- dataloader本质是一个可迭代对象，使用iter()访问，不能使用next()访问；
- 使用iter(dataloader)返回的是一个迭代器，然后可以使用next访问；
- 也可以使用`for inputs, labels in dataloaders`进行可迭代对象的访问；
- 一般我们实现一个datasets对象，传入到dataloader中；然后内部使用yeild返回每一次batch的数据；
① DataLoader本质上就是一个iterable（跟python的内置类型list等一样），并利用多进程来加速batch data的处理，使用yield来使用有限的内存 ② Queue的特点 当队列里面没有数据时： queue.get() 会阻塞， 阻塞的时候，其它进程/线程如果有queue.put() 操作，本线程/进程会被通知，然后就可以 get 成功。 当数据满了: queue.put() 会阻塞 ③ DataLoader是一个高效，简洁，直观的网络输入数据结构，便于使用和扩展
输入数据PipeLine
pytorch 的数据加载到模型的操作顺序是这样的：
① 创建一个 Dataset 对象
② 创建一个 DataLoader 对象
③ 循环这个 DataLoader 对象，将img, label加载到模型中进行训练
dataset = MyDataset()
dataloader = DataLoader(dataset)
num_epoches = 100
for epoch in range(num_epoches):
for img, label in dataloader:
....
所以，作为直接对数据进入模型中的关键一步， DataLoader非常重要。
首先简单介绍一下DataLoader，它是PyTorch中数据读取的一个重要接口，该接口定义在dataloader.py中，只要是用PyTorch来训练模型基本都会用到该接口（除非用户重写…），该接口的目的：将自定义的Dataset根据batch size大小、是否shuffle等封装成一个Batch Size大小的Tensor，用于后面的训练。
官方对DataLoader的说明是：“数据加载由数据集和采样器组成，基于python的单、多进程的iterators来处理数据。”关于iterator和iterable的区别和概念请自行查阅，在实现中的差别就是iterators有__iter__和__next__方法，而iterable只有__iter__方法。
1.DataLoader
先介绍一下DataLoader(object)的参数：
dataset(Dataset): 传入的数据集 batch_size(int, optional): 每个batch有多少个样本 shuffle(bool, optional): 在每个epoch开始的时候，对数据进行重新排序 sampler(Sampler, optional): 自定义从数据集中取样本的策略，如果指定这个参数，那么shuffle必须为False batch_sampler(Sampler, optional): 与sampler类似，但是一次只返回一个batch的indices（索引），需要注意的是，一旦指定了这个参数，那么batch_size,shuffle,sampler,drop_last就不能再制定了（互斥——Mutually exclusive） num_workers (int, optional): 这个参数决定了有几个进程来处理data loading。0意味着所有的数据都会被load进主进程。（默认为0） collate_fn (callable, optional): 将一个list的sample组成一个mini-batch的函数 pin_memory (bool, optional)： 如果设置为True，那么data loader将会在返回它们之前，将tensors拷贝到CUDA中的固定内存（CUDA pinned memory）中. drop_last (bool, optional): 如果设置为True：这个是对最后的未完成的batch来说的，比如你的batch_size设置为64，而一个epoch只有100个样本，那么训练的时候后面的36个就被扔掉了… 如果为False（默认），那么会继续正常执行，只是最后的batch_size会小一点。 timeout(numeric, optional): 如果是正数，表明等待从worker进程中收集一个batch等待的时间，若超出设定的时间还没有收集到，那就不收集这个内容了。这个numeric应总是大于等于0。默认为0 worker_init_fn (callable, optional): 每个worker初始化函数 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)
- 首先dataloader初始化时得到datasets的采样list
class DataLoader(object): r""" Data loader. Combines a dataset and a sampler, and provides single- or multi-process iterators over the dataset. 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, ``shuffle`` must be False. batch_sampler (Sampler, optional): like sampler, but returns a batch of indices at a time. Mutually exclusive with batch_size, shuffle, sampler, and 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. pin_memory (bool, optional): If ``True``, the data loader will copy tensors into CUDA pinned memory before returning them. 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) .. note:: By default, each worker will have its PyTorch seed set to ``base_seed + worker_id``, where ``base_seed`` is a long generated by main process using its RNG. However, seeds for other libraies may be duplicated upon initializing workers (w.g., NumPy), causing each worker to return identical random numbers. (See :ref:`dataloader-workers-random-seed` section in FAQ.) You may use ``torch.initial_seed()`` to access the PyTorch seed for each worker in :attr:`worker_init_fn`, and use it to set other seeds before data loading. .. warning:: If ``spawn`` start method is used, :attr:`worker_init_fn` cannot be an unpicklable object, e.g., a lambda function. """ __initialized = False def __init__(self, dataset, batch_size=1, shuffle=False, sampler=None, batch_sampler=None, num_workers=0, collate_fn=default_collate, pin_memory=False, drop_last=False, timeout=0, worker_init_fn=None): self.dataset = dataset self.batch_size = batch_size self.num_workers = num_workers self.collate_fn = collate_fn self.pin_memory = pin_memory self.drop_last = drop_last self.timeout = timeout self.worker_init_fn = worker_init_fn if timeout < 0: raise ValueError('timeout option should be non-negative') if batch_sampler is not None: 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') self.batch_size = None self.drop_last = None if sampler is not None and shuffle: raise ValueError('sampler option is mutually exclusive with ' 'shuffle') if self.num_workers < 0: raise ValueError('num_workers option cannot be negative; ' 'use num_workers=0 to disable multiprocessing.') if batch_sampler is None: if sampler is None: if shuffle: sampler = RandomSampler(dataset) //将list打乱 else: sampler = SequentialSampler(dataset) batch_sampler = BatchSampler(sampler, batch_size, drop_last) self.sampler = sampler self.batch_sampler = batch_sampler self.__initialized = True def __setattr__(self, attr, val): if self.__initialized and attr in ('batch_size', 'sampler', 'drop_last'): raise ValueError('{} attribute should not be set after {} is ' 'initialized'.format(attr, self.__class__.__name__)) super(DataLoader, self).__setattr__(attr, val) def __iter__(self): return _DataLoaderIter(self) def __len__(self): return len(self.batch_sampler)
其中：RandomSampler，BatchSampler已经得到了采用batch数据的index索引；yield batch机制已经在！！！
class RandomSampler(Sampler): r"""Samples elements randomly, without replacement. Arguments: data_source (Dataset): dataset to sample from """ def __init__(self, data_source): self.data_source = data_source def __iter__(self): return iter(torch.randperm(len(self.data_source)).tolist()) def __len__(self): return len(self.data_source)
class BatchSampler(Sampler): r"""Wraps another sampler to yield a mini-batch of indices. Args: sampler (Sampler): Base sampler. batch_size (int): Size of mini-batch. drop_last (bool): If ``True``, the sampler will drop the last batch if its size would be less than ``batch_size`` Example: >>> list(BatchSampler(SequentialSampler(range(10)), batch_size=3, drop_last=False)) [[0, 1, 2], [3, 4, 5], [6, 7, 8], [9]] >>> list(BatchSampler(SequentialSampler(range(10)), batch_size=3, drop_last=True)) [[0, 1, 2], [3, 4, 5], [6, 7, 8]] """ def __init__(self, sampler, batch_size, drop_last): if not isinstance(sampler, Sampler): raise ValueError("sampler should be an instance of " "torch.utils.data.Sampler, but got sampler={}" .format(sampler)) if not isinstance(batch_size, _int_classes) or isinstance(batch_size, bool) or batch_size <= 0: raise ValueError("batch_size should be a positive integeral value, " "but got batch_size={}".format(batch_size)) if not isinstance(drop_last, bool): raise ValueError("drop_last should be a boolean value, but got " "drop_last={}".format(drop_last)) self.sampler = sampler self.batch_size = batch_size self.drop_last = drop_last def __iter__(self): batch = [] for idx in self.sampler: batch.append(idx) if len(batch) == self.batch_size: yield batch batch = [] if len(batch) > 0 and not self.drop_last: yield batch def __len__(self): if self.drop_last: return len(self.sampler) // self.batch_size else: return (len(self.sampler) + self.batch_size - 1) // self.batch_size
- 其中 _DataLoaderIter(self)输入为一个dataloader对象；如果num_workers=0很好理解，num_workers！=0引入多线程机制，加速数据加载过程；
- 没有多线程时：batch = self.collate_fn([self.dataset[i] for i in indices])进行将index转化为data数据，返回(image,label);self.dataset[i]会调用datasets对象的
__getitem__()方法；
- 多线程下，会为每个线程创建一个索引队列index_queues；共享一个worker_result_queue数据队列！在_worker_loop方法中加载数据；
class _DataLoaderIter(object): r"""Iterates once over the DataLoader's dataset, as specified by the sampler""" def __init__(self, loader): self.dataset = loader.dataset self.collate_fn = loader.collate_fn self.batch_sampler = loader.batch_sampler self.num_workers = loader.num_workers self.pin_memory = loader.pin_memory and torch.cuda.is_available() self.timeout = loader.timeout self.done_event = threading.Event() self.sample_iter = iter(self.batch_sampler) base_seed = torch.LongTensor(1).random_().item() if self.num_workers > 0: self.worker_init_fn = loader.worker_init_fn self.index_queues = [multiprocessing.Queue() for _ in range(self.num_workers)] self.worker_queue_idx = 0 self.worker_result_queue = multiprocessing.SimpleQueue() self.batches_outstanding = 0 self.worker_pids_set = False self.shutdown = False self.send_idx = 0 self.rcvd_idx = 0 self.reorder_dict = {} self.workers = [ multiprocessing.Process( target=_worker_loop, args=(self.dataset, self.index_queues[i], self.worker_result_queue, self.collate_fn, base_seed + i, self.worker_init_fn, i)) for i in range(self.num_workers)] if self.pin_memory or self.timeout > 0: self.data_queue = queue.Queue() if self.pin_memory: maybe_device_id = torch.cuda.current_device() else: # do not initialize cuda context if not necessary maybe_device_id = None self.worker_manager_thread = threading.Thread( target=_worker_manager_loop, args=(self.worker_result_queue, self.data_queue, self.done_event, self.pin_memory, maybe_device_id)) self.worker_manager_thread.daemon = True self.worker_manager_thread.start() else: self.data_queue = self.worker_result_queue for w in self.workers: w.daemon = True # ensure that the worker exits on process exit w.start() _update_worker_pids(id(self), tuple(w.pid for w in self.workers)) _set_SIGCHLD_handler() self.worker_pids_set = True # prime the prefetch loop for _ in range(2 * self.num_workers): self._put_indices() def __len__(self): return len(self.batch_sampler) def _get_batch(self): if self.timeout > 0: try: return self.data_queue.get(timeout=self.timeout) except queue.Empty: raise RuntimeError('DataLoader timed out after {} seconds'.format(self.timeout)) else: return self.data_queue.get() def __next__(self): if self.num_workers == 0: # same-process loading indices = next(self.sample_iter) # may raise StopIteration batch = self.collate_fn([self.dataset[i] for i in indices]) if self.pin_memory: batch = pin_memory_batch(batch) return batch # check if the next sample has already been generated if self.rcvd_idx in self.reorder_dict: batch = self.reorder_dict.pop(self.rcvd_idx) return self._process_next_batch(batch) if self.batches_outstanding == 0: self._shutdown_workers() raise StopIteration while True: assert (not self.shutdown and self.batches_outstanding > 0) idx, batch = self._get_batch() self.batches_outstanding -= 1 if idx != self.rcvd_idx: # store out-of-order samples self.reorder_dict[idx] = batch continue return self._process_next_batch(batch) next = __next__ # Python 2 compatibility def __iter__(self): return self def _put_indices(self): assert self.batches_outstanding < 2 * self.num_workers indices = next(self.sample_iter, None) if indices is None: return self.index_queues[self.worker_queue_idx].put((self.send_idx, indices)) self.worker_queue_idx = (self.worker_queue_idx + 1) % self.num_workers self.batches_outstanding += 1 self.send_idx += 1 def _process_next_batch(self, batch): self.rcvd_idx += 1 self._put_indices() if isinstance(batch, ExceptionWrapper): raise batch.exc_type(batch.exc_msg) return batch
def _worker_loop(dataset, index_queue, data_queue, collate_fn, seed, init_fn, worker_id): global _use_shared_memory _use_shared_memory = True # Intialize C side signal handlers for SIGBUS and SIGSEGV. Python signal # module's handlers are executed after Python returns from C low-level # handlers, likely when the same fatal signal happened again already. # https://docs.python.org/3/library/signal.html Sec. 18.8.1.1 _set_worker_signal_handlers() torch.set_num_threads(1) random.seed(seed) torch.manual_seed(seed) if init_fn is not None: init_fn(worker_id) watchdog = ManagerWatchdog() while True: try: r = index_queue.get(timeout=MANAGER_STATUS_CHECK_INTERVAL) except queue.Empty: if watchdog.is_alive(): continue else: break if r is None: break idx, batch_indices = r try: samples = collate_fn([dataset[i] for i in batch_indices]) except Exception: data_queue.put((idx, ExceptionWrapper(sys.exc_info()))) else: data_queue.put((idx, samples)) del samples
- 需要对队列操作，缓存数据，使得加载提速！