pipelining tutorials

Author: H-Huang

index.rst

@@ -744,6 +744,13 @@ Welcome to PyTorch Tutorials
    :link: intermediate/rpc_param_server_tutorial.html
    :tags: Parallel-and-Distributed-Training
 
+.. customcarditem::
+   :header: Introduction to Distributed Pipeline Parallelism
+   :card_description: Demonstrate how to implement pipeline parallelism using torch.distributed.pipelining
+   :image: _static/img/thumbnails/cropped/Introduction-to-Distributed-Pipeline-Parallelism.png
+   :link: intermediate/pipelining_tutorial.html
+   :tags: Parallel-and-Distributed-Training
+
 .. customcarditem::
    :header: Implementing Batch RPC Processing Using Asynchronous Executions
    :card_description: Learn how to use rpc.functions.async_execution to implement batch RPC
@@ -1128,6 +1135,7 @@ Additional Resources
    intermediate/FSDP_tutorial
    intermediate/FSDP_adavnced_tutorial
    intermediate/TP_tutorial
+   intermediate/pipelining_tutorial
    intermediate/process_group_cpp_extension_tutorial
    intermediate/rpc_tutorial
    intermediate/rpc_param_server_tutorial

intermediate_source/pipelining_tutorial.rst

@@ -0,0 +1,233 @@
+Introduction to Distributed Pipeline Parallelism
+================================================
+**Authors**: `Howard Huang <https://github.com/H-Huang>`_
+
+.. note::
+   |edit| View and edit this tutorial in `github <https://github.com/pytorch/tutorials/blob/main/intermediate_source/pipelining_tutorial.rst>`__.
+
+This tutorial uses a gpt-style transformer model to demonstrate implementing distributed
+pipeline parallelism with `torch.distributed.pipelining <https://pytorch.org/docs/main/distributed.pipelining.html>`__
+APIs.
+
+.. grid:: 2
+
+   .. grid-item-card:: :octicon:`mortar-board;1em;` What you will learn
+
+      *  How to use ``torch.distributed.pipelining`` APIs
+      *  How to apply pipeline parallelism to a transformer model
+      *  How to utilize different schedules on a set of microbatches
+
+
+   .. grid-item-card:: :octicon:`list-unordered;1em;` Prerequisites
+
+      * Familiarity with `basic distributed training  <https://pytorch.org/tutorials/beginner/dist_overview.html>`__ in PyTorch
+
+Setup
+-----
+
+With ``torch.distributed.pipelining`` we will be partitioning the execution of a model and scheduling computation on micro-batches. We will be using a simplified version
+of a transformer decoder model. The model architecture is for educational purposes and has multiple transformer decoder layers as we want to demonstrate how to split the model into different
+chunks. First, let us define the model:
+
+.. code:: python
+
+   import torch
+   import torch.nn as nn
+   from dataclasses import dataclass
+
+   @dataclass
+   class ModelArgs:
+      dim: int = 512
+      n_layers: int = 8
+      n_heads: int = 8
+      vocab_size: int = 10000
+
+   class Transformer(nn.Module):
+      def __init__(self, model_args: ModelArgs):
+         super().__init__()
+
+         self.tok_embeddings = nn.Embedding(model_args.vocab_size, model_args.dim)
+
+         # Using a ModuleDict lets us delete layers witout affecting names,
+         # ensuring checkpoints will correctly save and load.
+         self.layers = torch.nn.ModuleDict()
+         for layer_id in range(model_args.n_layers):
+               self.layers[str(layer_id)] = nn.TransformerDecoderLayer(model_args.dim, model_args.n_heads)
+
+         self.norm = nn.LayerNorm(model_args.dim)
+         self.output = nn.Linear(model_args.dim, model_args.vocab_size)
+
+      def forward(self, tokens: torch.Tensor):
+         # Handling layers being 'None' at runtime enables easy pipeline splitting
+         h = self.tok_embeddings(tokens) if self.tok_embeddings else tokens
+
+         for layer in self.layers.values():
+               h = layer(h, h)
+
+         h = self.norm(h) if self.norm else h
+         output = self.output(h).float() if self.output else h
+         return output
+
+Then, we need to import the necessary libraries in our script and initialize the distributed training process. In this case, we are defining some global variables to use
+later in the script:
+
+.. code:: python
+
+   import os
+   import torch.distributed as dist
+   from torch.distributed.pipelining import pipeline, SplitPoint, PipelineStage, ScheduleGPipe
+
+   global rank, device, pp_group, stage_index, num_stages
+   def init_distributed():
+      global rank, device, pp_group, stage_index, num_stages
+      rank = int(os.environ["LOCAL_RANK"])
+      world_size = int(os.environ["WORLD_SIZE"])
+      device = torch.device(f"cuda:{rank}") if torch.cuda.is_available() else torch.device("cpu")
+      dist.init_process_group()
+
+      pp_group = dist.new_group()
+      stage_index = rank
+      num_stages = world_size
+
+The ``rank``, ``world_size``, and ``init_process_group()`` code should seem familiar to you as those are commonly used in
+all distributed programs. The globals specific to pipeline parallelism include ``pp_group`` which is the process
+group that will be used for send/recv communications, ``stage_index`` which, in this example, is a single rank
+per stage so the index is equivalent to the rank, and ``num_stages`` which is equivalent to world_size.
+
+The ``num_stages`` is used to set the number of stages that will be used in the pipeline parallelism schedule. For example,
+for ``num_stages=4``, a microbatch will need to go through 4 forwards and 4 backwards before it is completed. The ``stage_index``
+is necessary for the framework to know how to communicate between stages. For example, for the first stage (``stage_index=0``), it will
+use data from the dataloader and does not need to receive data from any previous peers to perform its computation.
+
+
+Step 1: Partition the Transformer Model
+---------------------------------------
+
+There are two different ways of partitioning the model:
+
+First is the manual mode in which we can manually create two instances of the model by deleting portions of
+attributes of the model. In this example for a 2 stage (2 ranks) the model is cut in half.
+
+.. code:: python
+
+   def manual_model_split(model, example_input_microbatch, model_args) -> PipelineStage:
+      if stage_index == 0:
+         # prepare the first stage model
+         for i in range(4, 8):
+               del model.layers[str(i)]
+         model.norm = None
+         model.output = None
+         stage_input_microbatch = example_input_microbatch
+
+      elif stage_index == 1:
+         # prepare the second stage model
+         for i in range(4):
+               del model.layers[str(i)]
+         model.tok_embeddings = None
+         stage_input_microbatch = torch.randn(example_input_microbatch.shape[0], model_args.dim)
+
+      stage = PipelineStage(
+         model,
+         stage_index,
+         num_stages,
+         device,
+         input_args=stage_input_microbatch,
+      )
+      return stage
+
+As we can see the first stage does not have the layer norm or the output layer, and it only includes the first four transformer blocks.
+The second stage does not have the input embedding layers, but includes the output layers and the final four transformer blocks. The function
+then returns the ``PipelineStage`` for the current rank.
+
+The second method is the tracer-based mode which automatically splits the model based on a ``split_spec`` argument. Using the pipeline specification, we can instruct
+``torch.distributed.pipelining`` where to split the model. In the following code block,
+we are splitting before the before 4th transformer decoder layer, mirroring the manual split described above. Similarly,
+we can retrieve a ``PipelineStage`` by calling ``build_stage`` after this splitting is done.
+
+.. code:: python
+   def tracer_model_split(model, example_input_microbatch) -> PipelineStage:
+      pipe = pipeline(
+         module=model,
+         mb_args=(example_input_microbatch,),
+         split_spec={
+            "layers.4": SplitPoint.BEGINNING,
+         }
+      )
+      stage = pipe.build_stage(stage_index, device, pp_group)
+      return stage
+
+
+Step 2: Define The Main Execution
+---------------------------------
+
+In the main function we will create a particular pipeline schedule that the stages should follow. ``torch.distributed.pipelining``
+supports multiple schedules including supports multiple schedules, including single-stage-per-rank schedules ``GPipe`` and ``1F1B``,
+as well as multiple-stage-per-rank schedules such as ``Interleaved1F1B`` and ``LoopedBFS``.
+
+.. code:: python
+
+   if __name__ == "__main__":
+      init_distributed()
+      num_microbatches = 4
+      model_args = ModelArgs()
+      model = Transformer(model_args)
+
+      # Dummy data
+      x = torch.ones(32, 500, dtype=torch.long)
+      y = torch.randint(0, model_args.vocab_size, (32, 500), dtype=torch.long)
+      example_input_microbatch = x.chunk(num_microbatches)[0]
+
+      # Option 1: Manual model splitting
+      stage = manual_model_split(model, example_input_microbatch, model_args)
+
+      # Option 2: Tracer model splitting
+      # stage = tracer_model_split(model, example_input_microbatch)
+
+      x = x.to(device)
+      y = y.to(device)
+
+      def tokenwise_loss_fn(outputs, targets):
+         loss_fn = nn.CrossEntropyLoss()
+         outputs = outputs.view(-1, model_args.vocab_size)
+         targets = targets.view(-1)
+         return loss_fn(outputs, targets)
+
+      schedule = ScheduleGPipe(stage, n_microbatches=num_microbatches, loss_fn=token_loss_fn)
+
+      if rank == 0:
+         schedule.step(x)
+      elif rank == 1:
+         losses = []
+         output = schedule.step(target=y, losses=losses)
+      dist.destroy_process_group()
+
+In the example above, we are using the manual method to split the model, but the code can be uncommented to also try the
+tracer-based model splitting function. In our schedule, we need to pass in the number of microbatches and
+the loss function used to evaluate the targets.
+
+The ``.step()`` function processes the entire minibatch and automatically splits it into microbatches based
+on the ``n_microbatches`` passed previously. The microbatches are then operated on according to the schedule class.
+In the example above, we are using GPipe, which follows a simple all-forwards and then all-backwards schedule. The output
+returned from rank 1 will be the same as if the model was on a single GPU and run with the entire batch. Similarly,
+we can pass in a ``losses`` container to store the corresponding losses for each microbatch.
+
+Step 3: Launch the Distributed Processes
+----------------------------------------
+
+Finally, we are ready to run the script. We will use ``torchrun`` to create a single host, 2-process job.
+Our script is already written in a way rank 0 that performs the required logic for pipeline stage 0, and rank 1
+performs the logic for pipeline stage 1.
+
+``torchrun --standalone --nnodes 1 --nproc_per_node 2 pipelining_tutorial.py``
+
+Conclusion
+----------
+
+In this tutorial, we have learned how to implement distributed pipeline parallelism using PyTorch's ``torch.distributed.pipelining`` APIs.
+We explored setting up the environment, defining a transformer model, and partitioning it for distributed training.
+We discussed two methods of model partitioning, manual and tracer-based, and demonstrated how to schedule computations on
+micro-batches across different stages. Finally, we covered the execution of the pipeline schedule and the launch of distributed
+processes using ``torchrun``.
+
+For a production ready usage of pipeline parallelism as well as composition with other distributed techniques, see also
+`TorchTitan end to end example of 3D parallelism <https://github.com/pytorch/torchtitan>`__.