torch.utils.data.DataLoader
r"""Definition of the DataLoader and associated iterators that subclass _BaseDataLoaderIter
To support these two classes, in `./_utils` we define many utility methods and
functions to be run in multiprocessing. E.g., the data loading worker loop is
in `./_utils/worker.py`.
"""
import threading
import itertools
import warnings
import multiprocessing as python_multiprocessing
import torch
import torch.multiprocessing as multiprocessing
from torch._utils import ExceptionWrapper
from torch._six import queue, string_classes
from . import IterableDataset, Sampler, SequentialSampler, RandomSampler, BatchSampler
from . import _utils
get_worker_info = _utils.worker.get_worker_info
# This function used to be defined in this file. However, it was moved to
# _utils/collate.py. Although it is rather hard to access this from user land
# (one has to explicitly directly `import torch.utils.data.dataloader`), there
# probably is user code out there using it. This aliasing maintains BC in this
# aspect.
default_collate = _utils.collate.default_collate
class _DatasetKind(object):
Map = 0
Iterable = 1
@staticmethod
def create_fetcher(kind, dataset, auto_collation, collate_fn, drop_last):
if kind == _DatasetKind.Map:
return _utils.fetch._MapDatasetFetcher(dataset, auto_collation, collate_fn, drop_last)
else:
return _utils.fetch._IterableDatasetFetcher(dataset, auto_collation, collate_fn, drop_last)
class _InfiniteConstantSampler(Sampler):
r"""Analogous to ``itertools.repeat(None, None)``.
Used as sampler for :class:`~torch.utils.data.IterableDataset`.
Arguments:
data_source (Dataset): dataset to sample from
"""
def __init__(self):
super(_InfiniteConstantSampler, self).__init__(None)
def __iter__(self):
while True:
yield None
class DataLoader(object):
r"""
Data loader. Combines a dataset and a sampler, and provides an iterable over
the given dataset.
The :class:`~torch.utils.data.DataLoader` supports both map-style and
iterable-style datasets with single- or multi-process loading, customizing
loading order and optional automatic batching (collation) and memory pinning.
See :py:mod:`torch.utils.data` documentation page for more details.
Arguments:
dataset (Dataset): dataset from which to load the data.
batch_size (int, optional): how many samples per batch to load
(default: ``1``).
shuffle (bool, optional): set to ``True`` to have the data reshuffled
at every epoch (default: ``False``).
sampler (Sampler, optional): defines the strategy to draw samples from
the dataset. If specified, :attr:`shuffle` must be ``False``.
batch_sampler (Sampler, optional): like :attr:`sampler`, but returns a batch of
indices at a time. Mutually exclusive with :attr:`batch_size`,
:attr:`shuffle`, :attr:`sampler`, and :attr:`drop_last`.
num_workers (int, optional): how many subprocesses to use for data
loading. ``0`` means that the data will be loaded in the main process.
(default: ``0``)
collate_fn (callable, optional): merges a list of samples to form a
mini-batch of Tensor(s). Used when using batched loading from a
map-style dataset.
pin_memory (bool, optional): If ``True``, the data loader will copy Tensors
into CUDA pinned memory before returning them. If your data elements
are a custom type, or your :attr:`collate_fn` returns a batch that is a custom type,
see the example below.
drop_last (bool, optional): set to ``True`` to drop the last incomplete batch,
if the dataset size is not divisible by the batch size. If ``False`` and
the size of dataset is not divisible by the batch size, then the last batch
will be smaller. (default: ``False``)
timeout (numeric, optional): if positive, the timeout value for collecting a batch
from workers. Should always be non-negative. (default: ``0``)
worker_init_fn (callable, optional): If not ``None``, this will be called on each
worker subprocess with the worker id (an int in ``[0, num_workers - 1]``) as
input, after seeding and before data loading. (default: ``None``)
.. warning:: If the ``spawn`` start method is used, :attr:`worker_init_fn`
cannot be an unpicklable object, e.g., a lambda function. See
:ref:`multiprocessing-best-practices` on more details related
to multiprocessing in PyTorch.
.. note:: ``len(dataloader)`` heuristic is based on the length of the sampler used.
When :attr:`dataset` is an :class:`~torch.utils.data.IterableDataset`,
``len(dataset)`` (if implemented) is returned instead, regardless
of multi-process loading configurations, because PyTorch trust
user :attr:`dataset` code in correctly handling multi-process
loading to avoid duplicate data. See `Dataset Types`_ for more
details on these two types of datasets and how
:class:`~torch.utils.data.IterableDataset` interacts with `Multi-process data loading`_.
"""
__initialized = False
def __init__(self, dataset, batch_size=1, shuffle=False, sampler=None,
batch_sampler=None, num_workers=0, collate_fn=None,
pin_memory=False, drop_last=False, timeout=0,
worker_init_fn=None, multiprocessing_context=None):
torch._C._log_api_usage_once("python.data_loader")
if num_workers < 0:
raise ValueError('num_workers option should be non-negative; '
'use num_workers=0 to disable multiprocessing.')
if timeout < 0:
raise ValueError('timeout option should be non-negative')
self.dataset = dataset
self.num_workers = num_workers
self.pin_memory = pin_memory
self.timeout = timeout
self.worker_init_fn = worker_init_fn
self.multiprocessing_context = multiprocessing_context
# Arg-check dataset related before checking samplers because we want to
# tell users that iterable-style datasets are incompatible with custom
# samplers first, so that they don't learn that this combo doesn't work
# after spending time fixing the custom sampler errors.
if isinstance(dataset, IterableDataset):
self._dataset_kind = _DatasetKind.Iterable
# NOTE [ Custom Samplers and `IterableDataset` ]
#
# `IterableDataset` does not support custom `batch_sampler` or
# `sampler` since the key is irrelevant (unless we support
# generator-style dataset one day...).
#
# For `sampler`, we always create a dummy sampler. This is an
# infinite sampler even when the dataset may have an implemented
# finite `__len__` because in multi-process data loading, naive
# settings will return duplicated data (which may be desired), and
# thus using a sampler with length matching that of dataset will
# cause data lost (you may have duplicates of the first couple
# batches, but never see anything afterwards). Therefore,
# `Iterabledataset` always uses an infinite sampler, an instance of
# `_InfiniteConstantSampler` defined above.
#
# A custom `batch_sampler` essentially only controls the batch size.
# However, it is unclear how useful it would be since an iterable-style
# dataset can handle that within itself. Moreover, it is pointless
# in multi-process data loading as the assignment order of batches
# to workers is an implementation detail so users can not control
# how to batchify each worker's iterable. Thus, we disable this
# option. If this turns out to be useful in future, we can re-enable
# this, and support custom samplers that specify the assignments to
# specific workers.
if shuffle is not False:
raise ValueError(
"DataLoader with IterableDataset: expected unspecified "
"shuffle option, but got shuffle={}".format(shuffle))
elif sampler is not None:
# See NOTE [ Custom Samplers and IterableDataset ]
raise ValueError(
"DataLoader with IterableDataset: expected unspecified "
"sampler option, but got sampler={}".format(sampler))
elif batch_sampler is not None:
# See NOTE [ Custom Samplers and IterableDataset ]
raise ValueError(
"DataLoader with IterableDataset: expected unspecified "
"batch_sampler option, but got batch_sampler={}".format(batch_sampler))
else:
self._dataset_kind = _DatasetKind.Map
if sampler is not None and shuffle:
raise ValueError('sampler option is mutually exclusive with '
'shuffle')
if batch_sampler is not None:
# auto_collation with custom batch_sampler
if batch_size != 1 or shuffle or sampler is not None or drop_last:
raise ValueError('batch_sampler option is mutually exclusive '
'with batch_size, shuffle, sampler, and '
'drop_last')
batch_size = None
drop_last = False
elif batch_size is None:
# no auto_collation
if shuffle or drop_last:
raise ValueError('batch_size=None option disables auto-batching '
'and is mutually exclusive with '
'shuffle, and drop_last')
if sampler is None: # give default samplers
if self._dataset_kind == _DatasetKind.Iterable:
# See NOTE [ Custom Samplers and IterableDataset ]
sampler = _InfiniteConstantSampler()
else: # map-style
if shuffle:
sampler = RandomSampler(dataset)
else:
sampler = SequentialSampler(dataset)
if batch_size is not None and batch_sampler is None:
# auto_collation without custom batch_sampler
batch_sampler = BatchSampler(sampler, batch_size, drop_last)
self.batch_size = batch_size
self.drop_last = drop_last
self.sampler = sampler
self.batch_sampler = batch_sampler
if collate_fn is None:
if self._auto_collation:
collate_fn = _utils.collate.default_collate
else:
collate_fn = _utils.collate.default_convert
self.collate_fn = collate_fn
self.__initialized = True
self._IterableDataset_len_called = None # See NOTE [ IterableDataset and __len__ ]
@property
def multiprocessing_context(self):
return self.__multiprocessing_context
@multiprocessing_context.setter
def multiprocessing_context(self, multiprocessing_context):
if multiprocessing_context is not None:
if self.num_workers > 0:
if not multiprocessing._supports_context:
raise ValueError('multiprocessing_context relies on Python >= 3.4, with '
'support for different start methods')
if isinstance(multiprocessing_context, string_classes):
valid_start_methods = multiprocessing.get_all_start_methods()
if multiprocessing_context not in valid_start_methods:
raise ValueError(
('multiprocessing_context option '
'should specify a valid start method in {}, but got '
'multiprocessing_context={}').format(valid_start_methods, multiprocessing_context))
multiprocessing_context = multiprocessing.get_context(multiprocessing_context)
if not isinstance(multiprocessing_context, python_multiprocessing.context.BaseContext):
raise ValueError(('multiprocessing_context option should be a valid context '
'object or a string specifying the start method, but got '
'multiprocessing_context={}').format(multiprocessing_context))
else:
raise ValueError(('multiprocessing_context can only be used with '
'multi-process loading (num_workers > 0), but got '
'num_workers={}').format(self.num_workers))
self.__multiprocessing_context = multiprocessing_context
def __setattr__(self, attr, val):
if self.__initialized and attr in ('batch_size', 'batch_sampler', 'sampler', 'drop_last', 'dataset'):
raise ValueError('{} attribute should not be set after {} is '
'initialized'.format(attr, self.__class__.__name__))
super(DataLoader, self).__setattr__(attr, val)
def __iter__(self):
if self.num_workers == 0:
return _SingleProcessDataLoaderIter(self)
else:
return _MultiProcessingDataLoaderIter(self)
@property
def _auto_collation(self):
return self.batch_sampler is not None
@property
def _index_sampler(self):
# The actual sampler used for generating indices for `_DatasetFetcher`
# (see _utils/fetch.py) to read data at each time. This would be
# `.batch_sampler` if in auto-collation mode, and `.sampler` otherwise.
# We can't change `.sampler` and `.batch_sampler` attributes for BC
# reasons.
if self._auto_collation:
return self.batch_sampler
else:
return self.sampler
def __len__(self):
if self._dataset_kind == _DatasetKind.Iterable:
# NOTE [ IterableDataset and __len__ ]
#
# For `IterableDataset`, `__len__` could be inaccurate when one naively
# does multi-processing data loading, since the samples will be duplicated.
# However, no real use case should be actually using that behavior, so
# it should count as a user error. We should generally trust user
# code to do the proper thing (e.g., configure each replica differently
# in `__iter__`), and give us the correct `__len__` if they choose to
# implement it (this will still throw if the dataset does not implement
# a `__len__`).
#
# To provide a further warning, we track if `__len__` was called on the
# `DataLoader`, save the returned value in `self._len_called`, and warn
# if the iterator ends up yielding more than this number of samples.
length = self._IterableDataset_len_called = len(self.dataset)
return length
else:
return len(self._index_sampler)
class _BaseDataLoaderIter(object):
def __init__(self, loader):
self._dataset = loader.dataset
self._dataset_kind = loader._dataset_kind
self._IterableDataset_len_called = loader._IterableDataset_len_called
self._auto_collation = loader._auto_collation
self._drop_last = loader.drop_last
self._index_sampler = loader._index_sampler
self._num_workers = loader.num_workers
self._pin_memory = loader.pin_memory and torch.cuda.is_available()
self._timeout = loader.timeout
self._collate_fn = loader.collate_fn
self._sampler_iter = iter(self._index_sampler)
self._base_seed = torch.empty((), dtype=torch.int64).random_().item()
self._num_yielded = 0
def __iter__(self):
return self
def _next_index(self):
return next(self._sampler_iter) # may raise StopIteration
def _next_data(self):
raise NotImplementedError
def __next__(self):
data = self._next_data()
self._num_yielded += 1
if self._dataset_kind == _DatasetKind.Iterable and \
self._IterableDataset_len_called is not None and \
self._num_yielded > self._IterableDataset_len_called:
warn_msg = ("Length of IterableDataset {} was reported to be {} (when accessing len(dataloader)), but {} "
"samples have been fetched. ").format(self._dataset, self._IterableDataset_len_called,
self._num_yielded)
if self._num_workers > 0:
warn_msg += ("For multiprocessing data-loading, this could be caused by not properly configuring the "
"IterableDataset replica at each worker. Please see "
"https://pytorch.org/docs/stable/data.html#torch.utils.data.IterableDataset for examples.")
warnings.warn(warn_msg)
return data
next = __next__ # Python 2 compatibility
def __len__(self):
return len(self._index_sampler)
def __getstate__(self):
# TODO: add limited pickling support for sharing an iterator
# across multiple threads for HOGWILD.
# Probably the best way to do this is by moving the sample pushing
# to a separate thread and then just sharing the data queue
# but signalling the end is tricky without a non-blocking API
raise NotImplementedError("{} cannot be pickled", self.__class__.__name__)
class _SingleProcessDataLoaderIter(_BaseDataLoaderIter):
def __init__(self, loader):
super(_SingleProcessDataLoaderIter, self).__init__(loader)
assert self._timeout == 0
assert self._num_workers == 0
self._dataset_fetcher = _DatasetKind.create_fetcher(
self._dataset_kind, self._dataset, self._auto_collation, self._collate_fn, self._drop_last)
def _next_data(self):
index = self._next_index() # may raise StopIteration
data = self._dataset_fetcher.fetch(index) # may raise StopIteration
if self._pin_memory:
data = _utils.pin_memory.pin_memory(data)
return data
class _MultiProcessingDataLoaderIter(_BaseDataLoaderIter):
r"""Iterates once over the DataLoader's dataset, as specified by the sampler"""
# NOTE [ Data Loader Multiprocessing Shutdown Logic ]
#
# Preliminary:
#
# Our data model looks like this (queues are indicated with curly brackets):
#
# main process ||
# | ||
# {index_queue} ||
# | ||
# worker processes || DATA
# | ||
# {worker_result_queue} || FLOW
# | ||
# pin_memory_thread of main process || DIRECTION
# | ||
# {data_queue} ||
# | ||
# data output \/
#
# P.S. `worker_result_queue` and `pin_memory_thread` part may be omitted if
# `pin_memory=False`.
#
#
# Terminating multiprocessing logic requires very careful design. In
# particular, we need to make sure that
#
# 1. The iterator gracefully exits the workers when its last reference is
# gone or it is depleted.
#
# In this case, the workers should be gracefully exited because the
# main process may still need to continue to run, and we want cleaning
# up code in the workers to be executed (e.g., releasing GPU memory).
# Naturally, we implement the shutdown logic in `__del__` of
# DataLoaderIterator.
#
# We delay the discussion on the logic in this case until later.
#
# 2. The iterator exits the workers when the loader process and/or worker
# processes exits normally or with error.
#
# We set all workers and `pin_memory_thread` to have `daemon=True`.
#
# You may ask, why can't we make the workers non-daemonic, and
# gracefully exit using the same logic as we have in `__del__` when the
# iterator gets deleted (see 1 above)?
#
# First of all, `__del__` is **not** guaranteed to be called when
# interpreter exits. Even if it is called, by the time it executes,
# many Python core library resources may alreay be freed, and even
# simple things like acquiring an internal lock of a queue may hang.
# Therefore, in this case, we actually need to prevent `__del__` from
# being executed, and rely on the automatic termination of daemonic
# children. Thus, we register an `atexit` hook that sets a global flag
# `_utils.python_exit_status`. Since `atexit` hooks are executed in the
# reverse order of registration, we are guaranteed that this flag is
# set before library resources we use are freed. (Hooks freeing those
# resources are registered at importing the Python core libraries at
# the top of this file.) So in `__del__`, we check if
# `_utils.python_exit_status` is set or `None` (freed), and perform
# no-op if so.
#
# Another problem with `__del__` is also related to the library cleanup
# calls. When a process ends, it shuts the all its daemonic children
# down with a SIGTERM (instead of joining them without a timeout).
# Simiarly for threads, but by a different mechanism. This fact,
# together with a few implementation details of multiprocessing, forces
# us to make workers daemonic. All of our problems arise when a
# DataLoader is used in a subprocess, and are caused by multiprocessing
# code which looks more or less like this:
#
# try:
# your_function_using_a_dataloader()
# finally:
# multiprocessing.util._exit_function()
#
# The joining/termination mentioned above happens inside
# `_exit_function()`. Now, if `your_function_using_a_dataloader()`
# throws, the stack trace stored in the exception will prevent the
# frame which uses `DataLoaderIter` to be freed. If the frame has any
# reference to the `DataLoaderIter` (e.g., in a method of the iter),
# its `__del__`, which starts the shutdown procedure, will not be
# called. That, in turn, means that workers aren't notified. Attempting
# to join in `_exit_function` will then result in a hang.
#
# For context, `_exit_function` is also registered as an `atexit` call.
# So it is unclear to me (@ssnl) why this is needed in a finally block.
# The code dates back to 2008 and there is no comment on the original
# PEP 371 or patch https://bugs.python.org/issue3050 (containing both
# the finally block and the `atexit` registration) that explains this.
#
# Another choice is to just shutdown workers with logic in 1 above
# whenever we see an error in `next`. This isn't ideal because
# a. It prevents users from using try-catch to resume data loading.
# b. It doesn't prevent hanging if users have references to the
# iterator.
#
# 3. All processes exit if any of them die unexpectedly by fatal signals.
#
# As shown above, the workers are set as daemonic children of the main
# process. However, automatic cleaning-up of such child processes only
# happens if the parent process exits gracefully (e.g., not via fatal
# signals like SIGKILL). So we must ensure that each process will exit
# even the process that should send/receive data to/from it were
# killed, i.e.,
#
# a. A process won't hang when getting from a queue.
#
# Even with carefully designed data dependencies (i.e., a `put()`
# always corresponding to a `get()`), hanging on `get()` can still
# happen when data in queue is corrupted (e.g., due to
# `cancel_join_thread` or unexpected exit).
#
# For child exit, we set a timeout whenever we try to get data
# from `data_queue`, and check the workers' status on each timeout
# and error.
# See `_DataLoaderiter._get_batch()` and
# `_DataLoaderiter._try_get_data()` for details.
#
# Additionally, for child exit on non-Windows platforms, we also
# register a SIGCHLD handler (which is supported on Windows) on
# the main process, which checks if any of the workers fail in the
# (Python) handler. This is more efficient and faster in detecting
# worker failures, compared to only using the above mechanism.
# See `DataLoader.cpp` and `_utils/signal_handling.py` for details.
#
# For `.get()` calls where the sender(s) is not the workers, we
# guard them with timeouts, and check the status of the sender
# when timeout happens:
# + in the workers, the `_utils.worker.ManagerWatchdog` class
# checks the status of the main process.
# + if `pin_memory=True`, when getting from `pin_memory_thread`,
# check `pin_memory_thread` status periodically until `.get()`
# returns or see that `pin_memory_thread` died.
#
# b. A process won't hang when putting into a queue;
#
# We use `mp.Queue` which has a separate background thread to put
# objects from an unbounded buffer array. The background thread is
# daemonic and usually automatically joined when the process
# exits.
#
# However, in case that the receiver has ended abruptly while
# reading from the pipe, the join will hang forever. Therefore,
# for both `worker_result_queue` (worker -> main process/pin_memory_thread)
# and each `index_queue` (main process -> worker), we use
# `q.cancel_join_thread()` in sender process before any `q.put` to
# prevent this automatic join.
#
# Moreover, having all queues called `cancel_join_thread` makes
# implementing graceful shutdown logic in `__del__` much easier.
# It won't need to get from any queue, which would also need to be
# guarded by periodic status checks.
#
# Nonetheless, `cancel_join_thread` must only be called when the
# queue is **not** going to be read from or write into by another
# process, because it may hold onto a lock or leave corrupted data
# in the queue, leading other readers/writers to hang.
#
# `pin_memory_thread`'s `data_queue` is a `queue.Queue` that does
# a blocking `put` if the queue is full. So there is no above
# problem, but we do need to wrap the `put` in a loop that breaks
# not only upon success, but also when the main process stops
# reading, i.e., is shutting down.
#
#
# Now let's get back to 1:
# how we gracefully exit the workers when the last reference to the
# iterator is gone.
#
# To achieve this, we implement the following logic along with the design
# choices mentioned above:
#
# `workers_done_event`:
# A `multiprocessing.Event` shared among the main process and all worker
# processes. This is used to signal the workers that the iterator is
# shutting down. After it is set, they will not send processed data to
# queues anymore, and only wait for the final `None` before exiting.
# `done_event` isn't strictly needed. I.e., we can just check for `None`
# from the input queue, but it allows us to skip wasting resources
# processing data if we are already shutting down.
#
# `pin_memory_thread_done_event`:
# A `threading.Event` for a similar purpose to that of
# `workers_done_event`, but is for the `pin_memory_thread`. The reason
# that separate events are needed is that `pin_memory_thread` reads from
# the output queue of the workers. But the workers, upon seeing that
# `workers_done_event` is set, only wants to see the final `None`, and is
# not required to flush all data in the output queue (e.g., it may call
# `cancel_join_thread` on that queue if its `IterableDataset` iterator
# happens to exhaust coincidentally, which is out of the control of the
# main process). Thus, since we will exit `pin_memory_thread` before the
# workers (see below), two separete events are used.
#
# NOTE: In short, the protocol is that the main process will set these
# `done_event`s and then the corresponding processes/threads a `None`,
# and that they may exit at any time after receiving the `None`.
#
# NOTE: Using `None` as the final signal is valid, since normal data will
# always be a 2-tuple with the 1st element being the index of the data
# transferred (different from dataset index/key), and the 2nd being
# either the dataset key or the data sample (depending on which part
# of the data model the queue is at).
#
# [ worker processes ]
# While loader process is alive:
# Get from `index_queue`.
# If get anything else,
# Check `workers_done_event`.
# If set, continue to next iteration
# i.e., keep getting until see the `None`, then exit.
# Otherwise, process data:
# If is fetching from an `IterableDataset` and the iterator
# is exhausted, send an `_IterableDatasetStopIteration`
# object to signal iteration end. The main process, upon
# receiving such an object, will send `None` to this
# worker and not use the corresponding `index_queue`
# anymore.
# If timed out,
# No matter `workers_done_event` is set (still need to see `None`)
# or not, must continue to next iteration.
# (outside loop)
# If `workers_done_event` is set, (this can be False with `IterableDataset`)
# `data_queue.cancel_join_thread()`. (Everything is ending here:
# main process won't read from it;
# other workers will also call
# `cancel_join_thread`.)
#
# [ pin_memory_thread ]
# # No need to check main thread. If this thread is alive, the main loader
# # thread must be alive, because this thread is set as daemonic.
# While `pin_memory_thread_done_event` is not set:
# Get from `index_queue`.
# If timed out, continue to get in the next iteration.
# Otherwise, process data.
# While `pin_memory_thread_done_event` is not set:
# Put processed data to `data_queue` (a `queue.Queue` with blocking put)
# If timed out, continue to put in the next iteration.
# Otherwise, break, i.e., continuing to the out loop.
#
# NOTE: we don't check the status of the main thread because
# 1. if the process is killed by fatal signal, `pin_memory_thread`
# ends.
# 2. in other cases, either the cleaning-up in __del__ or the
# automatic exit of daemonic thread will take care of it.
# This won't busy-wait either because `.get(timeout)` does not
# busy-wait.
#
# [ main process ]
# In the DataLoader Iter's `__del__`
# b. Exit `pin_memory_thread`
# i. Set `pin_memory_thread_done_event`.
# ii Put `None` in `worker_result_queue`.
# iii. Join the `pin_memory_thread`.
# iv. `worker_result_queue.cancel_join_thread()`.
#
# c. Exit the workers.
# i. Set `workers_done_event`.
# ii. Put `None` in each worker's `index_queue`.
# iii. Join the workers.
# iv. Call `.cancel_join_thread()` on each worker's `index_queue`.
#
# NOTE: (c) is better placed after (b) because it may leave corrupted
# data in `worker_result_queue`, which `pin_memory_thread`
# reads from, in which case the `pin_memory_thread` can only
# happen at timeing out, which is slow. Nonetheless, same thing
# happens if a worker is killed by signal at unfortunate times,
# but in other cases, we are better off having a non-corrupted
# `worker_result_queue` for `pin_memory_thread`.
#
# NOTE: If `pin_memory=False`, there is no `pin_memory_thread` and (b)
# can be omitted
#
# NB: `done_event`s isn't strictly needed. E.g., we can just check for
# `None` from `index_queue`, but it allows us to skip wasting resources
# processing indices already in `index_queue` if we are already shutting
# down.
def __init__(self, loader):
super(_MultiProcessingDataLoaderIter, self).__init__(loader)
assert self._num_workers > 0
if loader.multiprocessing_context is None:
multiprocessing_context = multiprocessing
else:
multiprocessing_context = loader.multiprocessing_context
self._worker_init_fn = loader.worker_init_fn
self._worker_queue_idx_cycle = itertools.cycle(range(self._num_workers))
self._worker_result_queue = multiprocessing_context.Queue()
self._worker_pids_set = False
self._shutdown = False
self._send_idx = 0 # idx of the next task to be sent to workers
self._rcvd_idx = 0 # idx of the next task to be returned in __next__
# information about data not yet yielded, i.e., tasks w/ indices in range [rcvd_idx, send_idx).
# map: task idx => - (worker_id,) if data isn't fetched (outstanding)
# \ (worker_id, data) if data is already fetched (out-of-order)
self._task_info = {}
self._tasks_outstanding = 0 # always equal to count(v for v in task_info.values() if len(v) == 1)
self._workers_done_event = multiprocessing_context.Event()
self._index_queues = []
self._workers = []
# A list of booleans representing whether each worker still has work to
# do, i.e., not having exhausted its iterable dataset object. It always
# contains all `True`s if not using an iterable-style dataset
# (i.e., if kind != Iterable).
self._workers_status = []
for i in range(self._num_workers):
index_queue = multiprocessing_context.Queue()
# index_queue.cancel_join_thread()
w = multiprocessing_context.Process(
target=_utils.worker._worker_loop,
args=(self._dataset_kind, self._dataset, index_queue,
self._worker_result_queue, self._workers_done_event,
self._auto_collation, self._collate_fn, self._drop_last,
self._base_seed + i, self._worker_init_fn, i, self._num_workers))
w.daemon = True
# NB: Process.start() actually take some time as it needs to
# start a process and pass the arguments over via a pipe.
# Therefore, we only add a worker to self._workers list after
# it started, so that we do not call .join() if program dies
# before it starts, and __del__ tries to join but will get:
# AssertionError: can only join a started process.
w.start()
self._index_queues.append(index_queue)
self._workers.append(w)
self._workers_status.append(True)
if self._pin_memory:
self._pin_memory_thread_done_event = threading.Event()
self._data_queue = queue.Queue()
pin_memory_thread = threading.Thread(
target=_utils.pin_memory._pin_memory_loop,
args=(self._worker_result_queue, self._data_queue,
torch.cuda.current_device(),
self._pin_memory_thread_done_event))
pin_memory_thread.daemon = True
pin_memory_thread.start()
# Similar to workers (see comment above), we only register
# pin_memory_thread once it is started.
self._pin_memory_thread = pin_memory_thread
else:
self._data_queue = self._worker_result_queue
_utils.signal_handling._set_worker_pids(id(self), tuple(w.pid for w in self._workers))
_utils.signal_handling._set_SIGCHLD_handler()
self._worker_pids_set = True
# prime the prefetch loop
for _ in range(2 * self._num_workers):
self._try_put_index()
def _try_get_data(self, timeout=_utils.MP_STATUS_CHECK_INTERVAL):
# Tries to fetch data from `self._data_queue` once for a given timeout.
# This can also be used as inner loop of fetching without timeout, with
# the sender status as the loop condition.
#
# This raises a `RuntimeError` if any worker died expectedly. This error
# can come from either the SIGCHLD handler in `_utils/signal_handling.py`
# (only for non-Windows platforms), or the manual check below on errors
# and timeouts.
#
# Returns a 2-tuple:
# (bool: whether successfully get data, any: data if successful else None)
try:
data = self._data_queue.get(timeout=timeout)
return (True, data)
except Exception as e:
# At timeout and error, we manually check whether any worker has
# failed. Note that this is the only mechanism for Windows to detect
# worker failures.
failed_workers = []
for worker_id, w in enumerate(self._workers):
if self._workers_status[worker_id] and not w.is_alive():
failed_workers.append(w)
self._shutdown_worker(worker_id)
if len(failed_workers) > 0:
pids_str = ', '.join(str(w.pid) for w in failed_workers)
raise RuntimeError('DataLoader worker (pid(s) {}) exited unexpectedly'.format(pids_str))
if isinstance(e, queue.Empty):
return (False, None)
raise
def _get_data(self):
# Fetches data from `self._data_queue`.
#
# We check workers' status every `MP_STATUS_CHECK_INTERVAL` seconds,
# which we achieve by running `self._try_get_data(timeout=MP_STATUS_CHECK_INTERVAL)`
# in a loop. This is the only mechanism to detect worker failures for
# Windows. For other platforms, a SIGCHLD handler is also used for
# worker failure detection.
#
# If `pin_memory=True`, we also need check if `pin_memory_thread` had
# died at timeouts.
if self._timeout > 0:
success, data = self._try_get_data(self._timeout)
if success:
return data
else:
raise RuntimeError('DataLoader timed out after {} seconds'.format(self._timeout))
elif self._pin_memory:
while self._pin_memory_thread.is_alive():
success, data = self._try_get_data()
if success:
return data
else:
# while condition is false, i.e., pin_memory_thread died.
raise RuntimeError('Pin memory thread exited unexpectedly')
# In this case, `self._data_queue` is a `queue.Queue`,. But we don't
# need to call `.task_done()` because we don't use `.join()`.
else:
while True:
success, data = self._try_get_data()
if success:
return data
def _next_data(self):
while True:
# If the worker responsible for `self._rcvd_idx` has already ended
# and was unable to fulfill this task (due to exhausting an `IterableDataset`),
# we try to advance `self._rcvd_idx` to find the next valid index.
#
# This part needs to run in the loop because both the `self._get_data()`
# call and `_IterableDatasetStopIteration` check below can mark
# extra worker(s) as dead.
while self._rcvd_idx < self._send_idx:
info = self._task_info[self._rcvd_idx]
worker_id = info[0]
if len(info) == 2 or self._workers_status[worker_id]: # has data or is still active
break
del self._task_info[self._rcvd_idx]
self._rcvd_idx += 1
else:
# no valid `self._rcvd_idx` is found (i.e., didn't break)
self._shutdown_workers()
raise StopIteration
# Now `self._rcvd_idx` is the batch index we want to fetch
# Check if the next sample has already been generated
if len(self._task_info[self._rcvd_idx]) == 2:
data = self._task_info.pop(self._rcvd_idx)[1]
return self._process_data(data)
assert not self._shutdown and self._tasks_outstanding > 0
idx, data = self._get_data()
self._tasks_outstanding -= 1
if self._dataset_kind == _DatasetKind.Iterable:
# Check for _IterableDatasetStopIteration
if isinstance(data, _utils.worker._IterableDatasetStopIteration):
self._shutdown_worker(data.worker_id)
self._try_put_index()
continue
if idx != self._rcvd_idx:
# store out-of-order samples
self._task_info[idx] += (data,)
else:
del self._task_info[idx]
return self._process_data(data)
def _try_put_index(self):
assert self._tasks_outstanding < 2 * self._num_workers
try:
index = self._next_index()
except StopIteration:
return
for _ in range(self._num_workers): # find the next active worker, if any
worker_queue_idx = next(self._worker_queue_idx_cycle)
if self._workers_status[worker_queue_idx]:
break
else:
# not found (i.e., didn't break)
return
self._index_queues[worker_queue_idx].put((self._send_idx, index))
self._task_info[self._send_idx] = (worker_queue_idx,)
self._tasks_outstanding += 1
self._send_idx += 1
def _process_data(self, data):
self._rcvd_idx += 1
self._try_put_index()
if isinstance(data, ExceptionWrapper):
data.reraise()
return data
def _shutdown_worker(self, worker_id):
# Mark a worker as having finished its work and dead, e.g., due to
# exhausting an `IterableDataset`. This should be used only when this
# `_MultiProcessingDataLoaderIter` is going to continue running.
assert self._workers_status[worker_id]
# Signal termination to that specific worker.
q = self._index_queues[worker_id]
# Indicate that no more data will be put on this queue by the current
# process.
q.put(None)
# Note that we don't actually join the worker here, nor do we remove the
# worker's pid from C side struct because (1) joining may be slow, and
# (2) since we don't join, the worker may still raise error, and we
# prefer capturing those, rather than ignoring them, even though they
# are raised after the worker has finished its job.
# Joinning is deferred to `_shutdown_workers`, which it is called when
# all workers finish their jobs (e.g., `IterableDataset` replicas) or
# when this iterator is garbage collected.
self._workers_status[worker_id] = False
def _shutdown_workers(self):
# Called when shutting down this `_MultiProcessingDataLoaderIter`.
# See NOTE [ Data Loader Multiprocessing Shutdown Logic ] for details on
# the logic of this function.
python_exit_status = _utils.python_exit_status
if python_exit_status is True or python_exit_status is None:
# See (2) of the note. If Python is shutting down, do no-op.
return
# Normal exit when last reference is gone / iterator is depleted.
# See (1) and the second half of the note.
if not self._shutdown:
self._shutdown = True
try:
# Exit `pin_memory_thread` first because exiting workers may leave
# corrupted data in `worker_result_queue` which `pin_memory_thread`
# reads from.
if hasattr(self, '_pin_memory_thread'):
# Use hasattr in case error happens before we set the attribute.
self._pin_memory_thread_done_event.set()
# Send something to pin_memory_thread in case it is waiting
# so that it can wake up and check `pin_memory_thread_done_event`
self._worker_result_queue.put((None, None))
self._pin_memory_thread.join()
self._worker_result_queue.close()
# Exit workers now.
self._workers_done_event.set()
for worker_id in range(len(self._workers)):
# Get number of workers from `len(self._workers)` instead of
# `self._num_workers` in case we error before starting all
# workers.
if self._workers_status[worker_id]:
self._shutdown_worker(worker_id)
for w in self._workers:
w.join()
for q in self._index_queues:
q.cancel_join_thread()
q.close()
finally:
# Even though all this function does is putting into queues that
# we have called `cancel_join_thread` on, weird things can
# happen when a worker is killed by a signal, e.g., hanging in
# `Event.set()`. So we need to guard this with SIGCHLD handler,
# and remove pids from the C side data structure only at the
# end.
#
# FIXME: Unfortunately, for Windows, we are missing a worker
# error detection mechanism here in this function, as it
# doesn't provide a SIGCHLD handler.
if self._worker_pids_set:
_utils.signal_handling._remove_worker_pids(id(self))
self._worker_pids_set = False
def __del__(self):
self._shutdown_workers()手机扫一扫
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