diff --git a/distributed_train.py b/distributed_train.py
index c3e137bc2..f3725a6e1 100644
--- a/distributed_train.py
+++ b/distributed_train.py
@@ -30,7 +30,7 @@ def main(args):
                 raise e
             except FileNotFoundError as e:  # Slurm is not installed
                 pass
-    if args.distributed_init_method is None:
+    if args.distributed_init_method is None and args.distributed_port is None:
         raise ValueError('--distributed-init-method or --distributed-port '
                          'must be specified for distributed training')
 
diff --git a/docs/conf.py b/docs/conf.py
index ac733584f..3888492c8 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -60,9 +60,9 @@ github_doc_root = 'https://github.com/pytorch/fairseq/tree/master/docs/'
 # built documents.
 #
 # The short X.Y version.
-version = '0.5.0'
+version = '0.6.0'
 # The full version, including alpha/beta/rc tags.
-release = '0.5.0'
+release = '0.6.0'
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
diff --git a/docs/data.rst b/docs/data.rst
index 24a123628..fed632780 100644
--- a/docs/data.rst
+++ b/docs/data.rst
@@ -36,5 +36,7 @@ Iterators
     :members:
 .. autoclass:: fairseq.data.EpochBatchIterator
     :members:
+.. autoclass:: fairseq.data.GroupedIterator
+    :members:
 .. autoclass:: fairseq.data.ShardedIterator
     :members:
diff --git a/fairseq/criterions/adaptive_loss.py b/fairseq/criterions/adaptive_loss.py
index e8529fd42..c684565c9 100644
--- a/fairseq/criterions/adaptive_loss.py
+++ b/fairseq/criterions/adaptive_loss.py
@@ -54,6 +54,7 @@ class AdaptiveLoss(FairseqCriterion):
         logging_output = {
             'loss': utils.item(loss.data) if reduce else loss.data,
             'ntokens': sample['ntokens'],
+            'nsentences': sample['target'].size(0),
             'sample_size': sample_size,
         }
         return loss, sample_size, logging_output
@@ -63,9 +64,12 @@ class AdaptiveLoss(FairseqCriterion):
         """Aggregate logging outputs from data parallel training."""
         loss_sum = sum(log.get('loss', 0) for log in logging_outputs)
         ntokens = sum(log.get('ntokens', 0) for log in logging_outputs)
+        nsentences = sum(log.get('nsentences', 0) for log in logging_outputs)
         sample_size = sum(log.get('sample_size', 0) for log in logging_outputs)
         agg_output = {
             'loss': loss_sum / sample_size / math.log(2),
+            'ntokens': ntokens,
+            'nsentences': nsentences,
             'sample_size': sample_size,
         }
         if sample_size != ntokens:
diff --git a/fairseq/criterions/cross_entropy.py b/fairseq/criterions/cross_entropy.py
index 152b5a613..8c94265f5 100644
--- a/fairseq/criterions/cross_entropy.py
+++ b/fairseq/criterions/cross_entropy.py
@@ -37,6 +37,7 @@ class CrossEntropyCriterion(FairseqCriterion):
         logging_output = {
             'loss': utils.item(loss.data) if reduce else loss.data,
             'ntokens': sample['ntokens'],
+            'nsentences': sample['target'].size(0),
             'sample_size': sample_size,
         }
         return loss, sample_size, logging_output
@@ -46,9 +47,12 @@ class CrossEntropyCriterion(FairseqCriterion):
         """Aggregate logging outputs from data parallel training."""
         loss_sum = sum(log.get('loss', 0) for log in logging_outputs)
         ntokens = sum(log.get('ntokens', 0) for log in logging_outputs)
+        nsentences = sum(log.get('nsentences', 0) for log in logging_outputs)
         sample_size = sum(log.get('sample_size', 0) for log in logging_outputs)
         agg_output = {
             'loss': loss_sum / sample_size / math.log(2),
+            'ntokens': ntokens,
+            'nsentences': nsentences,
             'sample_size': sample_size,
         }
         if sample_size != ntokens:
diff --git a/fairseq/criterions/label_smoothed_cross_entropy.py b/fairseq/criterions/label_smoothed_cross_entropy.py
index 970f7b9ce..e95b06674 100644
--- a/fairseq/criterions/label_smoothed_cross_entropy.py
+++ b/fairseq/criterions/label_smoothed_cross_entropy.py
@@ -40,6 +40,7 @@ class LabelSmoothedCrossEntropyCriterion(FairseqCriterion):
             'loss': utils.item(loss.data) if reduce else loss.data,
             'nll_loss': utils.item(nll_loss.data) if reduce else nll_loss.data,
             'ntokens': sample['ntokens'],
+            'nsentences': sample['target'].size(0),
             'sample_size': sample_size,
         }
         return loss, sample_size, logging_output
@@ -58,14 +59,16 @@ class LabelSmoothedCrossEntropyCriterion(FairseqCriterion):
         loss = (1. - self.eps) * nll_loss + eps_i * smooth_loss
         return loss, nll_loss
 
-
     @staticmethod
     def aggregate_logging_outputs(logging_outputs):
         """Aggregate logging outputs from data parallel training."""
         ntokens = sum(log.get('ntokens', 0) for log in logging_outputs)
+        nsentences = sum(log.get('nsentences', 0) for log in logging_outputs)
         sample_size = sum(log.get('sample_size', 0) for log in logging_outputs)
         return {
             'loss': sum(log.get('loss', 0) for log in logging_outputs) / sample_size / math.log(2),
             'nll_loss': sum(log.get('nll_loss', 0) for log in logging_outputs) / ntokens / math.log(2),
+            'ntokens': ntokens,
+            'nsentences': nsentences,
             'sample_size': sample_size,
         }
diff --git a/fairseq/data/__init__.py b/fairseq/data/__init__.py
index 7d80c44a9..c04c32487 100644
--- a/fairseq/data/__init__.py
+++ b/fairseq/data/__init__.py
@@ -12,18 +12,24 @@ from .language_pair_dataset import LanguagePairDataset
 from .monolingual_dataset import MonolingualDataset
 from .token_block_dataset import TokenBlockDataset
 
-from .iterators import CountingIterator, EpochBatchIterator, ShardedIterator
+from .iterators import (
+    CountingIterator,
+    EpochBatchIterator,
+    GroupedIterator,
+    ShardedIterator,
+)
 
 __all__ = [
     'CountingIterator',
     'Dictionary',
     'EpochBatchIterator',
     'FairseqDataset',
+    'GroupedIterator',
     'IndexedDataset',
     'IndexedInMemoryDataset',
     'IndexedRawTextDataset',
     'LanguagePairDataset',
     'MonolingualDataset',
-    'TokenBlockDataset',
     'ShardedIterator',
+    'TokenBlockDataset',
 ]
diff --git a/fairseq/data/iterators.py b/fairseq/data/iterators.py
index 073dab600..31331baa0 100644
--- a/fairseq/data/iterators.py
+++ b/fairseq/data/iterators.py
@@ -6,6 +6,7 @@
 # can be found in the PATENTS file in the same directory.
 
 import itertools
+import math
 
 import numpy as np
 import torch
@@ -150,6 +151,36 @@ class EpochBatchIterator(object):
         ))
 
 
+class GroupedIterator(object):
+    """Wrapper around an iterable that returns groups (chunks) of items.
+
+    Args:
+        iterable (iterable): iterable to wrap
+        chunk_size (int): size of each chunk
+    """
+
+    def __init__(self, iterable, chunk_size):
+        self._len = int(math.ceil(len(iterable) / float(chunk_size)))
+        self.itr = iter(iterable)
+        self.chunk_size = chunk_size
+
+    def __len__(self):
+        return self._len
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        chunk = []
+        try:
+            for _ in range(self.chunk_size):
+                chunk.append(next(self.itr))
+        except StopIteration as e:
+            if len(chunk) == 0:
+                raise e
+        return chunk
+
+
 class ShardedIterator(object):
     """A sharded wrapper around an iterable, padded to length.
 
diff --git a/fairseq/distributed_utils.py b/fairseq/distributed_utils.py
index a14e522a5..0ff0d0e5c 100644
--- a/fairseq/distributed_utils.py
+++ b/fairseq/distributed_utils.py
@@ -7,7 +7,9 @@
 
 import pickle
 
-import torch.distributed
+import torch
+from torch import distributed
+from torch.distributed import group
 
 from fairseq import utils
 
@@ -16,22 +18,39 @@ def is_master(args):
     return args.distributed_rank == 0
 
 
+_use_c10d = [None]
+
+
 def distributed_init(args):
     if args.distributed_world_size == 1:
         raise ValueError('Cannot initialize distributed with distributed_world_size=1')
 
+    if _use_c10d[0] is None:
+        _use_c10d[0] = not args.no_c10d
+
+    if _use_c10d[0] and not hasattr(torch.nn.parallel, '_DistributedDataParallelC10d'):
+        _use_c10d[0] = False
+        print('WARNING: cannot find DistributedDataParallelC10d, '
+              'falling back to standard DistributedDataParallel')
+
     print('| distributed init (rank {}): {}'.format(
         args.distributed_rank, args.distributed_init_method), flush=True)
-    if args.distributed_init_method.startswith('tcp://'):
-        torch.distributed.init_process_group(
-            backend=args.distributed_backend, init_method=args.distributed_init_method,
-            world_size=args.distributed_world_size, rank=args.distributed_rank)
-    else:
-        torch.distributed.init_process_group(
-            backend=args.distributed_backend, init_method=args.distributed_init_method,
-            world_size=args.distributed_world_size)
 
-    args.distributed_rank = torch.distributed.get_rank()
+    if _use_c10d[0]:
+        distributed.c10d.init_process_group(
+            backend=args.distributed_backend,
+            init_method=args.distributed_init_method,
+            world_size=args.distributed_world_size,
+            rank=args.distributed_rank,
+        )
+    else:
+        distributed.init_process_group(
+            backend=args.distributed_backend,
+            init_method=args.distributed_init_method,
+            world_size=args.distributed_world_size,
+            rank=args.distributed_rank,
+        )
+
     if not is_master(args):
         suppress_output()
 
@@ -52,35 +71,77 @@ def suppress_output():
     __builtin__.print = print
 
 
-def all_gather_list(data, max_size=16384):
-    """Gathers arbitrary data from all nodes into a list."""
-    world_size = torch.distributed.get_world_size()
-    if not hasattr(all_gather_list, '_in_buffer') or \
-            max_size != all_gather_list._in_buffer.size():
-        all_gather_list._in_buffer = torch.cuda.ByteTensor(max_size)
-        all_gather_list._out_buffers = [
-            torch.cuda.ByteTensor(max_size)
-            for i in range(world_size)
-        ]
-    in_buffer = all_gather_list._in_buffer
-    out_buffers = all_gather_list._out_buffers
+def get_rank():
+    if _use_c10d[0]:
+        return distributed.c10d.get_rank()
+    else:
+        return distributed.get_rank()
+
+
+def get_world_size():
+    if _use_c10d[0]:
+        return distributed.c10d.get_world_size()
+    else:
+        return distributed.get_world_size()
+
+
+def get_default_group():
+    if _use_c10d[0]:
+        return distributed.c10d.group.WORLD
+    else:
+        return distributed.group.WORLD
+
+
+def all_reduce(tensor, group=None):
+    if group is None:
+        group = get_default_group()
+    if _use_c10d[0]:
+        return distributed.c10d.all_reduce(tensor, group=group)
+    else:
+        return distributed.all_reduce(tensor, group=group)
+
+
+def all_gather_list(data, group=None, max_size=16384):
+    """Gathers arbitrary data from all nodes into a list.
+
+    Similar to :func:`~torch.distributed.all_gather` but for arbitrary Python
+    data. Note that *data* must be picklable.
+
+    Args:
+        data (Any): data from the local worker to be gathered on other workers
+        group (optional): group of the collective
+        max_size (int, optional): maximum size of the data to be gathered
+            across workers
+    """
+    rank = get_rank()
+    world_size = get_world_size()
+
+    buffer_size = max_size * world_size
+    if not hasattr(all_gather_list, '_buffer') or \
+            all_gather_list._buffer.numel() < buffer_size:
+        all_gather_list._buffer = torch.cuda.ByteTensor(buffer_size)
+    buffer = all_gather_list._buffer
+    buffer.zero_()
 
     enc = pickle.dumps(data)
     enc_size = len(enc)
     if enc_size + 2 > max_size:
         raise ValueError('encoded data exceeds max_size: {}'.format(enc_size + 2))
     assert max_size < 255*256
-    in_buffer[0] = enc_size // 255  # this encoding works for max_size < 65k
-    in_buffer[1] = enc_size % 255
-    in_buffer[2:enc_size+2] = torch.ByteTensor(list(enc))
 
-    torch.distributed.all_gather(out_buffers, in_buffer.cuda())
+    buffer_rank = buffer[rank * max_size : (rank + 1) * max_size]
+    buffer_rank[0] = enc_size // 255  # this encoding works for max_size < 65k
+    buffer_rank[1] = enc_size % 255
+    buffer_rank[2:enc_size+2] = torch.ByteTensor(list(enc))
+
+    all_reduce(buffer, group=group)
 
     result = []
     for i in range(world_size):
-        out_buffer = out_buffers[i]
+        out_buffer = buffer[i * max_size : (i + 1) * max_size]
         size = (255 * utils.item(out_buffer[0])) + utils.item(out_buffer[1])
-        result.append(
-            pickle.loads(bytes(out_buffer[2:size+2].tolist()))
-        )
+        if size > 0:
+            result.append(
+                pickle.loads(bytes(out_buffer[2:size+2].tolist()))
+            )
     return result
diff --git a/fairseq/fp16_trainer.py b/fairseq/fp16_trainer.py
deleted file mode 100644
index 5ae88ab1e..000000000
--- a/fairseq/fp16_trainer.py
+++ /dev/null
@@ -1,154 +0,0 @@
-# Copyright (c) 2017-present, Facebook, Inc.
-# All rights reserved.
-#
-# This source code is licensed under the license found in the LICENSE file in
-# the root directory of this source tree. An additional grant of patent rights
-# can be found in the PATENTS file in the same directory.
-
-"""
-Train a network on multiple GPUs.
-"""
-
-import torch
-
-from fairseq import optim, utils
-from fairseq.meters import AverageMeter
-from fairseq.optim import lr_scheduler
-from fairseq.trainer import Trainer
-
-
-class DynamicLossScaler:
-
-    def __init__(self, init_scale=2.**15, scale_factor=2., scale_window=2000):
-        self.loss_scale = init_scale
-        self.scale_factor = scale_factor
-        self.scale_window = scale_window
-        self._iter = 0
-        self._last_overflow_iter = -1
-
-    def update_scale(self, overflow):
-        if overflow:
-            self.loss_scale /= self.scale_factor
-            self._last_overflow_iter = self._iter
-        elif (self._iter - self._last_overflow_iter) % self.scale_window == 0:
-            self.loss_scale *= self.scale_factor
-        self._iter += 1
-
-    @staticmethod
-    def has_overflow(grad_norm):
-        # detect inf and nan
-        if grad_norm == float('inf') or grad_norm != grad_norm:
-            return True
-        return False
-
-
-class FP16Trainer(Trainer):
-    """Modified trainer for FP16.
-
-    We maintain two copies of the model's parameters, both in FP16 and FP32.
-    We do forward/backward with FP16 and compute the loss + optimize with FP32.
-    """
-
-    def __init__(self, args, task, model, criterion):
-        super().__init__(args, task, model, criterion)
-
-        # convert model to FP16 (but keep criterion FP32)
-        self.model.half()
-
-        # dynamically scale loss to reduce overflow
-        self.scaler = DynamicLossScaler(init_scale=2.**7)
-        self.meters['loss_scale'] = AverageMeter()
-
-    def _build_optimizer(self):
-        # create FP32 copy of parameters and grads
-        params = [p for p in self.model.parameters() if p.requires_grad]
-        total_param_size = sum(p.data.numel() for p in params)
-        self.fp32_params = params[0].new(0).float().new(total_param_size)
-        offset = 0
-        for p in params:
-            numel = p.data.numel()
-            self.fp32_params[offset:offset+numel].copy_(p.data.view(-1))
-            offset += numel
-        self.fp32_params = torch.nn.Parameter(self.fp32_params)
-        self.fp32_params.grad = self.fp32_params.data.new(total_param_size)
-
-        # create optimizer using the copied FP32 params
-        self._optimizer = optim.build_optimizer(self.args, [self.fp32_params])
-        self.lr_scheduler = lr_scheduler.build_lr_scheduler(self.args, self.optimizer)
-
-    def save_checkpoint(self, filename, extra_state):
-        """Save all training state in a checkpoint file."""
-        extra_state['loss_scale'] = self.scaler.loss_scale
-        super().save_checkpoint(filename, extra_state)
-
-    def load_checkpoint(self, filename, reset_optimizer=False, reset_lr_scheduler=False, optimizer_overrides=None):
-        """Load all training state from a checkpoint file."""
-        extra_state = super().load_checkpoint(filename, reset_optimizer, reset_lr_scheduler, optimizer_overrides)
-        if extra_state is not None and 'loss_scale' in extra_state:
-            self.scaler.loss_scale = extra_state['loss_scale']
-        return extra_state
-
-    def zero_grad(self):
-        # zero both the FP16 and FP32 grads
-        self.model.zero_grad()      # FP16
-        self.optimizer.zero_grad()  # FP32
-
-    def _backward(self, loss):
-        self.meters['loss_scale'].reset()
-        self.meters['loss_scale'].update(self.scaler.loss_scale)
-        if loss is not None:
-            # dynamically rescale loss to stay in FP16 range
-            loss = loss * self.scaler.loss_scale
-        return super()._backward(loss)
-
-    def _all_reduce_and_rescale(self, grad_denom):
-        # undo effect of dynamic loss scaling on gradients
-        grad_denom *= self.scaler.loss_scale
-
-        if self.args.distributed_world_size > 1:
-            # flatten grads into a single buffer
-            flat_grads = self._flat_grads = self._get_flat_grads(self._flat_grads)
-
-            # scale gradients to avoid overflow in all-reduce
-            flat_grads.div_(self.args.distributed_world_size)
-            grad_denom /= self.args.distributed_world_size
-
-            # all-reduce flat grads
-            torch.distributed.all_reduce(flat_grads)
-
-            # copy grads back to FP32
-            self.fp32_params.grad.data.copy_(flat_grads)
-        else:
-            # single worker: copy grads directly to FP32
-            self._get_flat_grads(out=self.fp32_params.grad.data)
-
-        # rescale and clip grads
-        self.fp32_params.grad.data.div_(grad_denom)
-        grad_norm = utils.clip_grad_norm_(self.fp32_params.grad.data, self.args.clip_norm)
-
-        # detect overflow and adjust loss scale
-        overflow = DynamicLossScaler.has_overflow(grad_norm)
-        self.scaler.update_scale(overflow)
-        if overflow:
-            if self.scaler.loss_scale <= self.args.min_loss_scale:
-                raise Exception((
-                    'Minimum loss scale reached ({}). Your loss is probably exploding. '
-                    'Try lowering the learning rate, using gradient clipping or '
-                    'increasing the batch size.'
-                ).format(self.args.min_loss_scale))
-            raise OverflowError('setting loss scale to: ' + str(self.scaler.loss_scale))
-
-        return grad_norm
-
-    def _opt(self):
-        # take an optimization step using the FP32 params and grads
-        super()._opt()
-
-        # copy FP32 params back into FP16 model
-        offset = 0
-        for p in self.model.parameters():
-            if not p.requires_grad:
-                continue
-            numel = p.data.numel()
-            p.data.copy_(self.fp32_params.data[offset:offset+numel].view_as(p.data))
-            offset += numel
diff --git a/fairseq/models/__init__.py b/fairseq/models/__init__.py
index b019fbb52..3a8646075 100644
--- a/fairseq/models/__init__.py
+++ b/fairseq/models/__init__.py
@@ -15,6 +15,7 @@ from .fairseq_incremental_decoder import FairseqIncrementalDecoder  # noqa: F401
 from .fairseq_model import BaseFairseqModel, FairseqModel, FairseqLanguageModel  # noqa: F401
 
 from .composite_encoder import CompositeEncoder  # noqa: F401
+from .distributed_fairseq_model import DistributedFairseqModel  # noqa: F401
 
 
 MODEL_REGISTRY = {}
diff --git a/fairseq/models/distributed_fairseq_model.py b/fairseq/models/distributed_fairseq_model.py
new file mode 100644
index 000000000..bf4c911d3
--- /dev/null
+++ b/fairseq/models/distributed_fairseq_model.py
@@ -0,0 +1,62 @@
+# Copyright (c) 2017-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under the license found in the LICENSE file in
+# the root directory of this source tree. An additional grant of patent rights
+# can be found in the PATENTS file in the same directory.
+
+from torch.distributed import c10d
+from torch.nn import parallel
+
+from . import BaseFairseqModel
+
+
+class DistributedFairseqModel(BaseFairseqModel):
+    """
+    A wrapper around a :class:`BaseFairseqModel` instance that adds support for
+    distributed training.
+
+    Anytime a method or attribute is called on this class we first try to
+    forward it to the underlying DistributedDataParallel instance, otherwise we
+    forward it to the original :class:`BaseFairseqModel` instance.
+
+    Args:
+        args (argparse.Namespace): fairseq args
+        model (BaseFairseqModel): model to wrap
+    """
+
+    def __init__(self, args, model):
+        super().__init__()
+        assert isinstance(model, BaseFairseqModel)
+        if args.no_c10d:
+            self.ddp_model = parallel.DistributedDataParallel(
+                module=model,
+                device_ids=[args.device_id],
+                output_device=args.device_id,
+                broadcast_buffers=False,
+            )
+        else:
+            self.ddp_model = parallel._DistributedDataParallelC10d(
+                module=model,
+                device_ids=[args.device_id],
+                output_device=args.device_id,
+                broadcast_buffers=False,
+                bucket_cap_mb=args.c10d_bucket_cap_mb,
+            )
+
+    def __call__(self, *args, **kwargs):
+        return self.ddp_model(*args, **kwargs)
+
+    def forward(self, *args, **kwargs):
+        return self.ddp_model.forward(*args, **kwargs)
+
+    def __getattr__(self, name):
+        try:
+            return super().__getattr__(name)
+        except AttributeError:
+            pass
+        try:
+            return self.ddp_model.__getattr__(name)
+        except AttributeError:
+            pass
+        return self.ddp_model.module.__getattr__(name)
diff --git a/fairseq/optim/__init__.py b/fairseq/optim/__init__.py
index 91934dce6..2424d7282 100644
--- a/fairseq/optim/__init__.py
+++ b/fairseq/optim/__init__.py
@@ -9,6 +9,7 @@ import importlib
 import os
 
 from .fairseq_optimizer import FairseqOptimizer
+from .fp16_optimizer import FP16Optimizer
 
 
 OPTIMIZER_REGISTRY = {}
@@ -16,7 +17,7 @@ OPTIMIZER_CLASS_NAMES = set()
 
 
 def build_optimizer(args, params):
-    params = filter(lambda p: p.requires_grad, params)
+    params = list(filter(lambda p: p.requires_grad, params))
     return OPTIMIZER_REGISTRY[args.optimizer](args, params)
 
 
diff --git a/fairseq/optim/fairseq_optimizer.py b/fairseq/optim/fairseq_optimizer.py
index 91760582a..984956d45 100644
--- a/fairseq/optim/fairseq_optimizer.py
+++ b/fairseq/optim/fairseq_optimizer.py
@@ -5,7 +5,9 @@
 # the root directory of this source tree. An additional grant of patent rights
 # can be found in the PATENTS file in the same directory.
 
-import torch.optim
+import math
+
+import torch
 
 
 class FairseqOptimizer(object):
@@ -13,7 +15,7 @@ class FairseqOptimizer(object):
     def __init__(self, args, params):
         super().__init__()
         self.args = args
-        self.params = params
+        self.params = list(params)
 
     @staticmethod
     def add_args(parser):
@@ -67,10 +69,25 @@ class FairseqOptimizer(object):
             for group in self.optimizer.param_groups:
                 group.update(optimizer_overrides)
 
+    def backward(self, loss):
+        loss.backward()
+
+    def multiply_grads(self, c):
+        """Multiplies grads by a constant ``c``."""
+        for p in self.params:
+            p.grad.data.mul_(c)
+
+    def clip_grad_norm(self, max_norm):
+        """Clips gradient norm."""
+        if max_norm > 0:
+            return torch.nn.utils.clip_grad_norm_(self.params, max_norm)
+        else:
+            return math.sqrt(sum(p.grad.data.norm()**2 for p in self.params))
+
     def step(self, closure=None):
         """Performs a single optimization step."""
-        return self.optimizer.step(closure)
+        self.optimizer.step(closure)
 
     def zero_grad(self):
         """Clears the gradients of all optimized parameters."""
-        return self.optimizer.zero_grad()
+        self.optimizer.zero_grad()
diff --git a/fairseq/optim/fp16_optimizer.py b/fairseq/optim/fp16_optimizer.py
new file mode 100644
index 000000000..a7293d0cd
--- /dev/null
+++ b/fairseq/optim/fp16_optimizer.py
@@ -0,0 +1,164 @@
+# Copyright (c) 2017-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under the license found in the LICENSE file in
+# the root directory of this source tree. An additional grant of patent rights
+# can be found in the PATENTS file in the same directory.
+
+import torch
+
+from fairseq import optim, utils
+
+
+class DynamicLossScaler:
+
+    def __init__(self, init_scale=2.**15, scale_factor=2., scale_window=2000):
+        self.loss_scale = init_scale
+        self.scale_factor = scale_factor
+        self.scale_window = scale_window
+        self._iter = 0
+        self._last_overflow_iter = -1
+
+    def update_scale(self, overflow):
+        if overflow:
+            self.loss_scale /= self.scale_factor
+            self._last_overflow_iter = self._iter
+        elif (self._iter - self._last_overflow_iter) % self.scale_window == 0:
+            self.loss_scale *= self.scale_factor
+        self._iter += 1
+
+    @staticmethod
+    def has_overflow(grad_norm):
+        # detect inf and nan
+        if grad_norm == float('inf') or grad_norm != grad_norm:
+            return True
+        return False
+
+
+class FP16Optimizer(optim.FairseqOptimizer):
+
+    def __init__(self, args, params, fp32_optimizer, fp32_params):
+        super().__init__(args, params)
+        self.fp32_optimizer = fp32_optimizer
+        self.fp32_params = fp32_params
+        self.scaler = DynamicLossScaler(
+            init_scale=2.**7,
+            scale_window=(2**14 / args.distributed_world_size),
+        )
+
+    @staticmethod
+    def build_optimizer(args, params):
+        # create FP32 copy of parameters and grads
+        total_param_size = sum(p.data.numel() for p in params)
+        fp32_params = params[0].new(0).float().new(total_param_size)
+        offset = 0
+        for p in params:
+            numel = p.data.numel()
+            fp32_params[offset:offset+numel].copy_(p.data.view(-1))
+            offset += numel
+        fp32_params = torch.nn.Parameter(fp32_params)
+        fp32_params.grad = fp32_params.data.new(total_param_size)
+
+        fp32_optimizer = optim.build_optimizer(args, [fp32_params])
+        return FP16Optimizer(args, params, fp32_optimizer, fp32_params)
+
+    @property
+    def optimizer(self):
+        return self.fp32_optimizer.optimizer
+
+    @property
+    def optimizer_config(self):
+        return self.fp32_optimizer.optimizer_config
+
+    def get_lr(self):
+        return self.fp32_optimizer.get_lr()
+
+    def set_lr(self, lr):
+        self.fp32_optimizer.set_lr(lr)
+
+    def state_dict(self):
+        """Return the optimizer's state dict."""
+        state_dict = self.fp32_optimizer.state_dict()
+        state_dict['loss_scale'] = self.scaler.loss_scale
+        return state_dict
+
+    def load_state_dict(self, state_dict, optimizer_overrides=None):
+        """Load an optimizer state dict.
+
+        In general we should prefer the configuration of the existing optimizer
+        instance (e.g., learning rate) over that found in the state_dict. This
+        allows us to resume training from a checkpoint using a new set of
+        optimizer args.
+        """
+        if 'loss_scale' in state_dict:
+            self.scaler.loss_scale = state_dict['loss_scale']
+        self.fp32_optimizer.load_state_dict(state_dict, optimizer_overrides)
+
+    def backward(self, loss):
+        loss = loss * self.scaler.loss_scale
+        loss.backward()
+        self._needs_sync = True
+
+    def _sync_fp16_grads_to_fp32(self, multiply_grads=1.):
+        if self._needs_sync:
+            # copy FP16 grads to FP32
+            offset = 0
+            for p in self.params:
+                if not p.requires_grad:
+                    continue
+                numel = p.grad.data.numel()
+                self.fp32_params.grad.data[offset:offset+numel].copy_(p.grad.data.view(-1))
+                offset += numel
+
+            # correct for dynamic loss scaler
+            self.fp32_params.grad.data.mul_(multiply_grads / self.scaler.loss_scale)
+
+            self._needs_sync = False
+
+    def multiply_grads(self, c):
+        """Multiplies grads by a constant ``c``."""
+        if self._needs_sync:
+            self._sync_fp16_grads_to_fp32(c)
+        else:
+            self.fp32_params.grad.data.mul_(c)
+
+    def clip_grad_norm(self, max_norm):
+        """Clips gradient norm and updates dynamic loss scaler."""
+        self._sync_fp16_grads_to_fp32()
+        grad_norm = utils.clip_grad_norm_(self.fp32_params.grad.data, max_norm)
+
+        # detect overflow and adjust loss scale
+        overflow = DynamicLossScaler.has_overflow(grad_norm)
+        self.scaler.update_scale(overflow)
+        if overflow:
+            if self.scaler.loss_scale <= self.args.min_loss_scale:
+                raise Exception((
+                    'Minimum loss scale reached ({}). Your loss is probably exploding. '
+                    'Try lowering the learning rate, using gradient clipping or '
+                    'increasing the batch size.'
+                ).format(self.args.min_loss_scale))
+            raise OverflowError('setting loss scale to: ' + str(self.scaler.loss_scale))
+        return grad_norm
+
+    def step(self, closure=None):
+        """Performs a single optimization step."""
+        self._sync_fp16_grads_to_fp32()
+        self.fp32_optimizer.step(closure)
+
+        # copy FP32 params back into FP16 model
+        offset = 0
+        for p in self.params:
+            if not p.requires_grad:
+                continue
+            numel = p.data.numel()
+            p.data.copy_(self.fp32_params.data[offset:offset+numel].view_as(p.data))
+            offset += numel
+
+    def zero_grad(self):
+        """Clears the gradients of all optimized parameters."""
+        self.fp32_optimizer.zero_grad()
+        for p in self.params:
+            if p.grad is not None:
+                p.grad.detach_()
+                p.grad.zero_()
+        self._needs_sync = False
diff --git a/fairseq/options.py b/fairseq/options.py
index 70fac7ca7..5ae694826 100644
--- a/fairseq/options.py
+++ b/fairseq/options.py
@@ -183,6 +183,10 @@ def add_distributed_training_args(parser):
                        help='port number (not required if using --distributed-init-method)')
     group.add_argument('--device-id', default=0, type=int,
                        help='which GPU to use (usually configured automatically)')
+    group.add_argument('--no-c10d', action='store_true',
+                       help='don\'t use c10d distributed backend')
+    group.add_argument('--c10d-bucket-cap-mb', default=150, metavar='MB',
+                       help='bucket size for c10d backend')
     return group
 
 
diff --git a/fairseq/trainer.py b/fairseq/trainer.py
index 56ba63420..07c5efcfa 100644
--- a/fairseq/trainer.py
+++ b/fairseq/trainer.py
@@ -15,7 +15,7 @@ from itertools import chain
 
 import torch
 
-from fairseq import distributed_utils, optim, utils
+from fairseq import distributed_utils, models, optim, utils
 from fairseq.meters import AverageMeter, StopwatchMeter, TimeMeter
 from fairseq.optim import lr_scheduler
 
@@ -23,22 +23,27 @@ from fairseq.optim import lr_scheduler
 class Trainer(object):
     """Main class for data parallel training.
 
-    This class supports data parallel training, where multiple workers each
-    have a full model replica and gradients are accumulated synchronously via
-    torch.distributed.all_reduce.
+    This class supports synchronous distributed data parallel training,
+    where multiple workers each have a full model replica and gradients
+    are accumulated across workers before each update. We use
+    :class:`~torch.nn.parallel.DistributedDataParallel` to handle
+    communication of the gradients across workers.
     """
 
-    def __init__(self, args, task, model, criterion):
+    def __init__(self, args, task, model, criterion, dummy_batch):
 
         if not torch.cuda.is_available():
             raise NotImplementedError('Training on CPU is not supported')
 
         self.args = args
+        self.task = task
 
         # copy model and criterion to current device
-        self.task = task
-        self.model = model.cuda()
         self.criterion = criterion.cuda()
+        if args.fp16:
+            self._model = model.half().cuda()
+        else:
+            self._model = model.cuda()
 
         # initialize meters
         self.meters = OrderedDict()
@@ -53,14 +58,27 @@ class Trainer(object):
         self.meters['gnorm'] = AverageMeter()  # gradient norm
         self.meters['clip'] = AverageMeter()   # % of updates clipped
         self.meters['oom'] = AverageMeter()    # out of memory
+        if args.fp16:
+            self.meters['loss_scale'] = AverageMeter()  # dynamic loss scale
         self.meters['wall'] = TimeMeter()      # wall time in seconds
         self.meters['train_wall'] = StopwatchMeter()  # train wall time in seconds
 
-        self._buffered_stats = defaultdict(lambda: [])
-        self._flat_grads = None
+        self._dummy_batch = dummy_batch
         self._num_updates = 0
         self._optim_history = None
         self._optimizer = None
+        self._wrapped_model = None
+
+    @property
+    def model(self):
+        if self._wrapped_model is None:
+            if self.args.distributed_world_size > 1:
+                self._wrapped_model = models.DistributedFairseqModel(
+                    self.args, self._model,
+                )
+            else:
+                self._wrapped_model = self._model
+        return self._wrapped_model
 
     @property
     def optimizer(self):
@@ -69,7 +87,17 @@ class Trainer(object):
         return self._optimizer
 
     def _build_optimizer(self):
-        self._optimizer = optim.build_optimizer(self.args, self.model.parameters())
+        if self.args.fp16:
+            if torch.cuda.get_device_capability(0)[0] < 7:
+                print('| WARNING: your device does NOT support faster training with --fp16, '
+                      'please switch to FP32 which is likely to be faster')
+            params = list(filter(lambda p: p.requires_grad, self.model.parameters()))
+            self._optimizer = optim.FP16Optimizer.build_optimizer(self.args, params)
+        else:
+            if torch.cuda.get_device_capability(0)[0] >= 7:
+                print('| NOTICE: your device may support faster training with --fp16')
+            self._optimizer = optim.build_optimizer(self.args, self.model.parameters())
+
         self.lr_scheduler = lr_scheduler.build_lr_scheduler(self.args, self._optimizer)
 
     def save_checkpoint(self, filename, extra_state):
@@ -77,31 +105,27 @@ class Trainer(object):
         if distributed_utils.is_master(self.args):  # only save one checkpoint
             extra_state['train_meters'] = self.meters
             utils.save_state(
-                filename, self.args, self.model, self.criterion, self.optimizer,
+                filename, self.args, self.get_model(), self.criterion, self.optimizer,
                 self.lr_scheduler, self._num_updates, self._optim_history, extra_state,
             )
 
     def load_checkpoint(self, filename, reset_optimizer=False, reset_lr_scheduler=False, optimizer_overrides=None):
         """Load all training state from a checkpoint file."""
         extra_state, self._optim_history, last_optim_state = \
-            utils.load_model_state(filename, self.model)
-
+            utils.load_model_state(filename, self.get_model())
         if last_optim_state is not None and not reset_optimizer:
             # rebuild optimizer after loading model, since params may have changed
             self._build_optimizer()
 
             # only reload optimizer and lr_scheduler if they match
             last_optim = self._optim_history[-1]
-
             assert last_optim['criterion_name'] == self.criterion.__class__.__name__, \
                 'criterion does not match; please reset the optimizer (--reset-optimizer)'
-
             assert last_optim['optimizer_name'] == self.optimizer.__class__.__name__, \
                 'optimizer does not match; please reset the optimizer (--reset-optimizer)'
 
             if not reset_lr_scheduler:
                 self.lr_scheduler.load_state_dict(last_optim['lr_scheduler_state'])
-
             self.optimizer.load_state_dict(last_optim_state, optimizer_overrides)
 
             self._num_updates = last_optim['num_updates']
@@ -117,7 +141,7 @@ class Trainer(object):
 
         return extra_state
 
-    def train_step(self, sample, update_params=True, dummy_batch=False):
+    def train_step(self, samples, dummy_batch=False):
         """Do forward, backward and parameter update."""
         # Set seed based on args.seed and the update number so that we get
         # reproducible results when resuming from checkpoints
@@ -125,230 +149,164 @@ class Trainer(object):
         torch.manual_seed(seed)
         torch.cuda.manual_seed(seed)
 
+        self.model.train()
+        self.zero_grad()
+
         if not dummy_batch:
             self.meters['train_wall'].start()
 
         # forward and backward pass
-        sample = self._prepare_sample(sample)
-        loss, sample_size, logging_output, oom_fwd = self._forward(sample)
-        oom_bwd = self._backward(loss)
+        logging_outputs, sample_sizes, ooms = [], [], 0
+        for i, sample in enumerate(samples):
+            sample = self._prepare_sample(sample)
+            if sample is None:
+                # when sample is None, run forward/backward on a dummy batch
+                # and ignore the resulting gradients
+                sample = self._prepare_sample(self._dummy_batch)
+                ignore_grad = True
+            else:
+                ignore_grad = False
 
-        # buffer stats and logging outputs
-        self._buffered_stats['sample_sizes'].append(sample_size)
-        self._buffered_stats['logging_outputs'].append(logging_output)
-        self._buffered_stats['ooms_fwd'].append(oom_fwd)
-        self._buffered_stats['ooms_bwd'].append(oom_bwd)
+            try:
+                # forward
+                loss, sample_size, logging_output = self.task.get_loss(
+                    self.model, self.criterion, sample,
+                )
+                if ignore_grad:
+                    loss *= 0
 
-        # update parameters
-        if update_params:
-            agg_logging_output = self._update_params()
-        else:
-            agg_logging_output = None  # buffering updates
+                if self.args.distributed_world_size > 1:
+                    # only all-reduce gradients in the last backwards pass
+                    if i < len(samples) - 1:
+                        self.model.need_reduction = False
+                    else:
+                        self.model.need_reduction = True
 
-        if not dummy_batch:
-            self.meters['train_wall'].stop()
+                # backward
+                self.optimizer.backward(loss)
 
-        return agg_logging_output
+                if not ignore_grad:
+                    logging_outputs.append(logging_output)
+                    sample_sizes.append(sample_size)
+            except RuntimeError as e:
+                if 'out of memory' in str(e):
+                    print('| WARNING: ran out of memory, skipping batch')
+                    ooms += 1
+                    self.zero_grad()
+                else:
+                    raise e
+
+        if dummy_batch:
+            return None
 
-    def _update_params(self):
         # gather logging outputs from all replicas
-        sample_sizes = self._buffered_stats['sample_sizes']
-        logging_outputs = self._buffered_stats['logging_outputs']
-        ooms_fwd = self._buffered_stats['ooms_fwd']
-        ooms_bwd = self._buffered_stats['ooms_bwd']
         if self.args.distributed_world_size > 1:
-            sample_sizes, logging_outputs, ooms_fwd, ooms_bwd = map(
-                lambda l: list(chain.from_iterable(l)),
-                zip(*distributed_utils.all_gather_list(
-                    (sample_sizes, logging_outputs, ooms_fwd, ooms_bwd)
-                ))
-            )
-        ooms_fwd = sum(ooms_fwd)
-        ooms_bwd = sum(ooms_bwd)
+            logging_outputs, sample_sizes, ooms = zip(*distributed_utils.all_gather_list(
+                [logging_outputs, sample_sizes, ooms],
+            ))
+            logging_outputs = list(chain.from_iterable(logging_outputs))
+            sample_sizes = list(chain.from_iterable(sample_sizes))
+            ooms = sum(ooms)
 
-        if ooms_fwd == self.args.distributed_world_size:
-            print('| WARNING: OOM in all workers, skipping batch')
+        if ooms == self.args.distributed_world_size:
+            print('| WARNING: OOM in all workers, skipping update')
             self.zero_grad()
             return None
 
-        # aggregate stats and logging outputs
-        ntokens = sum(log.get('ntokens', 0) for log in logging_outputs)
-        nsentences = sum(log.get('nsentences', 0) for log in logging_outputs)
-        agg_logging_output = self.criterion.__class__.aggregate_logging_outputs(logging_outputs)
-        grad_denom = self.criterion.__class__.grad_denom(sample_sizes)
+        # aggregate logging outputs and sample sizes
+        logging_output = self.criterion.__class__.aggregate_logging_outputs(logging_outputs)
+        sample_size = self.criterion.__class__.grad_denom(sample_sizes)
+
+        if not all(k in logging_output for k in ['ntokens', 'nsentences']):
+            raise Exception((
+                'Please update the {}.aggregate_logging_outputs() method to '
+                'return ntokens and nsentences'
+            ).format(self.criterion.__class__.__name__))
 
         try:
-            # all-reduce and rescale gradients, then take an optimization step
-            grad_norm = self._all_reduce_and_rescale(grad_denom)
-            self._opt()
+            # normalize grads by sample size
+            self.optimizer.multiply_grads(self.args.distributed_world_size / float(sample_size))
+
+            # clip grads
+            grad_norm = self.optimizer.clip_grad_norm(self.args.clip_norm)
+
+            # take an optimization step
+            self.optimizer.step()
+            self._num_updates += 1
+
+            # update learning rate
+            self.lr_scheduler.step_update(self._num_updates)
 
             # update meters
+            ntokens = logging_output.get('ntokens', 0)
+            nsentences = logging_output.get('nsentences', 0)
             self.meters['wps'].update(ntokens)
             self.meters['ups'].update(1.)
             self.meters['wpb'].update(ntokens)
             self.meters['bsz'].update(nsentences)
-            if grad_norm is not None:
-                self.meters['gnorm'].update(grad_norm)
-                self.meters['clip'].update(1. if grad_norm > self.args.clip_norm else 0.)
-            self.meters['oom'].update(ooms_fwd + ooms_bwd)
-
-            # update loss meters for training
-            if 'loss' in agg_logging_output:
-                self.meters['train_loss'].update(agg_logging_output['loss'], grad_denom)
-            # criterions can optionally log the NLL loss too
-            if 'nll_loss' in agg_logging_output:
-                self.meters['train_nll_loss'].update(agg_logging_output['nll_loss'], ntokens)
+            self.meters['gnorm'].update(grad_norm)
+            self.meters['clip'].update(
+                1. if grad_norm > self.args.clip_norm and self.args.clip_norm > 0 else 0.
+            )
+            self.meters['oom'].update(ooms)
+            self.meters['train_loss'].update(logging_output.get('loss', 0), sample_size)
+            self.meters['train_nll_loss'].update(logging_output.get('nll_loss', 0), ntokens)
         except OverflowError as e:
-            self.zero_grad()
             print('| WARNING: overflow detected, ' + str(e))
+            self.zero_grad()
+            logging_output = None
 
-        self.clear_buffered_stats()
+        if self.args.fp16:
+            self.meters['loss_scale'].reset()
+            self.meters['loss_scale'].update(self.optimizer.scaler.loss_scale)
 
-        return agg_logging_output
+        self.meters['train_wall'].stop()
 
-    def _forward(self, sample, eval=False):
-        loss = None
-        sample_size = 0
-        logging_output = {
-            'ntokens': sample['ntokens'] if sample is not None else 0,
-            'nsentences': sample['target'].size(0) if sample is not None else 0,
-        }
-        oom = 0
-        try:
-            # prepare model and optimizer
-            if eval:
-                self.model.eval()
-            else:
-                self.model.train()
-
-            if sample is not None:
-                with torch.no_grad() if eval else contextlib.ExitStack():
-                    # calculate loss and sample size
-                    loss, sample_size, logging_output_ = self.task.get_loss(self.model, self.criterion, sample)
-                    logging_output.update(logging_output_)
-        except RuntimeError as e:
-            if not eval and 'out of memory' in str(e):
-                print('| WARNING: ran out of memory, skipping batch')
-                oom = 1
-                loss = None
-            else:
-                raise e
-        return loss, sample_size, logging_output, oom
-
-    def _backward(self, loss):
-        oom = 0
-        if loss is not None:
-            try:
-                # backward pass
-                loss.backward()
-            except RuntimeError as e:
-                if 'out of memory' in str(e):
-                    print('| WARNING: ran out of memory, skipping batch')
-                    oom = 1
-                    self.zero_grad()
-                else:
-                    raise e
-        return oom
-
-    def _all_reduce_and_rescale(self, grad_denom):
-        # flatten grads into a single buffer and all-reduce
-        flat_grads = self._flat_grads = self._get_flat_grads(self._flat_grads)
-        if self.args.distributed_world_size > 1:
-            torch.distributed.all_reduce(flat_grads)
-
-        # rescale and clip gradients
-        flat_grads.div_(grad_denom)
-        grad_norm = utils.clip_grad_norm_(flat_grads, self.args.clip_norm)
-
-        # copy grads back into model parameters
-        self._set_flat_grads(flat_grads)
-
-        return grad_norm
-
-    def _get_grads(self):
-        grads = []
-        for name, p in self.model.named_parameters():
-            if not p.requires_grad:
-                continue
-            if p.grad is None:
-                print('WARNING: model parameter did not receive gradient: ' + name + '. '
-                      'Check that you\'re using the param in the forward pass or set requires_grad=False')
-                grads.append(p.new_zeros(p.shape))
-            else:
-                grads.append(p.grad.data)
-        return grads
-
-    def _get_flat_grads(self, out=None):
-        grads = self._get_grads()
-        if out is None:
-            grads_size = sum(g.numel() for g in grads)
-            out = grads[0].new(grads_size).zero_()
-        offset = 0
-        for g in grads:
-            numel = g.numel()
-            out[offset:offset+numel].copy_(g.view(-1))
-            offset += numel
-        return out[:offset]
-
-    def _set_flat_grads(self, new_grads):
-        grads = self._get_grads()
-        offset = 0
-        for g in grads:
-            numel = g.numel()
-            g.copy_(new_grads[offset:offset+numel].view_as(g))
-            offset += numel
-
-    def _opt(self):
-        # take an optimization step
-        self.optimizer.step()
-        self.zero_grad()
-        self._num_updates += 1
-
-        # update learning rate
-        self.lr_scheduler.step_update(self._num_updates)
+        return logging_output
 
     def valid_step(self, sample):
         """Do forward pass in evaluation mode."""
-        # forward pass
-        sample = self._prepare_sample(sample)
-        _loss, sample_size, logging_output, oom_fwd = self._forward(sample, eval=True)
-        assert not oom_fwd, 'Ran out of memory during validation'
+        self.model.eval()
 
-        # gather logging outputs from all GPUs
+        logging_output, sample_size = {}, 0
+        with torch.no_grad():
+            sample = self._prepare_sample(sample)
+            if sample is None:
+                sample = self._prepare_sample(self._dummy_batch)
+            _loss, sample_size, logging_output = self.task.get_loss(
+                self.model, self.criterion, sample,
+            )
+
+        # gather logging outputs from all replicas
         if self.args.distributed_world_size > 1:
-            sample_sizes, logging_outputs = zip(*distributed_utils.all_gather_list(
-                (sample_size, logging_output)
+            logging_output, sample_size = zip(*distributed_utils.all_gather_list(
+                [logging_output, sample_size],
             ))
+            logging_output = list(logging_output)
+            sample_size = list(sample_size)
         else:
-            sample_sizes = [sample_size]
-            logging_outputs = [logging_output]
+            logging_output = [logging_output]
+            sample_size = [sample_size]
 
-        # aggregate stats and logging outputs
-        ntokens = sum(log.get('ntokens', 0) for log in logging_outputs)
-        grad_denom = self.criterion.__class__.grad_denom(sample_sizes)
-        agg_logging_output = self.criterion.__class__.aggregate_logging_outputs(logging_outputs)
+        # aggregate logging outputs and sample sizes
+        logging_output = self.criterion.__class__.aggregate_logging_outputs(logging_output)
+        sample_size = self.criterion.__class__.grad_denom(sample_size)
 
-        # update loss meters for validation
-        if 'loss' in agg_logging_output:
-            self.meters['valid_loss'].update(agg_logging_output['loss'], grad_denom)
-        # criterions can optionally log the NLL loss too
-        if 'nll_loss' in agg_logging_output:
-            self.meters['valid_nll_loss'].update(agg_logging_output['nll_loss'], ntokens)
+        # update meters for validation
+        ntokens = logging_output.get('ntokens', 0)
+        self.meters['valid_loss'].update(logging_output.get('loss', 0), sample_size)
+        self.meters['valid_nll_loss'].update(logging_output.get('nll_loss', 0), ntokens)
 
-        return agg_logging_output
+        return logging_output
 
     def dummy_train_step(self, dummy_batch):
         """Dummy training step for warming caching allocator."""
-        self.train_step(dummy_batch, update_params=False, dummy_batch=True)
+        self.train_step(dummy_batch, dummy_batch=True)
         self.zero_grad()
-        self.clear_buffered_stats()
 
     def zero_grad(self):
         self.optimizer.zero_grad()
 
-    def clear_buffered_stats(self):
-        self._buffered_stats.clear()
-
     def lr_step(self, epoch, val_loss=None):
         """Adjust the learning rate based on the validation loss."""
         return self.lr_scheduler.step(epoch, val_loss)
@@ -362,8 +320,8 @@ class Trainer(object):
         return self.optimizer.get_lr()
 
     def get_model(self):
-        """Get the model replica."""
-        return self.model
+        """Get the (non-wrapped) model instance."""
+        return self._model
 
     def get_meter(self, name):
         """Get a specific meter by name."""
diff --git a/multiprocessing_train.py b/multiprocessing_train.py
index 062067adf..80c46706d 100644
--- a/multiprocessing_train.py
+++ b/multiprocessing_train.py
@@ -19,8 +19,10 @@ from train import main as single_process_main
 def main(args):
     # Set distributed training parameters for a single node.
     args.distributed_world_size = torch.cuda.device_count()
-    args.distributed_init_method = 'tcp://localhost:{port}'.format(
-        port=random.randint(10000, 20000))
+    port = random.randint(10000, 20000)
+    args.distributed_init_method = 'tcp://localhost:{port}'.format(port=port)
+    args.distributed_init_host = 'localhost'
+    args.distributed_port = port + 1
 
     mp = torch.multiprocessing.get_context('spawn')
 
diff --git a/setup.py b/setup.py
index 97c00f135..53e8eaf51 100644
--- a/setup.py
+++ b/setup.py
@@ -35,7 +35,7 @@ bleu = Extension(
 
 setup(
     name='fairseq',
-    version='0.5.0',
+    version='0.6.0',
     description='Facebook AI Research Sequence-to-Sequence Toolkit',
     long_description=readme,
     license=license,
diff --git a/train.py b/train.py
index e1d76bfe6..34b2a3e3d 100644
--- a/train.py
+++ b/train.py
@@ -16,7 +16,7 @@ import math
 import torch
 
 from fairseq import distributed_utils, options, progress_bar, tasks, utils
-from fairseq.fp16_trainer import FP16Trainer
+from fairseq.data import iterators
 from fairseq.trainer import Trainer
 from fairseq.meters import AverageMeter, StopwatchMeter
 
@@ -43,16 +43,17 @@ def main(args):
     print('| model {}, criterion {}'.format(args.arch, criterion.__class__.__name__))
     print('| num. model params: {}'.format(sum(p.numel() for p in model.parameters())))
 
+    # Make a dummy batch to (i) warm the caching allocator and (ii) as a
+    # placeholder DistributedDataParallel when there's an uneven number of
+    # batches per worker.
+    max_positions = utils.resolve_max_positions(
+        task.max_positions(),
+        model.max_positions(),
+    )
+    dummy_batch = task.dataset('train').get_dummy_batch(args.max_tokens, max_positions)
+
     # Build trainer
-    if args.fp16:
-        if torch.cuda.get_device_capability(0)[0] < 7:
-            print('| WARNING: your device does NOT support faster training with --fp16,'
-                  ' please switch to FP32 which is likely to be faster')
-        trainer = FP16Trainer(args, task, model, criterion)
-    else:
-        if torch.cuda.get_device_capability(0)[0] >= 7:
-            print('| NOTICE: your device may support faster training with --fp16')
-        trainer = Trainer(args, task, model, criterion)
+    trainer = Trainer(args, task, model, criterion, dummy_batch)
     print('| training on {} GPUs'.format(args.distributed_world_size))
     print('| max tokens per GPU = {} and max sentences per GPU = {}'.format(
         args.max_tokens,
@@ -60,10 +61,6 @@ def main(args):
     ))
 
     # Initialize dataloader
-    max_positions = utils.resolve_max_positions(
-        task.max_positions(),
-        trainer.get_model().max_positions(),
-    )
     epoch_itr = task.get_batch_iterator(
         dataset=task.dataset(args.train_subset),
         max_tokens=args.max_tokens,
@@ -78,9 +75,7 @@ def main(args):
 
     # Load the latest checkpoint if one is available
     if not load_checkpoint(args, trainer, epoch_itr):
-        # Send a dummy batch to warm the caching allocator
-        dummy_batch = task.dataset('train').get_dummy_batch(args.max_tokens, max_positions)
-        trainer.dummy_train_step(dummy_batch)
+        trainer.dummy_train_step([dummy_batch])
 
     # Train until the learning rate gets too small
     max_epoch = args.max_epoch or math.inf
@@ -110,32 +105,32 @@ def main(args):
 def train(args, trainer, task, epoch_itr):
     """Train the model for one epoch."""
 
-    # Initialize data iterator
-    itr = epoch_itr.next_epoch_itr()
-    progress = progress_bar.build_progress_bar(args, itr, epoch_itr.epoch, no_progress_bar='simple')
-
-    # update parameters every N batches
+    # Update parameters every N batches
     if epoch_itr.epoch <= len(args.update_freq):
         update_freq = args.update_freq[epoch_itr.epoch - 1]
     else:
         update_freq = args.update_freq[-1]
 
+    # Initialize data iterator
+    itr = epoch_itr.next_epoch_itr()
+    itr = iterators.GroupedIterator(itr, update_freq)
+    progress = progress_bar.build_progress_bar(
+        args, itr, epoch_itr.epoch, no_progress_bar='simple',
+    )
+
     extra_meters = collections.defaultdict(lambda: AverageMeter())
     first_valid = args.valid_subset.split(',')[0]
     max_update = args.max_update or math.inf
     num_batches = len(epoch_itr)
-    for i, sample in enumerate(progress, start=epoch_itr.iterations_in_epoch):
-        if i < num_batches - 1 and (i + 1) % update_freq > 0:
-            # buffer updates according to --update-freq
-            trainer.train_step(sample, update_params=False)
+    for i, samples in enumerate(progress, start=epoch_itr.iterations_in_epoch):
+        log_output = trainer.train_step(samples)
+        if log_output is None:
             continue
-        else:
-            log_output = trainer.train_step(sample, update_params=True)
 
         # log mid-epoch stats
         stats = get_training_stats(trainer)
         for k, v in log_output.items():
-            if k in ['loss', 'nll_loss', 'sample_size']:
+            if k in ['loss', 'nll_loss', 'ntokens', 'nsentences', 'sample_size']:
                 continue  # these are already logged above
             if 'loss' in k:
                 extra_meters[k].update(v, log_output['sample_size'])
@@ -163,7 +158,9 @@ def train(args, trainer, task, epoch_itr):
     progress.print(stats)
 
     # reset training meters
-    for k in ['train_loss', 'train_nll_loss', 'wps', 'ups', 'wpb', 'bsz', 'clip', 'gnorm']:
+    for k in [
+        'train_loss', 'train_nll_loss', 'wps', 'ups', 'wpb', 'bsz', 'gnorm', 'clip',
+    ]:
         meter = trainer.get_meter(k)
         if meter is not None:
             meter.reset()
@@ -230,7 +227,7 @@ def validate(args, trainer, task, epoch_itr, subsets):
             log_output = trainer.valid_step(sample)
 
             for k, v in log_output.items():
-                if k in ['loss', 'nll_loss', 'sample_size']:
+                if k in ['loss', 'nll_loss', 'ntokens', 'nsentences', 'sample_size']:
                     continue
                 extra_meters[k].update(v)