pipeline_tutorial.py

"""
Training Transformer models using Pipeline Parallelism
======================================================

**Author**: `Pritam Damania <https://github.com/pritamdamania87>`_

This tutorial demonstrates how to train a large Transformer model across
multiple GPUs using pipeline parallelism. This tutorial is an extension of the
`Sequence-to-Sequence Modeling with nn.Transformer and TorchText <https://pytorch.org/tutorials/beginner/transformer_tutorial.html>`__ tutorial
and scales up the same model to demonstrate how pipeline parallelism can be
used to train Transformer models.

Prerequisites:

    * `Pipeline Parallelism <https://pytorch.org/docs/stable/pipeline.html>`__
    * `Sequence-to-Sequence Modeling with nn.Transformer and TorchText <https://pytorch.org/tutorials/beginner/transformer_tutorial.html>`__
"""


######################################################################
# Define the model
# ----------------
#


######################################################################
# In this tutorial, we will split a Transformer model across two GPUs and use
# pipeline parallelism to train the model. The model is exactly the same model
# used in the `Sequence-to-Sequence Modeling with nn.Transformer and TorchText
# <https://pytorch.org/tutorials/beginner/transformer_tutorial.html>`__ tutorial,
# but is split into two stages. The largest number of parameters belong to the
# `nn.TransformerEncoder <https://pytorch.org/docs/stable/generated/torch.nn.TransformerEncoder.html>`__ layer.
# The `nn.TransformerEncoder <https://pytorch.org/docs/stable/generated/torch.nn.TransformerEncoder.html>`__
# itself consists of ``nlayers`` of `nn.TransformerEncoderLayer <https://pytorch.org/docs/stable/generated/torch.nn.TransformerEncoderLayer.html>`__.
# As a result, our focus is on ``nn.TransformerEncoder`` and we split the model
# such that half of the ``nn.TransformerEncoderLayer`` are on one GPU and the
# other half are on another. To do this, we pull out the ``Encoder`` and
# ``Decoder`` sections into seperate modules and then build an nn.Sequential
# representing the original Transformer module.

import sys
import math
import torch
import torch.nn as nn
import torch.nn.functional as F
import tempfile
from torch.nn import TransformerEncoder, TransformerEncoderLayer
from torch.nn import TransformerDecoder, TransformerDecoderLayer

if sys.platform == 'win32':
    print('Windows platform is not supported for pipeline parallelism')
    sys.exit(0)
if torch.cuda.device_count() < 2:
    print('Need at least two GPU devices for this tutorial')
    sys.exit(0)

class Encoder(nn.Module):
    def __init__(self, ntoken, ninp, dropout=0.5):
        super(Encoder, self).__init__()
        self.pos_encoder = PositionalEncoding(ninp, dropout)
        self.encoder = nn.Embedding(ntoken, ninp)
        self.ninp = ninp
        self.init_weights()

    def init_weights(self):
        initrange = 0.1
        self.encoder.weight.data.uniform_(-initrange, initrange)

    def forward(self, src):
        # Need (S, N) format for encoder.
        src = src.t()
        src = self.encoder(src) * math.sqrt(self.ninp)
        return self.pos_encoder(src)

class Decoder(nn.Module):
    def __init__(self, ntoken, ninp):
        super(Decoder, self).__init__()
        self.decoder = nn.Linear(ninp, ntoken)
        self.init_weights()

    def init_weights(self):
        initrange = 0.1
        self.decoder.bias.data.zero_()
        self.decoder.weight.data.uniform_(-initrange, initrange)

    def forward(self, inp):
        # Need batch dimension first for output of pipeline.
        return self.decoder(inp).permute(1, 0, 2)


######################################################################
# ``PositionalEncoding`` module injects some information about the
# relative or absolute position of the tokens in the sequence. The
# positional encodings have the same dimension as the embeddings so that
# the two can be summed. Here, we use ``sine`` and ``cosine`` functions of
# different frequencies.


class PositionalEncoding(nn.Module):

    def __init__(self, d_model, dropout=0.1, max_len=5000):
        super(PositionalEncoding, self).__init__()
        self.dropout = nn.Dropout(p=dropout)

        pe = torch.zeros(max_len, d_model)
        position = torch.arange(0, max_len, dtype=torch.float).unsqueeze(1)
        div_term = torch.exp(torch.arange(0, d_model, 2).float() * (-math.log(10000.0) / d_model))
        pe[:, 0::2] = torch.sin(position * div_term)
        pe[:, 1::2] = torch.cos(position * div_term)
        pe = pe.unsqueeze(0).transpose(0, 1)
        self.register_buffer('pe', pe)

    def forward(self, x):
        x = x + self.pe[:x.size(0), :]
        return self.dropout(x)



######################################################################
# Load and batch data
# -------------------
#


######################################################################
# The training process uses Wikitext-2 dataset from ``torchtext``. 
# To access torchtext datasets, please install torchdata following instructions at https://github.com/pytorch/data.
#
# The vocab object is built based on the train dataset and is used to numericalize
# tokens into tensors. Starting from sequential data, the ``batchify()``
# function arranges the dataset into columns, trimming off any tokens remaining
# after the data has been divided into batches of size ``batch_size``.
# For instance, with the alphabet as the sequence (total length of 26)
# and a batch size of 4, we would divide the alphabet into 4 sequences of
# length 6:
#
# .. math::
#   \begin{bmatrix}
#   \text{A} & \text{B} & \text{C} & \ldots & \text{X} & \text{Y} & \text{Z}
#   \end{bmatrix}
#   \Rightarrow
#   \begin{bmatrix}
#   \begin{bmatrix}\text{A} \\ \text{B} \\ \text{C} \\ \text{D} \\ \text{E} \\ \text{F}\end{bmatrix} &
#   \begin{bmatrix}\text{G} \\ \text{H} \\ \text{I} \\ \text{J} \\ \text{K} \\ \text{L}\end{bmatrix} &
#   \begin{bmatrix}\text{M} \\ \text{N} \\ \text{O} \\ \text{P} \\ \text{Q} \\ \text{R}\end{bmatrix} &
#   \begin{bmatrix}\text{S} \\ \text{T} \\ \text{U} \\ \text{V} \\ \text{W} \\ \text{X}\end{bmatrix}
#   \end{bmatrix}
#
# These columns are treated as independent by the model, which means that
# the dependence of ``G`` and ``F`` can not be learned, but allows more
# efficient batch processing.
#

import torch
from torchtext.datasets import WikiText2
from torchtext.data.utils import get_tokenizer
from torchtext.vocab import build_vocab_from_iterator

train_iter = WikiText2(split='train')
tokenizer = get_tokenizer('basic_english')
vocab = build_vocab_from_iterator(map(tokenizer, train_iter), specials=["<unk>"])
vocab.set_default_index(vocab["<unk>"]) 

def data_process(raw_text_iter):
  data = [torch.tensor(vocab(tokenizer(item)), dtype=torch.long) for item in raw_text_iter]
  return torch.cat(tuple(filter(lambda t: t.numel() > 0, data)))

train_iter, val_iter, test_iter = WikiText2()
train_data = data_process(train_iter)
val_data = data_process(val_iter)
test_data = data_process(test_iter)

device = torch.device("cuda")

def batchify(data, bsz):
    # Divide the dataset into bsz parts.
    nbatch = data.size(0) // bsz
    # Trim off any extra elements that wouldn't cleanly fit (remainders).
    data = data.narrow(0, 0, nbatch * bsz)
    # Evenly divide the data across the bsz batches.
    data = data.view(bsz, -1).t().contiguous()
    return data.to(device)

batch_size = 20
eval_batch_size = 10
train_data = batchify(train_data, batch_size)
val_data = batchify(val_data, eval_batch_size)
test_data = batchify(test_data, eval_batch_size)


######################################################################
# Functions to generate input and target sequence
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#


######################################################################
# ``get_batch()`` function generates the input and target sequence for
# the transformer model. It subdivides the source data into chunks of
# length ``bptt``. For the language modeling task, the model needs the
# following words as ``Target``. For example, with a ``bptt`` value of 2,
# we'd get the following two Variables for ``i`` = 0:
#
# .. image:: ../_static/img/transformer_input_target.png
#
# It should be noted that the chunks are along dimension 0, consistent
# with the ``S`` dimension in the Transformer model. The batch dimension
# ``N`` is along dimension 1.
#

bptt = 25
def get_batch(source, i):
    seq_len = min(bptt, len(source) - 1 - i)
    data = source[i:i+seq_len]
    target = source[i+1:i+1+seq_len].view(-1)
    # Need batch dimension first for pipeline parallelism.
    return data.t(), target

######################################################################
# Model scale and Pipe initialization
# -----------------------------------
#


######################################################################
# To demonstrate training large Transformer models using pipeline parallelism,
# we scale up the Transformer layers appropriately. We use an embedding
# dimension of 4096, hidden size of 4096, 16 attention heads and 12 total
# transformer layers (``nn.TransformerEncoderLayer``). This creates a model with
# **~1.4 billion** parameters.
#
# We need to initialize the `RPC Framework <https://pytorch.org/docs/stable/rpc.html>`__
# since Pipe depends on the RPC framework via `RRef <https://pytorch.org/docs/stable/rpc.html#rref>`__
# which allows for future expansion to cross host pipelining. We need to
# initialize the RPC framework with only a single worker since we're using a
# single process to drive multiple GPUs.
#
# The pipeline is then initialized with 8 transformer layers on one GPU and 8
# transformer layers on the other GPU.
#
# .. note::
#    For efficiency purposes we ensure that the ``nn.Sequential`` passed to
#    ``Pipe`` only consists of two elements (corresponding to two GPUs), this
#    allows the Pipe to work with only two partitions and avoid any
#    cross-partition overheads.

ntokens = len(vocab) # the size of vocabulary
emsize = 4096 # embedding dimension
nhid = 4096 # the dimension of the feedforward network model in nn.TransformerEncoder
nlayers = 32 # the number of nn.TransformerEncoderLayer in nn.TransformerEncoder
nhead = 16 # the number of heads in the multiheadattention models
dropout = 0.2 # the dropout value

from torch.distributed import rpc
tmpfile = tempfile.NamedTemporaryFile()
rpc.init_rpc(
    name="worker",
    rank=0,
    world_size=1,
    rpc_backend_options=rpc.TensorPipeRpcBackendOptions(
        init_method="file://{}".format(tmpfile.name),
        # Specifying _transports and _channels is a workaround and we no longer
        # will have to specify _transports and _channels for PyTorch
        # versions >= 1.8.1
        _transports=["ibv", "uv"],
        _channels=["cuda_ipc", "cuda_basic"],
    )
)

num_gpus = 8
partition_len = ((nlayers - 1) // num_gpus) + 1

# Add encoder in the beginning.
tmp_list = [Encoder(ntokens, emsize, dropout).cuda(0)]
module_list = []

# Add all the necessary transformer blocks.
for i in range(nlayers):
    transformer_block = TransformerEncoderLayer(emsize, nhead, nhid, dropout)
    if i != 0 and i % (partition_len) == 0:
        module_list.append(nn.Sequential(*tmp_list))
        tmp_list = []
    device = i // (partition_len)
    tmp_list.append(transformer_block.to(device))

# Add decoder in the end.
tmp_list.append(Decoder(ntokens, emsize).cuda(num_gpus - 1))
module_list.append(nn.Sequential(*tmp_list))

from torch.distributed.pipeline.sync import Pipe

# Build the pipeline.
chunks = 8
model = Pipe(torch.nn.Sequential(*module_list), chunks = chunks)


def get_total_params(module: torch.nn.Module):
    total_params = 0
    for param in module.parameters():
        total_params += param.numel()
    return total_params

print ('Total parameters in model: {:,}'.format(get_total_params(model)))

######################################################################
# Run the model
# -------------
#


######################################################################
# `CrossEntropyLoss <https://pytorch.org/docs/master/nn.html?highlight=crossentropyloss#torch.nn.CrossEntropyLoss>`__
# is applied to track the loss and
# `SGD <https://pytorch.org/docs/master/optim.html?highlight=sgd#torch.optim.SGD>`__
# implements stochastic gradient descent method as the optimizer. The initial
# learning rate is set to 5.0. `StepLR <https://pytorch.org/docs/master/optim.html?highlight=steplr#torch.optim.lr_scheduler.StepLR>`__ is
# applied to adjust the learn rate through epochs. During the
# training, we use
# `nn.utils.clip_grad_norm\_ <https://pytorch.org/docs/master/nn.html?highlight=nn%20utils%20clip_grad_norm#torch.nn.utils.clip_grad_norm_>`__
# function to scale all the gradient together to prevent exploding.
#

criterion = nn.CrossEntropyLoss()
lr = 5.0 # learning rate
optimizer = torch.optim.SGD(model.parameters(), lr=lr)
scheduler = torch.optim.lr_scheduler.StepLR(optimizer, 1.0, gamma=0.95)

import time
def train():
    model.train() # Turn on the train mode
    total_loss = 0.
    start_time = time.time()
    ntokens = len(vocab)

    # Train only for 50 batches to keep script execution time low.
    nbatches = min(50 * bptt, train_data.size(0) - 1)

    for batch, i in enumerate(range(0, nbatches, bptt)):
        data, targets = get_batch(train_data, i)
        optimizer.zero_grad()
        # Since the Pipe is only within a single host and process the ``RRef``
        # returned by forward method is local to this node and can simply
        # retrieved via ``RRef.local_value()``.
        output = model(data).local_value()
        # Need to move targets to the device where the output of the
        # pipeline resides.
        loss = criterion(output.view(-1, ntokens), targets.cuda(num_gpus - 1))
        loss.backward()
        torch.nn.utils.clip_grad_norm_(model.parameters(), 0.5)
        optimizer.step()

        total_loss += loss.item()
        log_interval = 10
        if batch % log_interval == 0 and batch > 0:
            cur_loss = total_loss / log_interval
            elapsed = time.time() - start_time
            print('| epoch {:3d} | {:5d}/{:5d} batches | '
                  'lr {:02.2f} | ms/batch {:5.2f} | '
                  'loss {:5.2f} | ppl {:8.2f}'.format(
                    epoch, batch, nbatches // bptt, scheduler.get_lr()[0],
                    elapsed * 1000 / log_interval,
                    cur_loss, math.exp(cur_loss)))
            total_loss = 0
            start_time = time.time()

def evaluate(eval_model, data_source):
    eval_model.eval() # Turn on the evaluation mode
    total_loss = 0.
    ntokens = len(vocab)
    # Evaluate only for 50 batches to keep script execution time low.
    nbatches = min(50 * bptt, data_source.size(0) - 1)
    with torch.no_grad():
        for i in range(0, nbatches, bptt):
            data, targets = get_batch(data_source, i)
            output = eval_model(data).local_value()
            output_flat = output.view(-1, ntokens)
            # Need to move targets to the device where the output of the
            # pipeline resides.
            total_loss += len(data) * criterion(output_flat, targets.cuda(num_gpus - 1)).item()
    return total_loss / (len(data_source) - 1)

######################################################################
# Loop over epochs. Save the model if the validation loss is the best
# we've seen so far. Adjust the learning rate after each epoch.

best_val_loss = float("inf")
epochs = 3 # The number of epochs
best_model = None

for epoch in range(1, epochs + 1):
    epoch_start_time = time.time()
    train()
    val_loss = evaluate(model, val_data)
    print('-' * 89)
    print('| end of epoch {:3d} | time: {:5.2f}s | valid loss {:5.2f} | '
          'valid ppl {:8.2f}'.format(epoch, (time.time() - epoch_start_time),
                                     val_loss, math.exp(val_loss)))
    print('-' * 89)

    if val_loss < best_val_loss:
        best_val_loss = val_loss
        best_model = model

    scheduler.step()


######################################################################
# Evaluate the model with the test dataset
# -------------------------------------
#


######################################################################
# Apply the best model to check the result with the test dataset.

test_loss = evaluate(best_model, test_data)
print('=' * 89)
print('| End of training | test loss {:5.2f} | test ppl {:8.2f}'.format(
    test_loss, math.exp(test_loss)))
print('=' * 89)