OpenXExperienceReplay¶
- class torchrl.data.datasets.OpenXExperienceReplay(dataset_id, batch_size: int | None = None, *, shuffle: bool = True, num_slices: int | None = None, slice_len: int | None = None, pad: float | bool | None = None, replacement: bool = None, streaming: bool | None = None, root: str | Path | None = None, download: bool | None = None, sampler: Sampler | None = None, writer: Writer | None = None, collate_fn: Callable | None = None, pin_memory: bool = False, prefetch: int | None = None, transform: 'torchrl.envs.Transform' | None = None, split_trajs: bool = False, strict_length: bool = True)[source]¶
Open X-Embodiment 数据集的经验回放。
Open X-Embodiment 数据集包含超过 100 万条真实机器人轨迹,涵盖 22 种机器人载体,由 21 家机构合作收集,展示了 527 种技能(160266 项任务)。
网站:https://robotics-transformer-x.github.io/
GitHub:https://github.com/google-deepmind/open_x_embodiment
论文:https://arxiv.org/abs/2310.08864
数据格式遵循 TED 约定。
注意
非张量数据将使用
NonTensorData
原语写入 tensordict 数据中。例如,数据中的 language_instruction 字段将存储在 data.get_non_tensor(“language_instruction”) 中(或等效地存储在 data.get(“language_instruction”).data 中)。有关如何与存储在TensorDict
中的非张量数据交互的更多信息,请参阅此类的文档。- 参数:
dataset_id (str) – 要下载的数据集。必须是
OpenXExperienceReplay.available_datasets
的一部分。batch_size (int) – 采样期间使用的批量大小。如有必要,可以通过调用 data.sample(batch_size) 覆盖此值。有关更精细的采样策略,请参阅
num_slices
和slice_len
关键字参数。如果batch_size
为None
(默认),遍历数据集将一次提供一条轨迹,而 调用sample()
仍然需要提供批量大小。
- 关键字参数:
shuffle (bool, optional) –
如果为
True
,遍历数据集时,轨迹将以随机顺序提供。如果为False
,数据集将以预定义顺序遍历。警告
shuffle=False 也会影响采样。如果用户希望在同一代码库中体验两种不同的行为(随机和非随机),我们建议他们创建一个数据集副本,并将采样器的
shuffle
属性设置为False
。num_slices (int, optional) – 一个批量中的切片数量。这对应于一个批量中存在的轨迹数量。收集后,批量将呈现为子轨迹的连接,可以通过 batch.reshape(num_slices, -1) 恢复。如果提供,batch_size 必须能被 num_slices 整除。此参数与
slice_len
互斥。如果num_slices
参数等于batch_size
,每个样本将属于一条不同的轨迹。如果既未提供slice_len
也未提供num_slices
:每当轨迹长度短于批量大小时,将采样其长度为 batch_size 的连续切片。如果轨迹长度不足,将引发异常,除非 pad 不是 None。slice_len (int, optional) –
一个批量中的切片长度。这对应于一个批量中存在的轨迹长度。收集后,批量将呈现为子轨迹的连接,可以通过 batch.reshape(-1, slice_len) 恢复。如果提供,batch_size 必须能被 slice_len 整除。此参数与
num_slices
互斥。如果slice_len
参数等于1
,每个样本将属于一条不同的轨迹。如果既未提供slice_len
也未提供num_slices
:每当轨迹长度短于批量大小时,将采样其长度为 batch_size 的连续切片。如果轨迹长度不足,将引发异常,除非 pad 不是 None。注意
slice_len
(但不能使用num_slices
)可在遍历数据集时使用,而无需在构造函数中传入批量大小。在这些情况下,将选择轨迹的随机子序列。replacement (bool, optional) – 如果为
False
,将进行无放回采样。对于已下载数据集默认为True
,对于流式数据集默认为False
。pad (bool,
float
or None) – 如果为True
,对于根据 slice_len 或 num_slices 参数长度不足的轨迹,将用 0 填充。如果提供了其他值,将用于填充。如果为False
或None
(默认),遇到长度不足的轨迹将引发异常。root (Path or str, optional) – OpenX 数据集的根目录。实际的数据集内存映射文件将保存在 <root>/<dataset_id> 下。如果未提供,默认为 ~/.cache/torchrl/atari.openx`。
streaming (bool, optional) –
如果为
True
,数据将不会被下载,而是从流中读取。注意
当 download=True 与 streaming=True 相比时,数据的格式__将 改变__。如果数据已下载且采样器未被触动(即 num_slices=None、slice_len=None 和 sampler=None),将从数据集中随机采样转换。使用 streaming=True 无法以合理的成本做到这一点:在这种情况下,轨迹将一次一个地被采样并交付(并进行裁剪以符合批量大小等)。当指定 num_slices 和 slice_len 时,这两种模式的行为更加相似,因为在这些情况下,两种情况都会返回子片段的视图。
download (bool or str, optional) – 如果未找到数据集,是否应该下载。默认为
True
。下载也可以传递为 “force”,在这种情况下,已下载的数据将被覆盖。sampler (Sampler, optional) – 要使用的采样器。如果未提供,将使用默认的 RandomSampler()。
writer (Writer, optional) – 要使用的写入器。如果未提供,将使用默认的
ImmutableDatasetWriter
。collate_fn (callable, optional) – 合并样本列表以形成 Tensor(s)/输出的迷你批量。在使用 map-style 数据集进行批量加载时使用。
pin_memory (bool) – 是否应该对 rb 样本调用
pin_memory()
。prefetch (int, optional) – 使用多线程预取的下一个批量的数量。
transform (Transform, optional) – 调用 sample() 时要执行的变换。要链式调用变换,请使用
Compose
类。split_trajs (bool, optional) – 如果为
True
,轨迹将沿第一个维度分割并填充以具有匹配的形状。为了分割轨迹,将使用"done"
信号,该信号通过done = truncated | terminated
恢复。换句话说,假定任何truncated
或terminated
信号都等同于轨迹的结束。默认为False
。strict_length (bool, optional) – 如果为
False
,允许长度短于 slice_len(或 batch_size // num_slices)的轨迹出现在批量中。请注意,这可能导致实际的 batch_size 短于请求的大小!可以使用torchrl.collectors.split_trajectories()
分割轨迹。默认为True
。
示例
>>> from torchrl.data.datasets import OpenXExperienceReplay >>> import tempfile >>> # Download the data, and sample 128 elements in each batch out of two trajectories >>> num_slices = 2 >>> with tempfile.TemporaryDirectory() as root: ... dataset = OpenXExperienceReplay("cmu_stretch", batch_size=128, ... num_slices=num_slices, download=True, streaming=False, ... root=root, ... ) ... for batch in dataset: ... print(batch.reshape(num_slices, -1)) ... break TensorDict( fields={ action: Tensor(shape=torch.Size([2, 64, 8]), device=cpu, dtype=torch.float64, is_shared=False), discount: Tensor(shape=torch.Size([2, 64]), device=cpu, dtype=torch.float32, is_shared=False), done: Tensor(shape=torch.Size([2, 64, 1]), device=cpu, dtype=torch.bool, is_shared=False), episode: Tensor(shape=torch.Size([2, 64]), device=cpu, dtype=torch.int32, is_shared=False), index: Tensor(shape=torch.Size([2, 64]), device=cpu, dtype=torch.int64, is_shared=False), is_init: Tensor(shape=torch.Size([2, 64]), device=cpu, dtype=torch.bool, is_shared=False), language_embedding: Tensor(shape=torch.Size([2, 64, 512]), device=cpu, dtype=torch.float64, is_shared=False), language_instruction: NonTensorData( data='lift open green garbage can lid', batch_size=torch.Size([2, 64]), device=cpu, is_shared=False), next: TensorDict( fields={ done: Tensor(shape=torch.Size([2, 64, 1]), device=cpu, dtype=torch.bool, is_shared=False), observation: TensorDict( fields={ image: Tensor(shape=torch.Size([2, 64, 3, 128, 128]), device=cpu, dtype=torch.uint8, is_shared=False), state: Tensor(shape=torch.Size([2, 64, 4]), device=cpu, dtype=torch.float64, is_shared=False)}, batch_size=torch.Size([2, 64]), device=cpu, is_shared=False), reward: Tensor(shape=torch.Size([2, 64, 1]), device=cpu, dtype=torch.float32, is_shared=False), terminated: Tensor(shape=torch.Size([2, 64, 1]), device=cpu, dtype=torch.bool, is_shared=False), truncated: Tensor(shape=torch.Size([2, 64, 1]), device=cpu, dtype=torch.bool, is_shared=False)}, batch_size=torch.Size([2, 64]), device=cpu, is_shared=False), observation: TensorDict( fields={ image: Tensor(shape=torch.Size([2, 64, 3, 128, 128]), device=cpu, dtype=torch.uint8, is_shared=False), state: Tensor(shape=torch.Size([2, 64, 4]), device=cpu, dtype=torch.float64, is_shared=False)}, batch_size=torch.Size([2, 64]), device=cpu, is_shared=False), terminated: Tensor(shape=torch.Size([2, 64, 1]), device=cpu, dtype=torch.bool, is_shared=False), truncated: Tensor(shape=torch.Size([2, 64, 1]), device=cpu, dtype=torch.bool, is_shared=False)}, batch_size=torch.Size([2, 64]), device=cpu, is_shared=False) >>> # Read data from a stream. Deliver entire trajectories when iterating >>> dataset = OpenXExperienceReplay("cmu_stretch", ... num_slices=num_slices, download=False, streaming=True) >>> for data in dataset: # data does not have a consistent shape ... break >>> # Define batch-size dynamically >>> data = dataset.sample(128) # delivers 2 sub-trajectories of length 64
- add(data: TensorDictBase) int ¶
向经验回放缓冲区添加一个元素。
- 参数:
data (Any) – 要添加到经验回放缓冲区的数据
- 返回:
数据在经验回放缓冲区中的索引。
- append_transform(transform: Transform, *, invert: bool = False) ReplayBuffer ¶
在末尾添加变换。
调用 sample 时,变换按顺序应用。
- 参数:
transform (Transform) – 要添加的变换
- 关键字参数:
invert (bool, optional) – 如果为
True
,变换将被反转(写入期间调用 forward,读取期间调用 inverse)。默认为False
。
示例
>>> rb = ReplayBuffer(storage=LazyMemmapStorage(10), batch_size=4) >>> data = TensorDict({"a": torch.zeros(10)}, [10]) >>> def t(data): ... data += 1 ... return data >>> rb.append_transform(t, invert=True) >>> rb.extend(data) >>> assert (data == 1).all()
- property data_path¶
数据集路径,包括分割信息。
- property data_path_root¶
数据集根目录路径。
- delete()¶
从磁盘删除数据集存储。
- dumps(path)¶
将经验回放缓冲区保存到指定路径的磁盘上。
- 参数:
path (Path or str) – 保存经验回放缓冲区的路径。
示例
>>> import tempfile >>> import tqdm >>> from torchrl.data import LazyMemmapStorage, TensorDictReplayBuffer >>> from torchrl.data.replay_buffers.samplers import PrioritizedSampler, RandomSampler >>> import torch >>> from tensordict import TensorDict >>> # Build and populate the replay buffer >>> S = 1_000_000 >>> sampler = PrioritizedSampler(S, 1.1, 1.0) >>> # sampler = RandomSampler() >>> storage = LazyMemmapStorage(S) >>> rb = TensorDictReplayBuffer(storage=storage, sampler=sampler) >>> >>> for _ in tqdm.tqdm(range(100)): ... td = TensorDict({"obs": torch.randn(100, 3, 4), "next": {"obs": torch.randn(100, 3, 4)}, "td_error": torch.rand(100)}, [100]) ... rb.extend(td) ... sample = rb.sample(32) ... rb.update_tensordict_priority(sample) >>> # save and load the buffer >>> with tempfile.TemporaryDirectory() as tmpdir: ... rb.dumps(tmpdir) ... ... sampler = PrioritizedSampler(S, 1.1, 1.0) ... # sampler = RandomSampler() ... storage = LazyMemmapStorage(S) ... rb_load = TensorDictReplayBuffer(storage=storage, sampler=sampler) ... rb_load.loads(tmpdir) ... assert len(rb) == len(rb_load)
- empty()¶
清空经验回放缓冲区并将游标重置为 0。
- extend(tensordicts: TensorDictBase) Tensor ¶
使用迭代器中包含的一个或多个元素扩展经验回放缓冲区。
如果存在,将调用逆变换。
- 参数:
data (iterable) – 要添加到经验回放缓冲区的数据集合。
- 返回:
添加到经验回放缓冲区的数据的索引。
警告
extend()
在处理值列表时可能具有模糊的签名,这些值列表应被解释为 PyTree(在这种情况下,列表中的所有元素将被放入存储中保存的 PyTree 的一个切片中)或要逐个添加的值列表。为了解决这个问题,TorchRL 明确区分了 list 和 tuple:tuple 将被视为 PyTree,list(在根级别)将被解释为要逐个添加到缓冲区的值堆栈。对于ListStorage
实例,只能提供未绑定的元素(不能提供 PyTrees)。
- insert_transform(index: int, transform: Transform, *, invert: bool = False) ReplayBuffer ¶
插入变换。
调用 sample 时,变换按顺序执行。
- 参数:
index (int) – 插入变换的位置。
transform (Transform) – 要添加的变换
- 关键字参数:
invert (bool, optional) – 如果为
True
,变换将被反转(写入期间调用 forward,读取期间调用 inverse)。默认为False
。
- loads(path)¶
从给定路径加载经验回放缓冲区状态。
缓冲区应具有匹配的组件并使用
dumps()
保存。- 参数:
path (Path or str) – 经验回放缓冲区保存的路径。
有关更多信息,请参阅
dumps()
。
- preprocess(fn: Callable[[TensorDictBase], TensorDictBase], dim: int = 0, num_workers: int | None = None, *, chunksize: int | None = None, num_chunks: int | None = None, pool: mp.Pool | None = None, generator: torch.Generator | None = None, max_tasks_per_child: int | None = None, worker_threads: int = 1, index_with_generator: bool = False, pbar: bool = False, mp_start_method: str | None = None, num_frames: int | None = None, dest: str | Path) TensorStorage ¶
预处理数据集并返回包含格式化数据的新存储。
数据转换必须是单元的(作用于数据集的单个样本)。
参数和关键字参数将转发给
map()
。随后可以使用
delete()
删除数据集。- 关键字参数:
dest (path 或 等效类型) – 新数据集的存储位置路径。
num_frames (int, 可选) – 如果提供,将仅转换前 num_frames 个帧。这对于初步调试转换很有用。
返回: 可在
ReplayBuffer
实例中使用的新的存储。示例
>>> from torchrl.data.datasets import MinariExperienceReplay >>> >>> data = MinariExperienceReplay( ... list(MinariExperienceReplay.available_datasets)[0], ... batch_size=32 ... ) >>> print(data) MinariExperienceReplay( storages=TensorStorage(TensorDict( fields={ action: MemoryMappedTensor(shape=torch.Size([1000000, 8]), device=cpu, dtype=torch.float32, is_shared=True), episode: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.int64, is_shared=True), info: TensorDict( fields={ distance_from_origin: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), forward_reward: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), goal: MemoryMappedTensor(shape=torch.Size([1000000, 2]), device=cpu, dtype=torch.float64, is_shared=True), qpos: MemoryMappedTensor(shape=torch.Size([1000000, 15]), device=cpu, dtype=torch.float64, is_shared=True), qvel: MemoryMappedTensor(shape=torch.Size([1000000, 14]), device=cpu, dtype=torch.float64, is_shared=True), reward_ctrl: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), reward_forward: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), reward_survive: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), success: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.bool, is_shared=True), x_position: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), x_velocity: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), y_position: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), y_velocity: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True)}, batch_size=torch.Size([1000000]), device=cpu, is_shared=False), next: TensorDict( fields={ done: MemoryMappedTensor(shape=torch.Size([1000000, 1]), device=cpu, dtype=torch.bool, is_shared=True), info: TensorDict( fields={ distance_from_origin: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), forward_reward: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), goal: MemoryMappedTensor(shape=torch.Size([1000000, 2]), device=cpu, dtype=torch.float64, is_shared=True), qpos: MemoryMappedTensor(shape=torch.Size([1000000, 15]), device=cpu, dtype=torch.float64, is_shared=True), qvel: MemoryMappedTensor(shape=torch.Size([1000000, 14]), device=cpu, dtype=torch.float64, is_shared=True), reward_ctrl: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), reward_forward: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), reward_survive: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), success: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.bool, is_shared=True), x_position: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), x_velocity: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), y_position: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True), y_velocity: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.float64, is_shared=True)}, batch_size=torch.Size([1000000]), device=cpu, is_shared=False), observation: TensorDict( fields={ achieved_goal: MemoryMappedTensor(shape=torch.Size([1000000, 2]), device=cpu, dtype=torch.float64, is_shared=True), desired_goal: MemoryMappedTensor(shape=torch.Size([1000000, 2]), device=cpu, dtype=torch.float64, is_shared=True), observation: MemoryMappedTensor(shape=torch.Size([1000000, 27]), device=cpu, dtype=torch.float64, is_shared=True)}, batch_size=torch.Size([1000000]), device=cpu, is_shared=False), reward: MemoryMappedTensor(shape=torch.Size([1000000, 1]), device=cpu, dtype=torch.float64, is_shared=True), terminated: MemoryMappedTensor(shape=torch.Size([1000000, 1]), device=cpu, dtype=torch.bool, is_shared=True), truncated: MemoryMappedTensor(shape=torch.Size([1000000, 1]), device=cpu, dtype=torch.bool, is_shared=True)}, batch_size=torch.Size([1000000]), device=cpu, is_shared=False), observation: TensorDict( fields={ achieved_goal: MemoryMappedTensor(shape=torch.Size([1000000, 2]), device=cpu, dtype=torch.float64, is_shared=True), desired_goal: MemoryMappedTensor(shape=torch.Size([1000000, 2]), device=cpu, dtype=torch.float64, is_shared=True), observation: MemoryMappedTensor(shape=torch.Size([1000000, 27]), device=cpu, dtype=torch.float64, is_shared=True)}, batch_size=torch.Size([1000000]), device=cpu, is_shared=False)}, batch_size=torch.Size([1000000]), device=cpu, is_shared=False)), samplers=RandomSampler, writers=ImmutableDatasetWriter(), batch_size=32, transform=Compose( ), collate_fn=<function _collate_id at 0x120e21dc0>) >>> from torchrl.envs import CatTensors, Compose >>> from tempfile import TemporaryDirectory >>> >>> cat_tensors = CatTensors( ... in_keys=[("observation", "observation"), ("observation", "achieved_goal"), ... ("observation", "desired_goal")], ... out_key="obs" ... ) >>> cat_next_tensors = CatTensors( ... in_keys=[("next", "observation", "observation"), ... ("next", "observation", "achieved_goal"), ... ("next", "observation", "desired_goal")], ... out_key=("next", "obs") ... ) >>> t = Compose(cat_tensors, cat_next_tensors) >>> >>> def func(td): ... td = td.select( ... "action", ... "episode", ... ("next", "done"), ... ("next", "observation"), ... ("next", "reward"), ... ("next", "terminated"), ... ("next", "truncated"), ... "observation" ... ) ... td = t(td) ... return td >>> with TemporaryDirectory() as tmpdir: ... new_storage = data.preprocess(func, num_workers=4, pbar=True, mp_start_method="fork", dest=tmpdir) ... rb = ReplayBuffer(storage=new_storage) ... print(rb) ReplayBuffer( storage=TensorStorage( data=TensorDict( fields={ action: MemoryMappedTensor(shape=torch.Size([1000000, 8]), device=cpu, dtype=torch.float32, is_shared=True), episode: MemoryMappedTensor(shape=torch.Size([1000000]), device=cpu, dtype=torch.int64, is_shared=True), next: TensorDict( fields={ done: MemoryMappedTensor(shape=torch.Size([1000000, 1]), device=cpu, dtype=torch.bool, is_shared=True), obs: MemoryMappedTensor(shape=torch.Size([1000000, 31]), device=cpu, dtype=torch.float64, is_shared=True), observation: TensorDict( fields={ }, batch_size=torch.Size([1000000]), device=cpu, is_shared=False), reward: MemoryMappedTensor(shape=torch.Size([1000000, 1]), device=cpu, dtype=torch.float64, is_shared=True), terminated: MemoryMappedTensor(shape=torch.Size([1000000, 1]), device=cpu, dtype=torch.bool, is_shared=True), truncated: MemoryMappedTensor(shape=torch.Size([1000000, 1]), device=cpu, dtype=torch.bool, is_shared=True)}, batch_size=torch.Size([1000000]), device=cpu, is_shared=False), obs: MemoryMappedTensor(shape=torch.Size([1000000, 31]), device=cpu, dtype=torch.float64, is_shared=True), observation: TensorDict( fields={ }, batch_size=torch.Size([1000000]), device=cpu, is_shared=False)}, batch_size=torch.Size([1000000]), device=cpu, is_shared=False), shape=torch.Size([1000000]), len=1000000, max_size=1000000), sampler=RandomSampler(), writer=RoundRobinWriter(cursor=0, full_storage=True), batch_size=None, collate_fn=<function _collate_id at 0x168406fc0>)
- register_load_hook(hook: Callable[[Any], Any])¶
注册存储的加载钩子。
注意
钩子目前在保存回放缓冲区时不会被序列化:每次创建缓冲区时必须手动重新初始化它们。
- register_save_hook(hook: Callable[[Any], Any])¶
注册存储的保存钩子。
注意
钩子目前在保存回放缓冲区时不会被序列化:每次创建缓冲区时必须手动重新初始化它们。
- sample(batch_size: int | None = None, return_info: bool = False, include_info: bool = None) TensorDictBase ¶
从回放缓冲区中采样一批数据。
使用 Sampler 采样索引,并从 Storage 中检索它们。
- 参数:
batch_size (int, 可选) – 要收集的数据大小。如果未提供,此方法将按采样器指示的批量大小进行采样。
return_info (bool) – 是否返回信息 (info)。如果为 True,结果为元组 (data, info)。如果为 False,结果为 data。
- 返回:
一个 TensorDict,包含在回放缓冲区中选择的一批数据。如果 return_info 标志设置为 True,则是一个包含此 TensorDict 和 info 的元组。
- set_storage(storage: Storage, collate_fn: Callable | None = None)¶
在回放缓冲区中设置新的存储并返回之前的存储。
- 参数:
storage (Storage) – 缓冲区的新存储。
collate_fn (callable, 可选) – 如果提供,则将 collate_fn 设置为此值。否则,它将被重置为默认值。
- property write_count¶
通过 add 和 extend 方法已写入缓冲区的总条目数。