---
name: pytorch-fsdp
description: Expert guidance for Fully Sharded Data Parallel training with PyTorch FSDP - parameter sharding, mixed precision, CPU offloading, FSDP2
---

# Pytorch-Fsdp Skill

Comprehensive assistance with pytorch-fsdp development, generated from official documentation.

## When to Use This Skill

This skill should be triggered when:
- Working with pytorch-fsdp
- Asking about pytorch-fsdp features or APIs
- Implementing pytorch-fsdp solutions
- Debugging pytorch-fsdp code
- Learning pytorch-fsdp best practices

## Quick Reference

### Common Patterns

**Pattern 1:** Generic Join Context Manager# Created On: Jun 06, 2025 | Last Updated On: Jun 06, 2025 The generic join context manager facilitates distributed training on uneven inputs. This page outlines the API of the relevant classes: Join, Joinable, and JoinHook. For a tutorial, see Distributed Training with Uneven Inputs Using the Join Context Manager. class torch.distributed.algorithms.Join(joinables, enable=True, throw_on_early_termination=False, **kwargs)[source]# This class defines the generic join context manager, which allows custom hooks to be called after a process joins. These hooks should shadow the collective communications of non-joined processes to prevent hanging and erroring and to ensure algorithmic correctness. Refer to JoinHook for details about the hook definition. Warning The context manager requires each participating Joinable to call the method notify_join_context() before its own per- iteration collective communications to ensure correctness. Warning The context manager requires that all process_group attributes in the JoinHook objects are the same. If there are multiple JoinHook objects, then the device of the first is used. The process group and device information is used for checking for non- joined processes and for notifying processes to throw an exception if throw_on_early_termination is enabled, both of which using an all- reduce. Parameters joinables (List[Joinable]) – a list of the participating Joinable s; their hooks are iterated over in the given order. enable (bool) – a flag enabling uneven input detection; setting to False disables the context manager’s functionality and should only be set when the user knows the inputs will not be uneven (default: True). throw_on_early_termination (bool) – a flag controlling whether to throw an exception upon detecting uneven inputs (default: False). Example: >>> import os >>> import torch >>> import torch.distributed as dist >>> import torch.multiprocessing as mp >>> import torch.nn.parallel.DistributedDataParallel as DDP >>> import torch.distributed.optim.ZeroRedundancyOptimizer as ZeRO >>> from torch.distributed.algorithms.join import Join >>> >>> # On each spawned worker >>> def worker(rank): >>> dist.init_process_group("nccl", rank=rank, world_size=2) >>> model = DDP(torch.nn.Linear(1, 1).to(rank), device_ids=[rank]) >>> optim = ZeRO(model.parameters(), torch.optim.Adam, lr=0.01) >>> # Rank 1 gets one more input than rank 0 >>> inputs = [torch.tensor([1.]).to(rank) for _ in range(10 + rank)] >>> with Join([model, optim]): >>> for input in inputs: >>> loss = model(input).sum() >>> loss.backward() >>> optim.step() >>> # All ranks reach here without hanging/erroring static notify_join_context(joinable)[source]# Notifies the join context manager that the calling process has not yet joined. Then, if throw_on_early_termination=True, checks if uneven inputs have been detected (i.e. if one process has already joined) and throws an exception if so. This method should be called from a Joinable object before its per-iteration collective communications. For example, this should be called at the beginning of the forward pass in DistributedDataParallel. Only the first Joinable object passed into the context manager performs the collective communications in this method, and for the others, this method is vacuous. Parameters joinable (Joinable) – the Joinable object calling this method. Returns An async work handle for the all-reduce meant to notify the context manager that the process has not yet joined if joinable is the first one passed into the context manager; None otherwise. class torch.distributed.algorithms.Joinable[source]# This defines an abstract base class for joinable classes. A joinable class (inheriting from Joinable) should implement join_hook(), which returns a JoinHook instance, in addition to join_device() and join_process_group() that return device and process group information, respectively. abstract property join_device: device# Return the device from which to perform collective communications needed by the join context manager. abstract join_hook(**kwargs)[source]# Return a JoinHook instance for the given Joinable. Parameters kwargs (dict) – a dict containing any keyword arguments to modify the behavior of the join hook at run time; all Joinable instances sharing the same join context manager are forwarded the same value for kwargs. Return type JoinHook abstract property join_process_group: Any# Returns the process group for the collective communications needed by the join context manager itself. class torch.distributed.algorithms.JoinHook[source]# This defines a join hook, which provides two entry points in the join context manager. Entry points : a main hook, which is called repeatedly while there exists a non-joined process, and a post-hook, which is called once all processes have joined. To implement a join hook for the generic join context manager, define a class that inherits from JoinHook and override main_hook() and post_hook() as appropriate. main_hook()[source]# Call this hook while there exists a non-joined process to shadow collective communications in a training iteration. Training iteration i.e., in one forward pass, backward pass, and optimizer step. post_hook(is_last_joiner)[source]# Call hook after all processes have joined. It is passed an additional bool argument is_last_joiner, which indicates if the rank is one of the last to join. Parameters is_last_joiner (bool) – True if the rank is one of the last to join; False otherwise.

```
Join
```

**Pattern 2:** Distributed communication package - torch.distributed# Created On: Jul 12, 2017 | Last Updated On: Sep 04, 2025 Note Please refer to PyTorch Distributed Overview for a brief introduction to all features related to distributed training. Backends# torch.distributed supports four built-in backends, each with different capabilities. The table below shows which functions are available for use with a CPU or GPU for each backend. For NCCL, GPU refers to CUDA GPU while for XCCL to XPU GPU. MPI supports CUDA only if the implementation used to build PyTorch supports it. Backend gloo mpi nccl xccl Device CPU GPU CPU GPU CPU GPU CPU GPU send ✓ ✘ ✓ ? ✘ ✓ ✘ ✓ recv ✓ ✘ ✓ ? ✘ ✓ ✘ ✓ broadcast ✓ ✓ ✓ ? ✘ ✓ ✘ ✓ all_reduce ✓ ✓ ✓ ? ✘ ✓ ✘ ✓ reduce ✓ ✓ ✓ ? ✘ ✓ ✘ ✓ all_gather ✓ ✓ ✓ ? ✘ ✓ ✘ ✓ gather ✓ ✓ ✓ ? ✘ ✓ ✘ ✓ scatter ✓ ✓ ✓ ? ✘ ✓ ✘ ✓ reduce_scatter ✓ ✓ ✘ ✘ ✘ ✓ ✘ ✓ all_to_all ✓ ✓ ✓ ? ✘ ✓ ✘ ✓ barrier ✓ ✘ ✓ ? ✘ ✓ ✘ ✓ Backends that come with PyTorch# PyTorch distributed package supports Linux (stable), MacOS (stable), and Windows (prototype). By default for Linux, 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.) Note As of PyTorch v1.8, Windows supports all collective communications backend but NCCL, If the init_method argument of init_process_group() points to a file it must adhere to the following schema: Local file system, init_method="file:///d:/tmp/some_file" Shared file system, init_method="file://////{machine_name}/{share_folder_name}/some_file" Same as on Linux platform, you can enable TcpStore by setting environment variables, MASTER_ADDR and MASTER_PORT. 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 training with CUDA GPU. Use the XCCL backend for distributed training with XPU GPU. Use the Gloo backend for distributed training with CPU. 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# Debugging - in case of NCCL failure, you can set NCCL_DEBUG=INFO to print an explicit warning message as well as basic NCCL initialization information. You may also use NCCL_DEBUG_SUBSYS to get more details about a specific aspect of NCCL. For example, NCCL_DEBUG_SUBSYS=COLL would print logs of collective calls, which may be helpful when debugging hangs, especially those caused by collective type or message size mismatch. In case of topology detection failure, it would be helpful to set NCCL_DEBUG_SUBSYS=GRAPH to inspect the detailed detection result and save as reference if further help from NCCL team is needed. Performance tuning - NCCL performs automatic tuning based on its topology detection to save users’ tuning effort. On some socket-based systems, users may still try tuning NCCL_SOCKET_NTHREADS and NCCL_NSOCKS_PERTHREAD to increase socket network bandwidth. These two environment variables have been pre-tuned by NCCL for some cloud providers, such as AWS or GCP. For a full list of NCCL environment variables, please refer to NVIDIA NCCL’s official documentation You can tune NCCL communicators even further using torch.distributed.ProcessGroupNCCL.NCCLConfig and torch.distributed.ProcessGroupNCCL.Options. Learn more about them using help (e.g. help(torch.distributed.ProcessGroupNCCL.NCCLConfig)) in the interpreter. 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() or torch.distributed.device_mesh.init_device_mesh() function before calling any other methods. Both block until all processes have joined. Warning Initialization is not thread-safe. Process group creation should be performed from a single thread, to prevent inconsistent ‘UUID’ assignment across ranks, and to prevent races during initialization that can lead to hangs. torch.distributed.is_available()[source]# Return True if the distributed package is available. Otherwise, torch.distributed does not expose any other APIs. Currently, torch.distributed is available on Linux, MacOS and Windows. Set USE_DISTRIBUTED=1 to enable it when building PyTorch from source. Currently, the default value is USE_DISTRIBUTED=1 for Linux and Windows, USE_DISTRIBUTED=0 for MacOS. Return type bool torch.distributed.init_process_group(backend=None, init_method=None, timeout=None, world_size=-1, rank=-1, store=None, group_name='', pg_options=None, device_id=None)[source]# Initialize the default distributed process group. This will also initialize the distributed package. There are 2 main ways to initialize a process group: Specify store, rank, and world_size explicitly. 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, optional) – The backend to use. Depending on build-time configurations, valid values include mpi, gloo, nccl, ucc, xccl or one that is registered by a third-party plugin. Since 2.6, if backend is not provided, c10d will use a backend registered for the device type indicated by the device_id kwarg (if provided). The known default registrations today are: nccl for cuda, gloo for cpu, xccl for xpu. If neither backend nor device_id is provided, c10d will detect the accelerator on the run-time machine and use a backend registered for that detected accelerator (or cpu). This field can 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 deadlock or NCCL invalid usage. ucc backend is experimental. Default backend for the device can be queried with get_default_backend_for_device(). 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 (it should be a number between 0 and world_size-1). 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 is 10 minutes for NCCL and 30 minutes for other backends. This is the duration after which collectives will be aborted asynchronously and the process will crash. This is done since CUDA execution is async and it is no longer safe to continue executing user code since failed async NCCL operations might result in subsequent CUDA operations running on corrupted data. When TORCH_NCCL_BLOCKING_WAIT is set, the process will block and wait for this timeout. group_name (str, optional, deprecated) – Group name. This argument is ignored pg_options (ProcessGroupOptions, optional) – process group options specifying what additional options need to be passed in during the construction of specific process groups. As of now, the only options we support is ProcessGroupNCCL.Options for the nccl backend, is_high_priority_stream can be specified so that the nccl backend can pick up high priority cuda streams when there’re compute kernels waiting. For other available options to config nccl, See https://docs.nvidia.com/deeplearning/nccl/user-guide/docs/api/types.html#ncclconfig-t device_id (torch.device | int, optional) – a single, specific device this process will work on, allowing for backend-specific optimizations. Currently this has two effects, only under NCCL: the communicator is immediately formed (calling ncclCommInit* immediately rather than the normal lazy call) and sub-groups will use ncclCommSplit when possible to avoid unnecessary overhead of group creation. If you want to know NCCL initialization error early, you can also use this field. If an int is provided, the API assumes that the accelerator type at compile time will be used. Note To enable backend == Backend.MPI, PyTorch needs to be built from source on a system that supports MPI. Note Support for multiple backends is experimental. Currently when no backend is specified, both gloo and nccl backends will be created. The gloo backend will be used for collectives with CPU tensors and the nccl backend will be used for collectives with CUDA tensors. A custom backend can be specified by passing in a string with format “<device_type>:<backend_name>,<device_type>:<backend_name>”, e.g. “cpu:gloo,cuda:custom_backend”. torch.distributed.device_mesh.init_device_mesh(device_type, mesh_shape, *, mesh_dim_names=None, backend_override=None)[source]# Initializes a DeviceMesh based on device_type, mesh_shape, and mesh_dim_names parameters. This creates a DeviceMesh with an n-dimensional array layout, where n is the length of mesh_shape. If mesh_dim_names is provided, each dimension is labeled as mesh_dim_names[i]. Note init_device_mesh follows SPMD programming model, meaning the same PyTorch Python program runs on all processes/ranks in the cluster. Ensure mesh_shape (the dimensions of the nD array describing device layout) is identical across all ranks. Inconsistent mesh_shape may lead to hanging. Note If no process group is found, init_device_mesh will initialize distributed process group/groups required for distributed communications behind the scene. Parameters device_type (str) – The device type of the mesh. Currently supports: “cpu”, “cuda/cuda-like”, “xpu”. Passing in a device type with a GPU index, such as “cuda:0”, is not allowed. mesh_shape (Tuple[int]) – A tuple defining the dimensions of the multi-dimensional array describing the layout of devices. mesh_dim_names (Tuple[str], optional) – A tuple of mesh dimension names to assign to each dimension of the multi-dimensional array describing the layout of devices. Its length must match the length of mesh_shape. Each string in mesh_dim_names must be unique. backend_override (Dict[int | str, tuple[str, Options] | str | Options], optional) – Overrides for some or all of the ProcessGroups that will be created for each mesh dimension. Each key can be either the index of a dimension or its name (if mesh_dim_names is provided). Each value can be a tuple containing the name of the backend and its options, or just one of these two components (in which case the other will be set to its default value). Returns A DeviceMesh object representing the device layout. Return type DeviceMesh Example: >>> from torch.distributed.device_mesh import init_device_mesh >>> >>> mesh_1d = init_device_mesh("cuda", mesh_shape=(8,)) >>> mesh_2d = init_device_mesh("cuda", mesh_shape=(2, 8), mesh_dim_names=("dp", "tp")) torch.distributed.is_initialized()[source]# Check if the default process group has been initialized. Return type bool torch.distributed.is_mpi_available()[source]# Check if the MPI backend is available. Return type bool torch.distributed.is_nccl_available()[source]# Check if the NCCL backend is available. Return type bool torch.distributed.is_gloo_available()[source]# Check if the Gloo backend is available. Return type bool torch.distributed.distributed_c10d.is_xccl_available()[source]# Check if the XCCL backend is available. Return type bool torch.distributed.is_torchelastic_launched()[source]# Check whether this process was launched with torch.distributed.elastic (aka torchelastic). The existence of TORCHELASTIC_RUN_ID environment variable is used as a proxy to determine whether the current process was launched with torchelastic. This is a reasonable proxy since TORCHELASTIC_RUN_ID maps to the rendezvous id which is always a non-null value indicating the job id for peer discovery purposes.. Return type bool torch.distributed.get_default_backend_for_device(device)[source]# Return the default backend for the given device. Parameters device (Union[str, torch.device]) – The device to get the default backend for. Returns The default backend for the given device as a lower case string. Return type str 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://). Improving initialization time# TORCH_GLOO_LAZY_INIT - establishes connections on demand rather than using a full mesh which can greatly improve initialization time for non all2all operations. Post-Initialization# Once torch.distributed.init_process_group() was run, the following functions can be used. To check whether the process group has already been initialized use torch.distributed.is_initialized(). class torch.distributed.Backend(name)[source]# An enum-like class for backends. Available backends: GLOO, NCCL, UCC, MPI, XCCL, and other registered backends. 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. classmethod register_backend(name, func, extended_api=False, devices=None)[source]# Register a new backend with the given name and instantiating function. This class method is used by 3rd party ProcessGroup extension to register new backends. Parameters name (str) – Backend name of the ProcessGroup extension. It should match the one in init_process_group(). func (function) – Function handler that instantiates the backend. The function should be implemented in the backend extension and takes four arguments, including store, rank, world_size, and timeout. extended_api (bool, optional) – Whether the backend supports extended argument structure. Default: False. If set to True, the backend will get an instance of c10d::DistributedBackendOptions, and a process group options object as defined by the backend implementation. device (str or list of str, optional) – device type this backend supports, e.g. “cpu”, “cuda”, etc. If None, assuming both “cpu” and “cuda” Note This support of 3rd party backend is experimental and subject to change. torch.distributed.get_backend(group=None)[source]# Return 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. Return type Backend torch.distributed.get_rank(group=None)[source]# Return the rank of the current process in the provided group, default otherwise. 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. If None, the default process group will be used. Returns The rank of the process group -1, if not part of the group Return type int torch.distributed.get_world_size(group=None)[source]# Return the number of processes in the current process group. Parameters group (ProcessGroup, optional) – The process group to work on. If None, the default process group will be used. Returns The world size of the process group -1, if not part of the group Return type int Shutdown# It is important to clean up resources on exit by calling destroy_process_group(). The simplest pattern to follow is to destroy every process group and backend by calling destroy_process_group() with the default value of None for the group argument, at a point in the training script where communications are no longer needed, usually near the end of main(). The call should be made once per trainer-process, not at the outer process-launcher level. if destroy_process_group() is not called by all ranks in a pg within the timeout duration, especially when there are multiple process-groups in the application e.g. for N-D parallelism, hangs on exit are possible. This is because the destructor for ProcessGroupNCCL calls ncclCommAbort, which must be called collectively, but the order of calling ProcessGroupNCCL’s destructor if called by python’s GC is not deterministic. Calling destroy_process_group() helps by ensuring ncclCommAbort is called in a consistent order across ranks, and avoids calling ncclCommAbort during ProcessGroupNCCL’s destructor. Reinitialization# destroy_process_group can also be used to destroy individual process groups. One use case could be fault tolerant training, where a process group may be destroyed and then a new one initialized during runtime. In this case, it’s critical to synchronize the trainer processes using some means other than torch.distribut\n\n[... truncated for storage; see full file on GitHub ...]