronous Executions tutorial demonstrates how to implement RPC batch processing using the @rpc.functions.async_execution decorator, which can help speed up inference and training. It uses similar RL and PS examples employed in the above tutorials 1 and 2.


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