"""
.. _model-graph-store:

Large-Scale Training of Graph Neural Networks
=============================================

**Author**: Da Zheng, Chao Ma, Zheng Zhang
"""
################################################################################################
#
# In real-world tasks, many graphs are very large. For example, a recent
# snapshot of the friendship network of Facebook contains 800 million
# nodes and over 100 billion links. We are facing challenges on
# large-scale training of graph neural networks.
#
# To accelerate training on a giant graph, DGL provides two additional
# components: sampler and graph store.
#
# -  A sampler constructs small subgraphs (``NodeFlow``) from a given
#    (giant) graph. The sampler can run on a local machine as well as on
#    remote machines. Also, DGL can launch multiple parallel samplers
#    across a set of machines.
#
# -  The graph store contains graph embeddings of a giant graph, as well
#    as the graph structure. So far, we provide a shared-memory graph
#    store to support multi-processing training, which is important for
#    training on multiple GPUs and on non-uniform memory access (NUMA)
#    machines. The shared-memory graph store has a similar interface to
#    ``DGLGraph`` for programming. DGL will also support a distributed
#    graph store that can store graph embeddings across machines in the
#    future release.
#
# The figure below shows the interaction of the trainer with the samplers
# and the graph store. The trainer takes subgraphs (``NodeFlow``) from the
# sampler and fetches graph embeddings from the graph store before
# training. The trainer can push new graph embeddings to the graph store
# afterward.
#
# |image0|
#
# In this tutorial, we use control-variate sampling to demonstrate how to
# use these three DGL components, extending `the original code of
# control-variate
# sampling <https://doc.dgl.ai/tutorials/models/5_giant_graph/1_sampling_mx.html#sphx-glr-tutorials-models-5-giant-graph-1-sampling-mx-py>`__.
# Because the graph store has a similar API to ``DGLGraph``, the code is
# similar. The tutorial will mainly focus on the difference.
#
# Graph Store
# -----------
#
# The graph store has two parts: the server and the client. We need to run
# the graph store server as a daemon before training. We provide a script
# ``run_store_server.py`` `(link) <https://github.com/dmlc/dgl/blob/master/examples/mxnet/sampling/run_store_server.py>`__
# that runs the graph store server and loads graph data. For example, the
# following command runs a graph store server that loads the reddit
# dataset and is configured to run with four trainers.
#
# ::
#
#    python3 run_store_server.py --dataset reddit --num-workers 4
#
# The trainer uses the graph store client to access data in the graph
# store from the trainer process. A user only needs to write code in the
# trainer. We first create the graph store client that connects with the
# server. We specify ``store_type`` as “shared_memory” to connect with the
# shared-memory graph store server.
#
# .. code:: python
#
#    g = dgl.contrib.graph_store.create_graph_from_store("reddit", store_type="shared_mem")
#
# The `sampling
# tutorial <https://doc.dgl.ai/tutorials/models/5_giant_graph/1_sampling_mx.html#sphx-glr-tutorials-models-5-giant-graph-1-sampling-mx-py>`__
# shows the detail of sampling methods and how they are used to train
# graph neural networks such as graph convolution network. As a recap, the
# graph convolution model performs the following computation in each
# layer.
#
# .. math::
#
#
#    z_v^{(l+1)} = \sum_{u \in \mathcal{N}^{(l)}(v)} \tilde{A}_{uv} h_u^{(l)} \qquad
#    h_v^{(l+1)} = \sigma ( z_v^{(l+1)} W^{(l)} )
#
# `Control variate sampling <https://arxiv.org/abs/1710.10568>`__
# approximates :math:`z_v^{(l+1)}` as follows:
#
# .. math::
#
#
#    \hat{z}_v^{(l+1)} = \frac{\vert \mathcal{N}(v) \vert }{\vert \hat{\mathcal{N}}^{(l)}(v) \vert} \sum_{u \in \hat{\mathcal{N}}^{(l)}(v)} \tilde{A}_{uv} ( \hat{h}_u^{(l)} - \bar{h}_u^{(l)} ) + \sum_{u \in \mathcal{N}(v)} \tilde{A}_{uv} \bar{h}_u^{(l)} \\
#    \hat{h}_v^{(l+1)} = \sigma ( \hat{z}_v^{(l+1)} W^{(l)} )
#
# In addition to the approximation, `Chen et.
# al. <https://arxiv.org/abs/1710.10568>`__ applies a preprocessing trick
# to reduce the number of hops for sampling neighbors by one. This trick
# works for models such as Graph Convolution Networks and GraphSage. It
# preprocesses the input layer. The original GCN takes :math:`X` as input.
# Instead of taking :math:`X` as the input of the model, the trick
# computes :math:`U^{(0)}=\tilde{A}X` and uses :math:`U^{(0)}` as the
# input of the first layer. In this way, the vertices in the first layer
# does not need to compute aggregation over their neighborhood and, thus,
# reduce the number of layers to sample by one.
#
# For a giant graph, both :math:`\tilde{A}` and :math:`X` can be very
# large. We need to perform this operation in a distributed fashion. That
# is, each trainer takes part of the computation and the computation is
# distributed among all trainers. We can use ``update_all`` in the graph
# store to perform this computation.
#
# .. code:: python
#
#    g.update_all(fn.copy_src(src='features', out='m'),
#                 fn.sum(msg='m', out='preprocess'),
#                 lambda node : {'preprocess': node.data['preprocess'] * node.data['norm']})
#
# ``update_all`` in the graph store runs in a distributed fashion. That
# is, all trainers need to invoke this function and take part of the
# computation. When a trainer completes its portion, it will wait for
# other trainers to complete before proceeding with its other computation.
#
# The node/edge data now live in the graph store and the access to the
# node/edge data is now a little different. The graph store no longer
# supports data access with ``g.ndata``/``g.edata``, which reads the
# entire node/edge data tensor. Instead, users have to use
# ``g.nodes[node_ids].data[embed_name]`` to access data on some nodes.
# (Note: this method is also allowed in ``DGLGraph`` and ``g.ndata`` is
# simply a short syntax for ``g.nodes[:].data``). In addition, the graph
# store supports ``get_n_repr``/``set_n_repr`` for node data and
# ``get_e_repr``/``set_e_repr`` for edge data.
#
# To initialize the node/edge tensors more efficiently, we provide two new
# methods in the graph store client to initialize node data and edge data
# (i.e., ``init_ndata`` for node data or ``init_edata`` for edge data).
# What happened under the hood is that these two methods send
# initialization commands to the server and the graph store server
# initializes the node/edge tensors on behalf of trainers.
#
# Here we show how we should initialize node data for control-variate
# sampling. ``h_i`` stores the history of nodes in layer ``i``;
# ``agg_h_i`` stores the aggregation of the history of neighbor nodes in
# layer ``i``.
#
# .. code:: python
#
#    for i in range(n_layers):
#        g.init_ndata('h_{}'.format(i), (features.shape[0], args.n_hidden), 'float32')
#        g.init_ndata('agg_h_{}'.format(i), (features.shape[0], args.n_hidden), 'float32')
#
# After we initialize node data, we train GCN with control-variate
# sampling as below. The training code takes advantage of preprocessed
# input data in the first layer and works identically to the
# single-process training procedure.
#
# .. code:: python
#
#    for nf in NeighborSampler(g, batch_size, num_neighbors,
#                              neighbor_type='in', num_hops=L-1,
#                              seed_nodes=labeled_nodes):
#        for i in range(nf.num_blocks):
#            # aggregate history on the original graph
#            g.pull(nf.layer_parent_nid(i+1),
#                   fn.copy_src(src='h_{}'.format(i), out='m'),
#                   lambda node: {'agg_h_{}'.format(i): node.data['m'].mean(axis=1)})
#        # We need to copy data in the NodeFlow to the right context.
#        nf.copy_from_parent(ctx=right_context)
#        nf.apply_layer(0, lambda node : {'h' : layer(node.data['preprocess'])})
#        h = nf.layers[0].data['h']
#
#        for i in range(nf.num_blocks):
#            prev_h = nf.layers[i].data['h_{}'.format(i)]
#            # compute delta_h, the difference of the current activation and the history
#            nf.layers[i].data['delta_h'] = h - prev_h
#            # refresh the old history
#            nf.layers[i].data['h_{}'.format(i)] = h.detach()
#            # aggregate the delta_h
#            nf.block_compute(i,
#                             fn.copy_src(src='delta_h', out='m'),
#                             lambda node: {'delta_h': node.data['m'].mean(axis=1)})
#            delta_h = nf.layers[i + 1].data['delta_h']
#            agg_h = nf.layers[i + 1].data['agg_h_{}'.format(i)]
#            # control variate estimator
#            nf.layers[i + 1].data['h'] = delta_h + agg_h
#            nf.apply_layer(i + 1, lambda node : {'h' : layer(node.data['h'])})
#            h = nf.layers[i + 1].data['h']
#        # update history
#        nf.copy_to_parent()
#
# The complete example code can be found
# `here <https://github.com/dmlc/dgl/tree/master/examples/mxnet/sampling>`__.
#
# After showing how the shared-memory graph store is used with
# control-variate sampling, let’s see how to use it for multi-GPU training
# and how to optimize the training on a non-uniform memory access (NUMA)
# machine. A NUMA machine here means a machine with multiple processors
# and large memory. It works for all backend frameworks as long as the
# framework supports multi-processing training. If we use MXNet as the
# backend, we can use the distributed MXNet kvstore to aggregate gradients
# among processes and use the MXNet launch tool to launch multiple workers
# that run the training script. The command below launches our example
# code for multi-processing GCN training with control variate sampling and
# it runs 4 trainers.
#
# ::
#
#    python3 ../incubator-mxnet/tools/launch.py -n 4 -s 1 --launcher local \
#        python3 examples/mxnet/sampling/multi_process_train.py \
#        --graph-name reddit \
#        --model gcn_cv --num-neighbors 1 \
#        --batch-size 2500 --test-batch-size 5000 \
#        --n-hidden 64
#
# ..
#
# It is fairly easy to enable multi-GPU training. All we need to do is to
# copy data to a right GPU context and invoke NodeFlow computation in that
# GPU context. As shown above, we specify a context ``right_context`` in
# ``copy_from_parent``.
#
# To optimize the computation on a NUMA machine, we need to configure each
# process properly. For example, we should use the same number of
# processes as the number of NUMA nodes (usually equivalent to the number
# of processors) and bind the processes to NUMA nodes. In addition, we
# should reduce the number of OpenMP threads to the number of CPU cores in
# a processor and reduce the number of threads of the MXNet kvstore to a
# small number such as 4.
#
# .. code:: python
#
#    import numa
#    import os
#    if 'DMLC_TASK_ID' in os.environ and int(os.environ['DMLC_TASK_ID']) < 4:
#        # bind the process to a NUMA node.
#        numa.bind([int(os.environ['DMLC_TASK_ID'])])
#        # Reduce the number of OpenMP threads to match the number of
#        # CPU cores of a processor.
#        os.environ['OMP_NUM_THREADS'] = '16'
#    else:
#        # Reduce the number of OpenMP threads in the MXNet KVstore server to 4.
#        os.environ['OMP_NUM_THREADS'] = '4'
#
# Given the configuration above, NUMA-aware multi-processing training can
# accelerate training almost by a factor of 4 as shown in the figure below
# on an X1.32xlarge instance where there are 4 processors, each of which
# has 16 physical CPU cores. We can see that NUMA-unaware training cannot
# take advantage of computation power of the machine. It is even slightly
# slower than just using one of the processors in the machine. NUMA-aware
# training, on the other hand, takes about only 20 seconds to converge to
# the accuracy of 96% with 20 iterations.
#
# |image1|
#
# Distributed Sampler
# -------------------
#
# For many tasks, we found that the sampling takes a significant amount of
# time for the training process on a giant graph. So DGL supports
# distributed samplers for speeding up the sampling process on giant
# graphs. DGL allows users to launch multiple samplers on different
# machines concurrently, and each sampler can send its sampled subgraph
# (``NodeFlow``) to trainer machines continuously.
#
# To use the distributed sampler on DGL, users start both trainer and
# sampler processes on different machines. Users can find the complete
# demo code and launch scripts `in this
# link <https://github.com/dmlc/dgl/tree/master/examples/mxnet/sampling/dis_sampling>`__
# and this tutorial will focus on the main difference between
# single-machine code and distributed code.
#
# For the trainer, developers can easily migrate the existing
# single-machine sampler code to the distributed setting seamlessly by
# just changing a few lines of code. First, users need to create a
# distributed ``SamplerReceiver`` object before training:
#
# .. code:: python
#
#    sampler = dgl.contrib.sampling.SamplerReceiver(graph, ip_addr, num_sampler)
#
# The ``SamplerReceiver`` class is used for receiving remote subgraph from
# other machines. This API has three arguments: ``parent_graph``,
# ``ip_address``, and ``number_of_samplers``.
#
# After that, developers can change just one line of existing
# single-machine training code like this:
#
# .. code:: python
#
#    for nf in sampler:
#        for i in range(nf.num_blocks):
#            # aggregate history on the original graph
#            g.pull(nf.layer_parent_nid(i+1),
#                   fn.copy_src(src='h_{}'.format(i), out='m'),
#                   lambda node: {'agg_h_{}'.format(i): node.data['m'].mean(axis=1)})
#
#    ...
#
# Here, we use the code ``for nf in sampler`` to replace the original
# single-machine sampling code:
#
# .. code:: python
#
#    for nf in NeighborSampler(g, batch_size, num_neighbors,
#                              neighbor_type='in', num_hops=L-1,
#                              seed_nodes=labeled_nodes):
#
# All the other parts of the original single-machine code is not changed.
#
# In addition, developers need to write sampling logic on the sampler
# machine. For neighbor-sampler, developers can just copy their existing
# single-machine code to sampler machines like this:
#
# .. code:: python
#
#    sender = dgl.contrib.sampling.SamplerSender(trainer_address)
#
#    ...
#
#    for n in num_epoch:
#        for nf in dgl.contrib.sampling.NeighborSampler(graph, batch_size, num_neighbors,
#                                                           neighbor_type='in',
#                                                           shuffle=shuffle,
#                                                           num_workers=num_workers,
#                                                           num_hops=num_hops,
#                                                           add_self_loop=add_self_loop,
#                                                           seed_nodes=seed_nodes):
#            sender.send(nf, trainer_id)
#        # tell trainer I have finished current epoch
#        sender.signal(trainer_id)
#
# The figure below shows the overall performance improvement of training
# GCN and GraphSage on the Reddit dataset after deploying the
# optimizations in this tutorial. Our NUMA optimization speeds up the
# training by a factor of 4. The distributed sampling achieves additional
# 20%-40% speed improvement for different tasks.
#
# |image2|
#
# Scale to giant graphs
# ---------------------
#
# Finally, we would like to demonstrate the scalability of DGL with giant
# synthetic graphs. We create three large power-law graphs with
# `RMAT <http://www.cs.cmu.edu/~christos/PUBLICATIONS/siam04.pdf>`__. Each
# node is associated with 100 features and we compute node embeddings with
# 64 dimensions. Below shows the training speed and memory consumption of
# GCN with neighbor sampling.
#
# ====== ====== ================== ===========
# #Nodes #Edges Time per epoch (s) Memory (GB)
# ====== ====== ================== ===========
# 5M     250M   4.7                8
# 50M    2.5B   46                 75
# 500M   25B    505                740
# ====== ====== ================== ===========
#
# We can see that DGL can scale to graphs with up to 500M nodes and 25B
# edges.
#
# .. |image0| image:: https://data.dgl.ai/tutorial/sampling/arch.png
# .. |image1| image:: https://data.dgl.ai/tutorial/sampling/NUMA_speedup.png
# .. |image2| image:: https://data.dgl.ai/tutorial/sampling/whole_speedup.png
#
