"""
Single Machine Multi-GPU Minibatch Graph Classification
=======================================================

In this tutorial, you will learn how to use multiple GPUs in training a
graph neural network (GNN) for graph classification. This tutorial assumes
knowledge in GNNs for graph classification and we recommend you to check
:doc:`Training a GNN for Graph Classification <../blitz/5_graph_classification>` otherwise.

(Time estimate: 8 minutes)

To use a single GPU in training a GNN, we need to put the model, graph(s), and other
tensors (e.g. labels) on the same GPU:

.. code:: python

    import torch

    # Use the first GPU
    device = torch.device("cuda:0")
    model = model.to(device)
    graph = graph.to(device)
    labels = labels.to(device)

The node and edge features in the graphs, if any, will also be on the GPU.
After that, the forward computation, backward computation and parameter
update will take place on the GPU. For graph classification, this repeats
for each minibatch gradient descent.

Using multiple GPUs allows performing more computation per unit of time. It
is like having a team work together, where each GPU is a team member. We need
to distribute the computation workload across GPUs and let them synchronize
the efforts regularly. PyTorch provides convenient APIs for this task with
multiple processes, one per GPU, and we can use them in conjunction with DGL.

Intuitively, we can distribute the workload along the dimension of data. This
allows multiple GPUs to perform the forward and backward computation of
multiple gradient descents in parallel. To distribute a dataset across
multiple GPUs, we need to partition it into multiple mutually exclusive
subsets of a similar size, one per GPU. We need to repeat the random
partition every epoch to guarantee randomness. We can use
:func:`~dgl.dataloading.pytorch.GraphDataLoader`, which wraps some PyTorch 
APIs and does the job for graph classification in data loading.

Once all GPUs have finished the backward computation for its minibatch,
we need to synchronize the model parameter update across them. Specifically,
this involves collecting gradients from all GPUs, averaging them and updating
the model parameters on each GPU. We can wrap a PyTorch model with
:func:`~torch.nn.parallel.DistributedDataParallel` so that the model
parameter update will invoke gradient synchronization first under the hood.

.. image:: https://data.dgl.ai/tutorial/mgpu_gc.png
  :width: 450px
  :align: center

That’s the core behind this tutorial. We will explore it more in detail with
a complete example below.

.. note::

   See `this tutorial <https://pytorch.org/tutorials/intermediate/ddp_tutorial.html>`__
   from PyTorch for general multi-GPU training with ``DistributedDataParallel``.

Distributed Process Group Initialization
----------------------------------------

For communication between multiple processes in multi-gpu training, we need
to start the distributed backend at the beginning of each process. We use
`world_size` to refer to the number of processes and `rank` to refer to the
process ID, which should be an integer from `0` to `world_size - 1`.
""" 

import torch.distributed as dist

def init_process_group(world_size, rank):
    dist.init_process_group(
        backend='nccl',
        init_method='tcp://127.0.0.1:12345',
        world_size=world_size,
        rank=rank)

###############################################################################
# Data Loader Preparation
# -----------------------
#
# We split the dataset into training, validation and test subsets. In dataset
# splitting, we need to use a same random seed across processes to ensure a
# same split. We follow the common practice to train with multiple GPUs and
# evaluate with a single GPU, thus only set `use_ddp` to True in the
# :func:`~dgl.dataloading.pytorch.GraphDataLoader` for the training set, where 
# `ddp` stands for :func:`~torch.nn.parallel.DistributedDataParallel`.
#

from dgl.data import split_dataset
from dgl.dataloading import GraphDataLoader

def get_dataloaders(dataset, seed, batch_size=32):
    # Use a 80:10:10 train-val-test split
    train_set, val_set, test_set = split_dataset(dataset,
                                                 frac_list=[0.8, 0.1, 0.1],
                                                 shuffle=True,
                                                 random_state=seed)
    train_loader = GraphDataLoader(train_set, use_ddp=True, batch_size=batch_size, shuffle=True)
    val_loader = GraphDataLoader(val_set, batch_size=batch_size)
    test_loader = GraphDataLoader(test_set, batch_size=batch_size)

    return train_loader, val_loader, test_loader

###############################################################################
# Model Initialization
# --------------------
#
# For this tutorial, we use a simplified Graph Isomorphism Network (GIN).
#

import torch.nn as nn
import torch.nn.functional as F
from dgl.nn.pytorch import GINConv, SumPooling

class GIN(nn.Module):
    def __init__(self, input_size=1, num_classes=2):
        super(GIN, self).__init__()

        self.conv1 = GINConv(nn.Linear(input_size, num_classes), aggregator_type='sum')
        self.conv2 = GINConv(nn.Linear(num_classes, num_classes), aggregator_type='sum')
        self.pool = SumPooling()

    def forward(self, g, feats):
        feats = self.conv1(g, feats)
        feats = F.relu(feats)
        feats = self.conv2(g, feats)

        return self.pool(g, feats)

###############################################################################
# To ensure same initial model parameters across processes, we need to set the
# same random seed before model initialization. Once we construct a model
# instance, we wrap it with :func:`~torch.nn.parallel.DistributedDataParallel`.
#

import torch
from torch.nn.parallel import DistributedDataParallel

def init_model(seed, device):
    torch.manual_seed(seed)
    model = GIN().to(device)
    model = DistributedDataParallel(model, device_ids=[device], output_device=device)

    return model

###############################################################################
# Main Function for Each Process
# -----------------------------
#
# Define the model evaluation function as in the single-GPU setting.
#

def evaluate(model, dataloader, device):
    model.eval()

    total = 0
    total_correct = 0

    for bg, labels in dataloader:
        bg = bg.to(device)
        labels = labels.to(device)
        # Get input node features
        feats = bg.ndata.pop('attr')
        with torch.no_grad():
            pred = model(bg, feats)
        _, pred = torch.max(pred, 1)
        total += len(labels)
        total_correct += (pred == labels).sum().cpu().item()

    return 1.0 * total_correct / total

###############################################################################
# Define the main function for each process.
#

from torch.optim import Adam

def main(rank, world_size, dataset, seed=0):
    init_process_group(world_size, rank)
    # Assume the GPU ID to be the same as the process ID
    device = torch.device('cuda:{:d}'.format(rank))
    torch.cuda.set_device(device)

    model = init_model(seed, device)
    criterion = nn.CrossEntropyLoss()
    optimizer = Adam(model.parameters(), lr=0.01)

    train_loader, val_loader, test_loader = get_dataloaders(dataset,
                                                            seed)
    for epoch in range(5):
        model.train()
        # The line below ensures all processes use a different
        # random ordering in data loading for each epoch.
        train_loader.set_epoch(epoch)

        total_loss = 0
        for bg, labels in train_loader:
            bg = bg.to(device)
            labels = labels.to(device)
            feats = bg.ndata.pop('attr')
            pred = model(bg, feats)

            loss = criterion(pred, labels)
            total_loss += loss.cpu().item()
            optimizer.zero_grad()
            loss.backward()
            optimizer.step()
        loss = total_loss
        print('Loss: {:.4f}'.format(loss))

        val_acc = evaluate(model, val_loader, device)
        print('Val acc: {:.4f}'.format(val_acc))

    test_acc = evaluate(model, test_loader, device)
    print('Test acc: {:.4f}'.format(test_acc))
    dist.destroy_process_group()

###############################################################################
# Finally we load the dataset and launch the processes.
# 
# .. note::
# 
#    You will need to use ``dgl.multiprocessing`` instead of the Python
#    ``multiprocessing`` package. ``dgl.multiprocessing`` is identical to
#    Python’s built-in ``multiprocessing`` except that it handles the
#    subtleties between forking and multithreading in Python.
#

if __name__ == '__main__':
    import dgl.multiprocessing as mp

    from dgl.data import GINDataset

    num_gpus = 4
    procs = []
    dataset = GINDataset(name='IMDBBINARY', self_loop=False)
    for rank in range(num_gpus):
        p = mp.Process(target=main, args=(rank, num_gpus, dataset))
        p.start()
        procs.append(p)
    for p in procs:
        p.join()
