class torchdata.dataloader2.MultiProcessingReadingService(num_workers: int = 0, multiprocessing_context: Optional[str] = None, worker_prefetch_cnt: int = 10, main_prefetch_cnt: int = 10, worker_init_fn: Optional[Callable[[Union[IterDataPipe, MapDataPipe], WorkerInfo], Union[IterDataPipe, MapDataPipe]]] = None, worker_reset_fn: Optional[Callable[[Union[IterDataPipe, MapDataPipe], WorkerInfo, SeedGenerator], Union[IterDataPipe, MapDataPipe]]] = None)

Spawns multiple worker processes to load data from the DataPipe graph. If any non-replicable DataPipe (sharding_round_robin_dispatch) is presented in the graph, a separate dispatching process will be created to load data from the lowest common ancestor of all non-replicable DataPipes and distributes data to each worker process in the round-robin manner Then, the subsequent DataPipe graph in each worker process will process the data from the dispatching process and eventually return the result to the main process.

  • num_workers (int, optional) – How many subprocesses to use for data loading. 0 will be replaced by InProcessReadingService in the future.

  • multiprocessing_context (str, optional) – Multiprocessing starting method. If method is None then the default context is returned. Otherwise, method should be ‘fork’, ‘spawn’.

  • worker_prefetch_cnt – (int, 10 by default): Number of data will be prefetched at the end of each worker process.

  • main_prefetch_cnt – (int, 10 by default): Number of data will be prefetched at the end of the whole pipeline in the main process.

  • worker_init_fn – (Callable, optional): Function to be called when each worker process launches with DataPipe and WorkerInfo as the expected arguments.

  • worker_reset_fn – (Callable, optional): Function to be called at the beginning of each epoch in each worker process with DataPipe, WorkerInfo and SeedGenerator as the expected arguments.

finalize() None

MultiProcessingReadingService invalidate states & properly exits all subprocesses.

initialize(datapipe: Union[IterDataPipe, MapDataPipe]) Union[IterDataPipe, MapDataPipe]

MultiProcessingReadingService finds information about sharding, separates graph by multiple pieces and reconnects it using queues. creates subprocesses.

initialize_iteration(seed_generator: SeedGenerator, iter_reset_fn: Optional[Callable[[Union[IterDataPipe, MapDataPipe]], Union[IterDataPipe, MapDataPipe]]] = None) Optional[Callable[[Union[IterDataPipe, MapDataPipe]], Union[IterDataPipe, MapDataPipe]]]

ReadingService spins up service for an epoch. Called at the beginning of every time getting DataLoader2 iterator.

  • seed_generator – SeedGenerator object created and managed by DataLoader2. As the single source of randomness, it will govern the determinism for all of random operations with the graph of DataPipes.

  • iter_reset_fn – Optional reset function from the prior ReadingServcie when SequentialReadingService chains multiple ReadingServices


A new iter_reset_fn to be used by subseqeuent ReadingService


MultiProcessingReadingService starts setting worker seeds per process and prefetching items from the graph.


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