---
title: "Pipeline Parallelism"
tags: training large-model
---

DeepSpeed v0.3 includes new support for pipeline parallelism! Pipeline
parallelism improves both the memory and compute efficiency of deep learning
training by partitioning the layers of a model into stages that can be
processed in parallel.
DeepSpeed's training engine provides hybrid data and pipeline parallelism and
can be further combined with model parallelism such as
[Megatron-LM](https://github.com/NVIDIA/Megatron-LM).
An illustration of
3D parallelism is shown below. Our latest [results]({{ site.press_release_v3 }})
demonstrate that this 3D parallelism enables training models with over a
**trillion** parameters.

![3D parallelism in DeepSpeed](/assets/images/3d-parallelism.png)

DeepSpeed uses *gradient accumulation* to extract pipeline parallelism (shown
below). Each batch of training data is divided into micro-batches that can be
processed in parallel by the pipeline stages. Once a stage completes the
forward pass for a micro-batch, the activation memory is communicated to the
next stage in the pipeline. Similarly, as the next stage completes its
backward pass on a micro-batch, the gradient with respect to the activation
is communicated backwards through the pipeline. Each backward pass
accumulates gradients locally. Next, all data parallel groups perform
reductions of the gradients in parallel. Lastly, the optimizer updates the
model weights.


Below is an illustration of how DeepSpeed will train a batch with eight
micro-batches using hybrid two-way data parallelism and two-stage pipeline
parallelism. GPUs 0 and 2 are arranged in a pipeline and will alternate
forward (F) and backward (B) passes. They will then all-reduce (AR) gradients
with their data parallel counterparts, GPUs 1 and 3, respectively. Finally,
the two pipeline stages update their model weights.

![Pipeline Schedule](/assets/images/pipe-schedule.png)


## Getting Starting with Pipeline Parallelism

DeepSpeed strives to accelerate *and* simplify the process of pipeline
parallel training. This section provides first steps with hybrid data and
pipeline parallel training by preparing `torchvision`'s
[AlexNet](https://pytorch.org/docs/1.2.0/_modules/torchvision/models/alexnet.html)
model.

### Expressing Pipeline Models
Pipeline parallelism requires models to be expressed as a sequence of layers.
In the forward pass, each layer consumes the output of the previous
layer. In fact, there is no need to specify a `forward()` for a pipeline
parallel model! The forward pass of a pipeline parallel model implicitly
takes the form:
```python
def forward(self, inputs):
    x = inputs
    for layer in self.layers:
        x = layer(x)
    return x
```

PyTorch's
[`torch.nn.Sequential`](https://pytorch.org/docs/stable/generated/torch.nn.Sequential.html)
is a convenient container for expressing pipeline parallel models and can be
parallelized by DeepSpeed with no modification:
```python
net = nn.Sequential(
    nn.Linear(in_features, hidden_dim),
    nn.ReLU(inplace=True),
    nn.Linear(hidden_dim, out_features)
)
from deepspeed.pipe import PipelineModule
net = PipelineModule(layers=net, num_stages=2)
```
`PipelineModule` uses its `layers` argument as the sequence of layers that
comprise the model. After initialization, `net` is divided into two pipeline
stages and its layers moved to the corresponding GPUs. If more than two GPUs
are present, DeepSpeed will also use hybrid data parallelism.

**Note:** The total number of GPUs must be divisible by the number of pipeline
stages.
{: .notice--info}

**Note:** For large model training, see [memory-efficient model construction](#memory-efficient-model-construction).
{: .notice--info}

### AlexNet
Let's look at an abbreviated implementation of `torchvision`'s
[AlexNet](https://pytorch.org/docs/1.2.0/_modules/torchvision/models/alexnet.html):

```python
class AlexNet(nn.Module):
    def __init__(self, num_classes=1000):
        super(AlexNet, self).__init__()
        self.features = nn.Sequential(
            nn.Conv2d(3, 64, kernel_size=11, stride=4, padding=2),
            ...
            nn.MaxPool2d(kernel_size=3, stride=2),
        )
        self.avgpool = nn.AdaptiveAvgPool2d((6, 6))
        self.classifier = nn.Sequential(
            nn.Dropout(),
            ...
            nn.Linear(4096, num_classes),
        )

    def forward(self, x):
        x = self.features(x)
        x = self.avgpool(x)
        x = torch.flatten(x, 1)
        x = self.classifier(x)
        return x
```
`AlexNet` is mostly a composition of several `Sequential` submodules. We can
turn this into a `PipelineModule` by flattening its submodules into a single
sequence of layers:
```python
class AlexNetPipe(AlexNet):
    def to_layers(self):
        layers = [
            *self.features,
            self.avgpool,
            lambda x: torch.flatten(x, 1),
            *self.classifier
        ]
        return layers

from deepspeed.pipe import PipelineModule
net = AlexNetPipe()
net = PipelineModule(layers=net.to_layers(), num_stages=2)
```

**Note:**
the `lambda` in the middle of `layers` above is not a `torch.nn.Module`
type. Any object that implements `__call__()` can be a layer in a
`PipelineModule`: this allows for convenient data transformations in the
pipeline.
{: .notice--info}


### Inputs and Outputs
Following `torch.nn.Sequential`, the inputs and outputs of each layer must be
either a single `torch.Tensor` or a `tuple` of tensors. In practice, some
models may need to modify their forward pass to pack and unpack arguments to
`forward()`. Consider an abbreviated implementation of a stack of Transformer
blocks:
```python
class TransformerBlock(nn.Module)
    ...
    def forward(self, hidden, mask):
        output = self.compute(hidden, mask)
        return output
    ...

stack = [ TransformerBlock() for _ in range(num_layers) ]
```
Two modifications to `TransformerBlock` are required:

1. The arguments must be collected into a `tuple`.
2. `mask` must also be returned from `forward()` to pass to the next layer.

These modifications can be accomplished with a short subclass:
```python
class TransformerBlockPipe(TransformerBlock)
    def forward(self, inputs):
        hidden, mask = inputs
        output = super().forward(hidden, mask)
        return (output, mask)
stack = [ TransformerBlockPipe() for _ in range(num_layers) ]
```

### Training Loops

Pipeline parallelism interleaves forward and backward passes, and thus the
training loop cannot be divided into separate stages of `forward()`,
`backward()` and `step()`.
Instead, DeepSpeed's pipeline engine provides a `train_batch()` method that
advances the pipeline engine until the next batch of training data is
consumed and the model weights updated.
```python
train_iter = iter(train_loader)
loss = engine.train_batch(data_iter=train_iter)
```

The above `train_batch()` example is equivalent to the following with
traditional data parallel DeepSpeed:
```python
train_iter = iter(train_loader)
for micro_batch in engine.gradient_accumulation_steps():
    batch = next(data_iter)
    loss = engine(batch)
    engine.backward(loss)
    engine.step()
```

### Dealing with Data

Data parallel training typically has each worker perform IO independently at
the start of each batch. However, in a pipeline parallel environment, only the
first stage uses the input data, and only the last stage uses labels for loss
calculation.

**Note:**
The pipeline engine expects data loaders to return a `tuple` of two items. The
first returned item is the input batch data, and the second item is the data
to be used in the loss calculation. As before, inputs and labels should be
either `torch.Tensor` type or a `tuple` of tensors.
{: .notice--info}

For convenience, the DeepSpeed pipeline engine can construct a distributed
data loader when a dataset is provided to `deepspeed.initialize()`. DeepSpeed
handles the rest of the complexity of data loading, and so the pipeline
training loop becomes:
```python
engine, _, _, _ = deepspeed.initialize(
    args=args,
    model=net,
    model_parameters=[p for p in net.parameters() if p.requires_grad],
    training_data=cifar_trainset())

for step in range(args.steps):
    loss = engine.train_batch()
```

Of course, DeepSpeed will work with any data loader that you wish to use.
Data loaders should be constructed by the first and last stages in the
pipeline. Each worker should load micro-batches of size
`engine.train_micro_batch_size_per_gpu()` and will be queried
a total of `engine.gradient_accumulation_steps()` times per `train_batch()`.

**Watch out!**
The pipeline engine *pulls* data from an iterator instead of iterating over
it. It's critical that the data stream does not empty in the middle of a
training batch. Each invocation of `train_batch()` will pull
a total of `engine.gradient_accumulation_steps()` micro-batches of data from
the data iterator.
{: .notice--warning}

DeepSpeed provides a convenience class `deepspeed.utils.RepeatingLoader` that
simply wraps an iterable such as a data loader and restarts it whenever the
end is reached:
```python
train_loader = deepspeed.utils.RepeatingLoader(train_loader)
train_iter = iter(train_loader)
for step in range(args.steps):
    loss = engine.train_batch(data_iter=trainiter)
```


## Advanced Topics


### Load Balancing Pipeline Modules
The performance of pipeline parallel training strongly relies on load
balance. DeepSpeed provides several mechanisms for partitioning the model
across GPUs. These strategies can be set with the `partition_method` keyword
argument to `PipelineModule`. Here are partitioning methods currently provided
by DeepSpeed:

* `partition_method="parameters"` (**default**)
   balances the number of trainable parameters on each pipeline stage . This is
   especially useful in memory-constrained environments and when the size of a
   layer is proportional to the computation time.
* `partition_method="type:[regex]"`
  balances layers whose class names match `[regex]`. The regular expression
  is not case sensitive. For example, `partition_method="type:transformer"`
  would balance the number of transformer layers per stage.
* `partition_method="uniform"` balances the number of layers per stage.

### Memory-Efficient Model Construction
Building a `Sequential` container and providing it to a `PipelineModule` is a convenient way
of specifying a pipeline parallel model. However, this approach encounters scalability issues
for massive models because each worker replicates the whole model in CPU memory.
For example, a machine with 16 GPUs must have as much local CPU memory as 16 times the model size.

DeepSpeed provides a `LayerSpec` class that delays the construction of
modules until the model layers have been partitioned across workers.
Then each worker will allocate only the layers it's assigned to. So, comparing to the
example from the previous paragraph, using `LayerSpec` a machine with 16 GPUs will need to
allocate a total of 1x model size on its CPU memory and not 16x.

Here is an example of the abbreviated AlexNet model, but expressed only
with `LayerSpec`s. Note that the syntax is almost unchanged: `nn.ReLU(inplace=True)`
simply becomes `LayerSpec(nn.ReLU, inplace=True)`.
```python
from deepspeed.pipe import PipelineModule, LayerSpec
class AlexNetPipe(PipelineModule):
    def __init__(self, num_classes=10, **kwargs):
        self.num_classes = num_classes
        specs = [
            LayerSpec(nn.Conv2d, 3, 64, kernel_size=11, stride=4, padding=2),
            LayerSpec(nn.ReLU, inplace=True),
            ...
            LayerSpec(nn.ReLU, inplace=True),
            LayerSpec(nn.Linear, 4096, self.num_classes),
        ]
        super().__init__(layers=specs, loss_fn=nn.CrossEntropyLoss(), **kwargs)
```

### Tied Layers
Some models cannot be entirely expressed as pipeline parallel models because
some layers are reused in the pipeline. For example, Transformer based
language models commonly use an embedding layer early in the pipeline to map
vocabulary to hidden states, and then use the embedding to map hidden states
back to vocabulary at the end of the pipeline. If the model was restricted to
pure pipeline parallelism, this embedding reuse would prohibit pipeline
parallelism.

DeepSpeed provides a `TiedLayerSpec` that is an extension of
`LayerSpec`. `TiedLayerSpec` requires an additional argument: `key`.
Each reuse of a layer is specified with a `TiedLayerSpec`, and the `key` field
is used to identify where a layer is reused.

Tied layers are replicated on every pipeline stage that owns an instance of
reuse. Training then proceeds as normal, but an additional all-reduce of the
tied gradients is added after all backward passes complete. The all-reduce
ensures that the weights of the tied layer remain in sync across pipeline stages.
