Multi-GPU training of AllenNLP coreference resolution
I'm trying to replicate (or come close) to the results obtained by the End-to-end Neural Coreference Resolution paper on the CoNLL-2012 shared task. I intend to do some enhancements on top of this, so I decided to use AllenNLP's CoreferenceResolver. This is how I'm initialising &amp; training the model:

import torch
from allennlp.common import Params
from allennlp.data import Vocabulary
from allennlp.data.dataset_readers import ConllCorefReader
from allennlp.data.dataset_readers.dataset_utils import Ontonotes
from allennlp.data.iterators import BasicIterator, MultiprocessIterator
from allennlp.data.token_indexers import SingleIdTokenIndexer, TokenCharactersIndexer
from allennlp.models import CoreferenceResolver
from allennlp.modules import Embedding, FeedForward
from allennlp.modules.seq2seq_encoders import PytorchSeq2SeqWrapper
from allennlp.modules.seq2vec_encoders import CnnEncoder
from allennlp.modules.text_field_embedders import BasicTextFieldEmbedder
from allennlp.modules.token_embedders import TokenCharactersEncoder
from allennlp.training import Trainer
from allennlp.training.learning_rate_schedulers import LearningRateScheduler
from torch.nn import LSTM, ReLU
from torch.optim import Adam


def read_data(directory_path):
    data = []
    for file_path in Ontonotes().dataset_path_iterator(directory_path):
        data += dataset_reader.read(file_path)
    return data


INPUT_FILE_PATH_TEMPLATE = "data/CoNLL-2012/v4/data/%s"
dataset_reader = ConllCorefReader(10, {"tokens": SingleIdTokenIndexer(),
                                       "token_characters": TokenCharactersIndexer()})
training_data = read_data(INPUT_FILE_PATH_TEMPLATE % "train")
validation_data = read_data(INPUT_FILE_PATH_TEMPLATE % "development")

vocabulary = Vocabulary.from_instances(training_data + validation_data)
model = CoreferenceResolver(vocab=vocabulary,
                            text_field_embedder=BasicTextFieldEmbedder({"tokens": Embedding.from_params(vocabulary, Params({"embedding_dim": embeddings_dimension, "pretrained_file": "glove.840B.300d.txt"})),
                                                                        "token_characters": TokenCharactersEncoder(embedding=Embedding(num_embeddings=vocabulary.get_vocab_size("token_characters"), embedding_dim=8, vocab_namespace="token_characters"),
                                                                                                                   encoder=CnnEncoder(embedding_dim=8, num_filters=50, ngram_filter_sizes=(3, 4, 5), output_dim=100))}),
                            context_layer=PytorchSeq2SeqWrapper(LSTM(input_size=400, hidden_size=200, num_layers=1, dropout=0.2, bidirectional=True, batch_first=True)),
                            mention_feedforward=FeedForward(input_dim=1220, num_layers=2, hidden_dims=[150, 150], activations=[ReLU(), ReLU()], dropout=[0.2, 0.2]),
                            antecedent_feedforward=FeedForward(input_dim=3680, num_layers=2, hidden_dims=[150, 150], activations=[ReLU(), ReLU()], dropout=[0.2, 0.2]),
                            feature_size=20,
                            max_span_width=10,
                            spans_per_word=0.4,
                            max_antecedents=250,
                            lexical_dropout=0.5)

if torch.cuda.is_available():
    cuda_device = 0
    model = model.cuda(cuda_device)
else:
    cuda_device = -1

iterator = BasicIterator(batch_size=1)
iterator.index_with(vocabulary)
optimiser = Adam(model.parameters(), weight_decay=0.1)
Trainer(model=model,
        train_dataset=training_data,
        validation_dataset=validation_data,
        optimizer=optimiser,
        learning_rate_scheduler=LearningRateScheduler.from_params(optimiser, Params({"type": "step", "step_size": 100})),
        iterator=iterator,
        num_epochs=150,
        patience=1,
        cuda_device=cuda_device).train()


After reading the data I've trained the model but ran out of GPU memory: RuntimeError: CUDA out of memory. Tried to allocate 4.43 GiB (GPU 0; 11.17 GiB total capacity; 3.96 GiB already allocated; 3.40 GiB free; 3.47 GiB cached). Therefore, I attempted to make use of multiple GPUs to train this model. I'm making use of Tesla K80s (which have 12GiB memory).

I've tried making use of AllenNLP's MultiprocessIterator, by itialising the iterator as MultiprocessIterator(BasicIterator(batch_size=1), num_workers=torch.cuda.device_count()). However, only 1 GPU is being used (by monitoring the memory usage through the nvidia-smi command) &amp; got the error below. I also tried fiddling with its parameters (increasing num_workers or decreasing output_queue_size) &amp; the ulimit (as mentioned by this PyTorch issue) to no avail.

Process Process-3:
Traceback (most recent call last):
  File "/usr/lib/python3.6/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
Traceback (most recent call last):
  File "/usr/lib/python3.6/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python3.6/multiprocessing/process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File "/home/user/.local/lib/python3.6/site-packages/allennlp/data/iterators/multiprocess_iterator.py", line 32, in _create_tensor_dicts
    output_queue.put(tensor_dict)
  File "/usr/lib/python3.6/multiprocessing/process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File "/home/user/.local/lib/python3.6/site-packages/allennlp/data/iterators/multiprocess_iterator.py", line 32, in _create_tensor_dicts
    output_queue.put(tensor_dict)
  File "&lt;string&gt;", line 2, in put
  File "&lt;string&gt;", line 2, in put
  File "/usr/lib/python3.6/multiprocessing/managers.py", line 772, in _callmethod
    raise convert_to_error(kind, result)
  File "/usr/lib/python3.6/multiprocessing/managers.py", line 772, in _callmethod
    raise convert_to_error(kind, result)
multiprocessing.managers.RemoteError: 
---------------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.6/multiprocessing/managers.py", line 228, in serve_client
    request = recv()
  File "/usr/lib/python3.6/multiprocessing/connection.py", line 251, in recv
    return _ForkingPickler.loads(buf.getbuffer())
  File "/home/user/.local/lib/python3.6/site-packages/torch/multiprocessing/reductions.py", line 276, in rebuild_storage_fd
    fd = df.detach()
  File "/usr/lib/python3.6/multiprocessing/resource_sharer.py", line 58, in detach
    return reduction.recv_handle(conn)
  File "/usr/lib/python3.6/multiprocessing/reduction.py", line 182, in recv_handle
    return recvfds(s, 1)[0]
  File "/usr/lib/python3.6/multiprocessing/reduction.py", line 161, in recvfds
    len(ancdata))
RuntimeError: received 0 items of ancdata
---------------------------------------------------------------------------


I also tried achieving this through PyTorch's DataParallel, by wrapping the model's context_layer, mention_feedforward, antecedent_feedforward with a custom DataParallelWrapper (to provide compatibility with the AllenNLP-assumed class functions). Still, only 1 GPU is used &amp; it eventually runs out of memory as before.

class DataParallelWrapper(DataParallel):
    def __init__(self, module):
        super().__init__(module)

    def get_output_dim(self):
        return self.module.get_output_dim()

    def get_input_dim(self):
        return self.module.get_input_dim()

    def forward(self, *inputs):
        return self.module.forward(inputs)


After some digging through the code I found out that AllenNLP does this under the hood directly through its Trainer. The cuda_device can either be a single int (in the case of single-processing) or a list of ints (in the case of multi-processing):


  cuda_device : Union[int, List[int]], optional (default = -1)
  An integer or list of integers specifying the CUDA device(s) to use. If -1, the CPU is used.


So all GPU devices needed should be passed on instead:

if torch.cuda.is_available():
    cuda_device = list(range(torch.cuda.device_count()))
    model = model.cuda(cuda_device[0])
else:
    cuda_device = -1


Note that the model still has to be manually moved to the GPU (via model.cuda(...)), as it would otherwise try to use multiple CPUs instead.
