# Multiprocessing package - torch.multiprocessing¶

torch.multiprocessing is a wrapper around the native multiprocessing module. It registers custom reducers, that use shared memory to provide shared views on the same data in different processes. Once the tensor/storage is moved to shared_memory (see share_memory_()), it will be possible to send it to other processes without making any copies.

The API is 100% compatible with the original module - it’s enough to change import multiprocessing to import torch.multiprocessing to have all the tensors sent through the queues or shared via other mechanisms, moved to shared memory.

Because of the similarity of APIs we do not document most of this package contents, and we recommend referring to very good docs of the original module.

Warning

If the main process exits abruptly (e.g. because of an incoming signal), Python’s multiprocessing sometimes fails to clean up its children. It’s a known caveat, so if you’re seeing any resource leaks after interrupting the interpreter, it probably means that this has just happened to you.

## Strategy management¶

torch.multiprocessing.get_all_sharing_strategies()[source]

Returns a set of sharing strategies supported on a current system.

torch.multiprocessing.get_sharing_strategy()[source]

Returns the current strategy for sharing CPU tensors.

torch.multiprocessing.set_sharing_strategy(new_strategy)[source]

Sets the strategy for sharing CPU tensors.

Parameters: new_strategy (str) – Name of the selected strategy. Should be one of the values returned by get_all_sharing_strategies().

## Sharing CUDA tensors¶

Sharing CUDA tensors between processes is supported only in Python 3, using a spawn or forkserver start methods. multiprocessing in Python 2 can only create subprocesses using fork, and it’s not supported by the CUDA runtime.

Warning

CUDA API requires that the allocation exported to other processes remains valid as long as it’s used by them. You should be careful and ensure that CUDA tensors you shared don’t go out of scope as long as it’s necessary. This shouldn’t be a problem for sharing model parameters, but passing other kinds of data should be done with care. Note that this restriction doesn’t apply to shared CPU memory.

## Sharing strategies¶

This section provides a brief overview into how different sharing strategies work. Note that it applies only to CPU tensor - CUDA tensors will always use the CUDA API, as that’s the only way they can be shared.

### File descriptor - file_descriptor¶

Note

This is the default strategy (except for macOS and OS X where it’s not supported).

This strategy will use file descriptors as shared memory handles. Whenever a storage is moved to shared memory, a file descriptor obtained from shm_open is cached with the object, and when it’s going to be sent to other processes, the file descriptor will be transferred (e.g. via UNIX sockets) to it. The receiver will also cache the file descriptor and mmap it, to obtain a shared view onto the storage data.

Note that if there will be a lot of tensors shared, this strategy will keep a large number of file descriptors open most of the time. If your system has low limits for the number of open file descriptors, and you can’t rise them, you should use the file_system strategy.

### File system - file_system¶

This strategy will use file names given to shm_open to identify the shared memory regions. This has a benefit of not requiring the implementation to cache the file descriptors obtained from it, but at the same time is prone to shared memory leaks. The file can’t be deleted right after its creation, because other processes need to access it to open their views. If the processes fatally crash, or are killed, and don’t call the storage destructors, the files will remain in the system. This is very serious, because they keep using up the memory until the system is restarted, or they’re freed manually.

To counter the problem of shared memory file leaks, torch.multiprocessing will spawn a daemon named torch_shm_manager that will isolate itself from the current process group, and will keep track of all shared memory allocations. Once all processes connected to it exit, it will wait a moment to ensure there will be no new connections, and will iterate over all shared memory files allocated by the group. If it finds that any of them still exist, they will be deallocated. We’ve tested this method and it proved to be robust to various failures. Still, if your system has high enough limits, and file_descriptor is a supported strategy, we do not recommend switching to this one.