DataLoader 与 Dataset_prefetch_factor option could only be specified in

一、总体概览

二、具体详解

DataLoader源码

class DataLoader(Generic[T_co]):
    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 or Iterable, optional): defines the strategy to draw
            samples from the dataset. Can be any ``Iterable`` with ``__len__``
            implemented. If specified, :attr:`shuffle` must not be specified.
        batch_sampler (Sampler or Iterable, 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``)
        prefetch_factor (int, optional, keyword-only arg): Number of sample loaded
            in advance by each worker. ``2`` means there will be a total of
            2 * num_workers samples prefetched across all workers. (default: ``2``)
        persistent_workers (bool, optional): If ``True``, the data loader will not shutdown
            the worker processes after a dataset has been consumed once. This allows to 
            maintain the workers `Dataset` instances alive. (default: ``False``)
    .. 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.
    .. warning:: ``len(dataloader)`` heuristic is based on the length of the sampler used.
                 When :attr:`dataset` is an :class:`~torch.utils.data.IterableDataset`,
                 it instead returns an estimate based on ``len(dataset) / batch_size``, with proper
                 rounding depending on :attr:`drop_last`, regardless of multi-process loading
                 configurations. This represents the best guess PyTorch can make because PyTorch
                 trusts user :attr:`dataset` code in correctly handling multi-process
                 loading to avoid duplicate data.
                 However, if sharding results in multiple workers having incomplete last batches,
                 this estimate can still be inaccurate, because (1) an otherwise complete batch can
                 be broken into multiple ones and (2) more than one batch worth of samples can be
                 dropped when :attr:`drop_last` is set. Unfortunately, PyTorch can not detect such
                 cases in general.
                 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`_.
    """
    dataset: Dataset[T_co]
    batch_size: Optional[int]
    num_workers: int
    pin_memory: bool
    drop_last: bool
    timeout: float
    sampler: Sampler
    prefetch_factor: int
    _iterator : Optional['_BaseDataLoaderIter']
    __initialized = False
 
    def __init__(self, dataset: Dataset[T_co], batch_size: Optional[int] = 1,
                 shuffle: bool = False, sampler: Optional[Sampler[int]] = None,
                 batch_sampler: Optional[Sampler[Sequence[int]]] = None,
                 num_workers: int = 0, collate_fn: _collate_fn_t = None,
                 pin_memory: bool = False, drop_last: bool = False,
                 timeout: float = 0, worker_init_fn: _worker_init_fn_t = None,
                 multiprocessing_context=None, generator=None,
                 *, prefetch_factor: int = 2,
                 persistent_workers: bool = False):
        torch._C._log_api_usage_once("python.data_loader")  # type: ignore
 
        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')
 
        if num_workers == 0 and prefetch_factor != 2:
            raise ValueError('prefetch_factor option could only be specified in multiprocessing.'
                             'let num_workers > 0 to enable multiprocessing.')
        assert prefetch_factor > 0
 
        if persistent_workers and num_workers == 0:
            raise ValueError('persistent_workers option needs num_workers > 0')
 
        self.dataset = dataset
        self.num_workers = num_workers
        self.prefetch_factor = prefetch_factor
        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 drop_last:
                raise ValueError('batch_size=None option disables auto-batching '
                                 'and is mutually exclusive with 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:
                    # Cannot statically verify that dataset is Sized
                    # Somewhat related: see NOTE [ Lack of Default `__len__` in Python Abstract Base Classes ]
                    sampler = RandomSampler(dataset, generator=generator)  # type: ignore
                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
        self.generator = generator
 
        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.persistent_workers = persistent_workers
 
        self.__initialized = True
        self._IterableDataset_len_called = None  # See NOTE [ IterableDataset and __len__ ]
 
        self._iterator = None

源码传入参数主要如下所示:

DataLoader(dataset,  
           batch_size=1, # 每一批数据大小
           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)
 
# 功能: 构建可迭代的数据装载器
 
# dataset: Dataset类，决定数据从哪读取以及如何读取
# batchsize: 批大小
# num_works: 是否多进程读取数据
# shuffle: 每个epoch是否乱序
# drop_list: 当样本数不能被batchsize整除时，是否舍弃最后一批数据
 
 
 
# Epoch: 所有训练样本都以输入到模型中，称为一个Epoch
# Iteration: 一批样本输入到模型中，为一个Iteration
# Batchsize: 批大小，主要是决定一个Epoch有多少个Iteration
 
样本81， Batchsize=8;
 
1 Epoch = 10  drop_last=True
1 Epoch = 11  drop_last=False
 

Dataset
 
torch.utils.data.Dataset
 
功能: Dataset抽象类，所有自定义的Dataset需要继承它，并且复写
 
getitem: 接收一个索引，返回一个样本
 
 
class Dataset(Generic[T_co]):
    r"""An abstract class representing a :class:`Dataset`.
    All datasets that represent a map from keys to data samples should subclass
    it. All subclasses should overwrite :meth:`__getitem__`, supporting fetching a
    data sample for a given key. Subclasses could also optionally overwrite
    :meth:`__len__`, which is expected to return the size of the dataset by many
    :class:`~torch.utils.data.Sampler` implementations and the default options
    of :class:`~torch.utils.data.DataLoader`.
    .. note::
      :class:`~torch.utils.data.DataLoader` by default constructs a index
      sampler that yields integral indices.  To make it work with a map-style
      dataset with non-integral indices/keys, a custom sampler must be provided.
    """
 
    def __getitem__(self, index) -> T_co:
        raise NotImplementedError
 
    def __add__(self, other: 'Dataset[T_co]') -> 'ConcatDataset[T_co]':
        return ConcatDataset([self, other])
 
 
# 例子
class Dataset(object):
    
    def __getitem__(self, index):
 
        path_img, label = self.data_info[index]
        img = Image.open(path_img).convert('RGB')     # 0~255
 
        if self.transform is not None:
            img = self.transform(img)
 
        return img, label

1. 读那些数据 - Sampler输出的Index

2. 从哪读数据 - Dataset中的data_dir

3. 怎么读数据 - Dataset中的getitem