Distributed communication package - torch.distributed

Backends

torch.distributed supports three backends, each with different capabilities. The table below shows which functions are available for use with CPU / CUDA tensors. MPI supports CUDA only if the implementation used to build PyTorch supports it.

Backend	gloo		mpi		nccl
Device	CPU	GPU	CPU	GPU	CPU	GPU
send	✓	✘	✓	?	✘	✘
recv	✓	✘	✓	?	✘	✘
broadcast	✓	✓	✓	?	✘	✓
all_reduce	✓	✓	✓	?	✘	✓
reduce	✓	✘	✓	?	✘	✓
all_gather	✓	✘	✓	?	✘	✓
gather	✓	✘	✓	?	✘	✘
scatter	✓	✘	✓	?	✘	✘
barrier	✓	✘	✓	?	✘	✓

Backends that come with PyTorch

PyTorch distributed currently only supports Linux. By default, the Gloo and NCCL backends are built and included in PyTorch distributed (NCCL only when building with CUDA). MPI is an optional backend that can only be included if you build PyTorch from source. (e.g. building PyTorch on a host that has MPI installed.)

Which backend to use?

In the past, we were often asked: “which backend should I use?”.

• Rule of thumb

  • Use the NCCL backend for distributed GPU training

  • Use the Gloo backend for distributed CPU training.

• GPU hosts with InfiniBand interconnect

  • Use NCCL, since it’s the only backend that currently supports InfiniBand and GPUDirect.

• GPU hosts with Ethernet interconnect

  • Use NCCL, since it currently provides the best distributed GPU training performance, especially for multiprocess single-node or multi-node distributed training. If you encounter any problem with NCCL, use Gloo as the fallback option. (Note that Gloo currently runs slower than NCCL for GPUs.)

• CPU hosts with InfiniBand interconnect

  • If your InfiniBand has enabled IP over IB, use Gloo, otherwise, use MPI instead. We are planning on adding InfiniBand support for Gloo in the upcoming releases.

• CPU hosts with Ethernet interconnect

  • Use Gloo, unless you have specific reasons to use MPI.

Common environment variables

Choosing the network interface to use

By default, both the NCCL and Gloo backends will try to find the right network interface to use. If the automatically detected interface is not correct, you can override it using the following environment variables (applicable to the respective backend):

• NCCL_SOCKET_IFNAME, for example export NCCL_SOCKET_IFNAME=eth0

• GLOO_SOCKET_IFNAME, for example export GLOO_SOCKET_IFNAME=eth0

If you’re using the Gloo backend, you can specify multiple interfaces by separating them by a comma, like this: export GLOO_SOCKET_IFNAME=eth0,eth1,eth2,eth3. The backend will dispatch operations in a round-robin fashion across these interfaces. It is imperative that all processes specify the same number of interfaces in this variable.

Other NCCL environment variables

NCCL has also provided a number of environment variables for fine-tuning purposes.

Commonly used ones include the following for debugging purposes:

• export NCCL_DEBUG=INFO

• export NCCL_DEBUG_SUBSYS=ALL

For the full list of NCCL environment variables, please refer to NVIDIA NCCL’s official documentation

Basics

The torch.distributed package provides PyTorch support and communication primitives for multiprocess parallelism across several computation nodes running on one or more machines. The class torch.nn.parallel.DistributedDataParallel() builds on this functionality to provide synchronous distributed training as a wrapper around any PyTorch model. This differs from the kinds of parallelism provided by Multiprocessing package - torch.multiprocessing and torch.nn.DataParallel() in that it supports multiple network-connected machines and in that the user must explicitly launch a separate copy of the main training script for each process.

In the single-machine synchronous case, torch.distributed or the torch.nn.parallel.DistributedDataParallel() wrapper may still have advantages over other approaches to data-parallelism, including torch.nn.DataParallel():

• Each process maintains its own optimizer and performs a complete optimization step with each iteration. While this may appear redundant, since the gradients have already been gathered together and averaged across processes and are thus the same for every process, this means that no parameter broadcast step is needed, reducing time spent transferring tensors between nodes.

• Each process contains an independent Python interpreter, eliminating the extra interpreter overhead and “GIL-thrashing” that comes from driving several execution threads, model replicas, or GPUs from a single Python process. This is especially important for models that make heavy use of the Python runtime, including models with recurrent layers or many small components.

Initialization

The package needs to be initialized using the torch.distributed.init_process_group() function before calling any other methods. This blocks until all processes have joined.

torch.distributed.init_process_group(backend, init_method=None, timeout=datetime.timedelta(0, 1800), world_size=-1, rank=-1, store=None, group_name='')

Initializes the default distributed process group, and this will also initialize the distributed package.

There are 2 main ways to initialize a process group:

1. Specify store, rank, and world_size explicitly.

2. Specify init_method (a URL string) which indicates where/how to discover peers. Optionally specify rank and world_size, or encode all required parameters in the URL and omit them.

If neither is specified, init_method is assumed to be “env://”.

Parameters

• backend (str or Backend) – The backend to use. Depending on build-time configurations, valid values include mpi, gloo, and nccl. This field should be given as a lowercase string (e.g., "gloo"), which can also be accessed via Backend attributes (e.g., Backend.GLOO). If using multiple processes per machine with nccl backend, each process must have exclusive access to every GPU it uses, as sharing GPUs between processes can result in deadlocks.

• init_method (str, optional) – URL specifying how to initialize the process group. Default is “env://” if no init_method or store is specified. Mutually exclusive with store.

• world_size (int, optional) – Number of processes participating in the job. Required if store is specified.

• rank (int, optional) – Rank of the current process. Required if store is specified.

• store (Store, optional) – Key/value store accessible to all workers, used to exchange connection/address information. Mutually exclusive with init_method.

• timeout (timedelta, optional) – Timeout for operations executed against the process group. Default value equals 30 minutes. This is applicable for the gloo backend. For nccl, this is applicable only if the environment variable NCCL_BLOCKING_WAIT is set to 1.

• group_name (str, optional, deprecated) – Group name.

To enable backend == Backend.MPI, PyTorch needs to built from source on a system that supports MPI.

class torch.distributed.Backend

An enum-like class of available backends: GLOO, NCCL, and MPI.

The values of this class are lowercase strings, e.g., "gloo". They can be accessed as attributes, e.g., Backend.NCCL.

This class can be directly called to parse the string, e.g., Backend(backend_str) will check if backend_str is valid, and return the parsed lowercase string if so. It also accepts uppercase strings, e.g., Backend("GLOO") returns "gloo".

Note

The entry Backend.UNDEFINED is present but only used as initial value of some fields. Users should neither use it directly nor assume its existence.

torch.distributed.get_backend(group=<object object>)

Returns the backend of the given process group.

Parameters

group (ProcessGroup, optional) – The process group to work on. The default is the general main process group. If another specific group is specified, the calling process must be part of group.

Returns

The backend of the given process group as a lower case string.

torch.distributed.get_rank(group=<object object>)

Returns the rank of current process group

Rank is a unique identifier assigned to each process within a distributed process group. They are always consecutive integers ranging from 0 to world_size.

Parameters

group (ProcessGroup, optional) – The process group to work on

Returns

The rank of the process group -1, if not part of the group

torch.distributed.get_world_size(group=<object object>)

Returns the number of processes in the current process group

Parameters

group (ProcessGroup, optional) – The process group to work on

Returns

The world size of the process group -1, if not part of the group

torch.distributed.is_initialized()

Checking if the default process group has been initialized

torch.distributed.is_mpi_available()

Checks if the MPI backend is available.

torch.distributed.is_nccl_available()

Checks if the NCCL backend is available.

---

Currently three initialization methods are supported:

TCP initialization

There are two ways to initialize using TCP, both requiring a network address reachable from all processes and a desired world_size. The first way requires specifying an address that belongs to the rank 0 process. This initialization method requires that all processes have manually specified ranks.

Note that multicast address is not supported anymore in the latest distributed package. group_name is deprecated as well.

import torch.distributed as dist

# Use address of one of the machines
dist.init_process_group(backend, init_method='tcp://10.1.1.20:23456',
                        rank=args.rank, world_size=4)

Shared file-system initialization

Another initialization method makes use of a file system that is shared and visible from all machines in a group, along with a desired world_size. The URL should start with file:// and contain a path to a non-existent file (in an existing directory) on a shared file system. File-system initialization will automatically create that file if it doesn’t exist, but will not delete the file. Therefore, it is your responsibility to make sure that the file is cleaned up before the next init_process_group() call on the same file path/name.

Note that automatic rank assignment is not supported anymore in the latest distributed package and group_name is deprecated as well.

Warning

This method assumes that the file system supports locking using fcntl - most local systems and NFS support it.

Warning

This method will always create the file and try its best to clean up and remove the file at the end of the program. In other words, each initialization with the file init method will need a brand new empty file in order for the initialization to succeed. If the same file used by the previous initialization (which happens not to get cleaned up) is used again, this is unexpected behavior and can often cause deadlocks and failures. Therefore, even though this method will try its best to clean up the file, if the auto-delete happens to be unsuccessful, it is your responsibility to ensure that the file is removed at the end of the training to prevent the same file to be reused again during the next time. This is especially important if you plan to call init_process_group() multiple times on the same file name. In other words, if the file is not removed/cleaned up and you call init_process_group() again on that file, failures are expected. The rule of thumb here is that, make sure that the file is non-existent or empty every time init_process_group() is called.

import torch.distributed as dist

# rank should always be specified
dist.init_process_group(backend, init_method='file:///mnt/nfs/sharedfile',
                        world_size=4, rank=args.rank)

Environment variable initialization

This method will read the configuration from environment variables, allowing one to fully customize how the information is obtained. The variables to be set are:

• MASTER_PORT - required; has to be a free port on machine with rank 0

• MASTER_ADDR - required (except for rank 0); address of rank 0 node

• WORLD_SIZE - required; can be set either here, or in a call to init function

• RANK - required; can be set either here, or in a call to init function

The machine with rank 0 will be used to set up all connections.

This is the default method, meaning that init_method does not have to be specified (or can be env://).

Groups

By default collectives operate on the default group (also called the world) and require all processes to enter the distributed function call. However, some workloads can benefit from more fine-grained communication. This is where distributed groups come into play. new_group() function can be used to create new groups, with arbitrary subsets of all processes. It returns an opaque group handle that can be given as a group argument to all collectives (collectives are distributed functions to exchange information in certain well-known programming patterns).

torch.distributed.new_group(ranks=None, timeout=datetime.timedelta(0, 1800), backend=None)

Creates a new distributed group.

This function requires that all processes in the main group (i.e. all processes that are part of the distributed job) enter this function, even if they are not going to be members of the group. Additionally, groups should be created in the same order in all processes.

Parameters

• ranks (list[int]) – List of ranks of group members.

• timeout (timedelta, optional) – Timeout for operations executed against the process group. Default value equals 30 minutes. This is only applicable for the gloo backend.

• backend (str or Backend, optional) – The backend to use. Depending on build-time configurations, valid values are gloo and nccl. By default uses the same backend as the global group. This field should be given as a lowercase string (e.g., "gloo"), which can also be accessed via Backend attributes (e.g., Backend.GLOO).

Returns

A handle of distributed group that can be given to collective calls.

Point-to-point communication

torch.distributed.send(tensor, dst, group=<object object>, tag=0)

Sends a tensor synchronously.

Parameters

• tensor (Tensor) – Tensor to send.

• dst (int) – Destination rank.

• group (ProcessGroup, optional) – The process group to work on

• tag (int, optional) – Tag to match send with remote recv

torch.distributed.recv(tensor, src=None, group=<object object>, tag=0)

Receives a tensor synchronously.

Parameters

• tensor (Tensor) – Tensor to fill with received data.

• src (int, optional) – Source rank. Will receive from any process if unspecified.

• group (ProcessGroup, optional) – The process group to work on

• tag (int, optional) – Tag to match recv with remote send

Returns

Sender rank -1, if not part of the group

isend() and irecv() return distributed request objects when used. In general, the type of this object is unspecified as they should never be created manually, but they are guaranteed to support two methods:

• is_completed() - returns True if the operation has finished

• wait() - will block the process until the operation is finished. is_completed() is guaranteed to return True once it returns.

torch.distributed.isend(tensor, dst, group=<object object>, tag=0)

Sends a tensor asynchronously.

Parameters

• tensor (Tensor) – Tensor to send.

• dst (int) – Destination rank.

• group (ProcessGroup, optional) – The process group to work on

• tag (int, optional) – Tag to match send with remote recv

Returns

A distributed request object. None, if not part of the group

torch.distributed.irecv(tensor, src, group=<object object>, tag=0)

Receives a tensor asynchronously.

Parameters

• tensor (Tensor) – Tensor to fill with received data.

• src (int) – Source rank.

• group (ProcessGroup, optional) – The process group to work on

• tag (int, optional) – Tag to match recv with remote send

Returns

A distributed request object. None, if not part of the group

Synchronous and asynchronous collective operations

Every collective operation function supports the following two kinds of operations:

synchronous operation - the default mode, when async_op is set to False. when the function returns, it is guaranteed that the collective operation is performed (not necessarily completed if it’s a CUDA op since all CUDA ops are asynchronous), and any further function calls depending on the data of the collective operation can be called. In the synchronous mode, the collective function does not return anything

asynchronous operation - when async_op is set to True. The collective operation function returns a distributed request object. In general, you don’t need to create it manually and it is guaranteed to support two methods:

• is_completed() - returns True if the operation has finished

• wait() - will block the process until the operation is finished.

Collective functions

torch.distributed.broadcast(tensor, src, group=<object object>, async_op=False)

Broadcasts the tensor to the whole group.

tensor must have the same number of elements in all processes participating in the collective.

Parameters

• tensor (Tensor) – Data to be sent if src is the rank of current process, and tensor to be used to save received data otherwise.

• src (int) – Source rank.

• group (ProcessGroup, optional) – The process group to work on

• async_op (bool, optional) – Whether this op should be an async op

Returns

Async work handle, if async_op is set to True. None, if not async_op or if not part of the group

torch.distributed.all_reduce(tensor, op=ReduceOp.SUM, group=<object object>, async_op=False)

Reduces the tensor data across all machines in such a way that all get the final result.

After the call tensor is going to be bitwise identical in all processes.

Parameters

• tensor (Tensor) – Input and output of the collective. The function operates in-place.

• op (optional) – One of the values from torch.distributed.ReduceOp enum. Specifies an operation used for element-wise reductions.

• group (ProcessGroup, optional) – The process group to work on

• async_op (bool, optional) – Whether this op should be an async op

Returns

Async work handle, if async_op is set to True. None, if not async_op or if not part of the group

torch.distributed.reduce(tensor, dst, op=ReduceOp.SUM, group=<object object>, async_op=False)

Reduces the tensor data across all machines.

Only the process with rank dst is going to receive the final result.

Parameters

• tensor (Tensor) – Input and output of the collective. The function operates in-place.

• dst (int) – Destination rank

• op (optional) – One of the values from torch.distributed.ReduceOp enum. Specifies an operation used for element-wise reductions.

• group (ProcessGroup, optional) – The process group to work on

• async_op (bool, optional) – Whether this op should be an async op

Returns

Async work handle, if async_op is set to True. None, if not async_op or if not part of the group

torch.distributed.all_gather(tensor_list, tensor, group=<object object>, async_op=False)

Gathers tensors from the whole group in a list.

Parameters

• tensor_list (list[Tensor]) – Output list. It should contain correctly-sized tensors to be used for output of the collective.

• tensor (Tensor) – Tensor to be broadcast from current process.

• group (ProcessGroup, optional) – The process group to work on

• async_op (bool, optional) – Whether this op should be an async op

Returns

Async work handle, if async_op is set to True. None, if not async_op or if not part of the group

torch.distributed.gather(tensor, gather_list=None, dst=0, group=<object object>, async_op=False)

Gathers a list of tensors in a single process.

Parameters

• tensor (Tensor) – Input tensor.

• gather_list (list[Tensor], optional) – List of appropriately-sized tensors to use for gathered data (default is None, must be specified on the destination rank)

• dst (int, optional) – Destination rank (default is 0)

• group (ProcessGroup, optional) – The process group to work on

• async_op (bool, optional) – Whether this op should be an async op

Returns

Async work handle, if async_op is set to True. None, if not async_op or if not part of the group

torch.distributed.scatter(tensor, scatter_list=None, src=0, group=<object object>, async_op=False)

Scatters a list of tensors to all processes in a group.

Each process will receive exactly one tensor and store its data in the tensor argument.

Parameters

• tensor (Tensor) – Output tensor.

• scatter_list (list[Tensor]) – List of tensors to scatter (default is None, must be specified on the source rank)

• src (int) – Source rank (default is 0)

• group (ProcessGroup, optional) – The process group to work on

• async_op (bool, optional) – Whether this op should be an async op

Returns

Async work handle, if async_op is set to True. None, if not async_op or if not part of the group

torch.distributed.barrier(group=<object object>, async_op=False)

Synchronizes all processes.

This collective blocks processes until the whole group enters this function, if async_op is False, or if async work handle is called on wait().

Parameters

• group (ProcessGroup, optional) – The process group to work on

• async_op (bool, optional) – Whether this op should be an async op

Returns

Async work handle, if async_op is set to True. None, if not async_op or if not part of the group

class torch.distributed.ReduceOp

An enum-like class for available reduction operations: SUM, PRODUCT, MIN, MAX, BAND, BOR, and BXOR.

The values of this class can be accessed as attributes, e.g., ReduceOp.SUM. They are used in specifying strategies for reduction collectives, e.g., reduce(), all_reduce_multigpu(), etc.

Members:

SUM

PRODUCT

MIN

MAX

BAND

BOR

BXOR

class torch.distributed.reduce_op

Deprecated enum-like class for reduction operations: SUM, PRODUCT, MIN, and MAX.

ReduceOp is recommended to use instead.

Multi-GPU collective functions

If you have more than one GPU on each node, when using the NCCL and Gloo backend, broadcast_multigpu() all_reduce_multigpu() reduce_multigpu() and all_gather_multigpu() support distributed collective operations among multiple GPUs within each node. These functions can potentially improve the overall distributed training performance and be easily used by passing a list of tensors. Each Tensor in the passed tensor list needs to be on a separate GPU device of the host where the function is called. Note that the length of the tensor list needs to be identical among all the distributed processes. Also note that currently the multi-GPU collective functions are only supported by the NCCL backend.

For example, if the system we use for distributed training has 2 nodes, each of which has 8 GPUs. On each of the 16 GPUs, there is a tensor that we would like to all-reduce. The following code can serve as a reference:

Code running on Node 0

import torch
import torch.distributed as dist

dist.init_process_group(backend="nccl",
                        init_method="file:///distributed_test",
                        world_size=2,
                        rank=0)
tensor_list = []
for dev_idx in range(torch.cuda.device_count()):
    tensor_list.append(torch.FloatTensor([1]).cuda(dev_idx))

dist.all_reduce_multigpu(tensor_list)

Code running on Node 1

import torch
import torch.distributed as dist

dist.init_process_group(backend="nccl",
                        init_method="file:///distributed_test",
                        world_size=2,
                        rank=1)
tensor_list = []
for dev_idx in range(torch.cuda.device_count()):
    tensor_list.append(torch.FloatTensor([1]).cuda(dev_idx))

dist.all_reduce_multigpu(tensor_list)

After the call, all 16 tensors on the two nodes will have the all-reduced value of 16

torch.distributed.broadcast_multigpu(tensor_list, src, group=<object object>, async_op=False, src_tensor=0)

Broadcasts the tensor to the whole group with multiple GPU tensors per node.

tensor must have the same number of elements in all the GPUs from all processes participating in the collective. each tensor in the list must be on a different GPU

Only nccl and gloo backend are currently supported tensors should only be GPU tensors

Parameters

• tensor_list (List[Tensor]) – Tensors that participate in the collective operation. If src is the rank, then the specified src_tensor element of tensor_list (tensor_list[src_tensor]) will be broadcast to all other tensors (on different GPUs) in the src process and all tensors in tensor_list of other non-src processes. You also need to make sure that len(tensor_list) is the same for all the distributed processes calling this function.

• src (int) – Source rank.

• group (ProcessGroup, optional) – The process group to work on

• async_op (bool, optional) – Whether this op should be an async op

• src_tensor (int, optional) – Source tensor rank within tensor_list

Returns

Async work handle, if async_op is set to True. None, if not async_op or if not part of the group

torch.distributed.all_reduce_multigpu(tensor_list, op=ReduceOp.SUM, group=<object object>, async_op=False)

Reduces the tensor data across all machines in such a way that all get the final result. This function reduces a number of tensors on every node, while each tensor resides on different GPUs. Therefore, the input tensor in the tensor list needs to be GPU tensors. Also, each tensor in the tensor list needs to reside on a different GPU.

After the call, all tensor in tensor_list is going to be bitwise identical in all processes.

Only nccl and gloo backend is currently supported tensors should only be GPU tensors

Parameters

• list (tensor) – List of input and output tensors of the collective. The function operates in-place and requires that each tensor to be a GPU tensor on different GPUs. You also need to make sure that len(tensor_list) is the same for all the distributed processes calling this function.

• op (optional) – One of the values from torch.distributed.ReduceOp enum. Specifies an operation used for element-wise reductions.

• group (ProcessGroup, optional) – The process group to work on

• async_op (bool, optional) – Whether this op should be an async op

Returns

Async work handle, if async_op is set to True. None, if not async_op or if not part of the group

torch.distributed.reduce_multigpu(tensor_list, dst, op=ReduceOp.SUM, group=<object object>, async_op=False, dst_tensor=0)

Reduces the tensor data on multiple GPUs across all machines. Each tensor in tensor_list should reside on a separate GPU

Only the GPU of tensor_list[dst_tensor] on the process with rank dst is going to receive the final result.

Only nccl backend is currently supported tensors should only be GPU tensors

Parameters

• tensor_list (List[Tensor]) – Input and output GPU tensors of the collective. The function operates in-place. You also need to make sure that len(tensor_list) is the same for all the distributed processes calling this function.

• dst (int) – Destination rank

• op (optional) – One of the values from torch.distributed.ReduceOp enum. Specifies an operation used for element-wise reductions.

• group (ProcessGroup, optional) – The process group to work on

• async_op (bool, optional) – Whether this op should be an async op

• dst_tensor (int, optional) – Destination tensor rank within tensor_list

Returns

Async work handle, if async_op is set to True. None, otherwise

torch.distributed.all_gather_multigpu(output_tensor_lists, input_tensor_list, group=<object object>, async_op=False)

Gathers tensors from the whole group in a list. Each tensor in tensor_list should reside on a separate GPU

Only nccl backend is currently supported tensors should only be GPU tensors

Parameters

• output_tensor_lists (List[List[Tensor]]) –

  Output lists. It should contain correctly-sized tensors on each GPU to be used for output of the collective, e.g. output_tensor_lists[i] contains the all_gather result that resides on the GPU of input_tensor_list[i].

  Note that each element of output_tensor_lists has the size of world_size * len(input_tensor_list), since the function all gathers the result from every single GPU in the group. To interpret each element of output_tensor_lists[i], note that input_tensor_list[j] of rank k will be appear in output_tensor_lists[i][k * world_size + j]

  Also note that len(output_tensor_lists), and the size of each element in output_tensor_lists (each element is a list, therefore len(output_tensor_lists[i])) need to be the same for all the distributed processes calling this function.

• input_tensor_list (List[Tensor]) – List of tensors(on different GPUs) to be broadcast from current process. Note that len(input_tensor_list) needs to be the same for all the distributed processes calling this function.

• group (ProcessGroup, optional) – The process group to work on

• async_op (bool, optional) – Whether this op should be an async op

Returns

Async work handle, if async_op is set to True. None, if not async_op or if not part of the group

Launch utility

The torch.distributed package also provides a launch utility in torch.distributed.launch. This helper utility can be used to launch multiple processes per node for distributed training. This utility also supports both python2 and python3.

Spawn utility

The Multiprocessing package - torch.multiprocessing package also provides a spawn function in torch.multiprocessing.spawn(). This helper function can be used to spawn multiple processes. It works by passing in the function that you want to run and spawns N processes to run it. This can be used for multiprocess distributed training as well.

For references on how to use it, please refer to PyTorch example - ImageNet implementation

Note that this function requires Python 3.4 or higher.