PyTorch Distributed Training: Fix 'Default Process Group Not Initialized' Error

This error occurs when the default process group has not been initialized in PyTorch distributed training. The default process group is essential for coordinating communication between different processes. To resolve this, you need to initialize the default process group before calling the 'restart_from_checkpoint' function. This can be achieved by using the 'init_process_group' function from the 'torch.distributed' package.

Here's an example of initializing the default process group:

import torch
import torch.distributed as dist

dist.init_process_group(backend='nccl', init_method='env://')

This code initializes the process group using the NCCL backend and an environment variable for the 'init_method'.

After initializing the process group, you should be able to call 'restart_from_checkpoint' without encountering the 'RuntimeError'.

Why It Might Work Initially and Fail Later

The issue might arise because the default process group was initialized during the first run. However, in subsequent runs, the process group might be closed or occupied by another process, leading to initialization failure. To handle this, consider:

• Manually Closing the Process Group: Explicitly close the process group after each run using 'dist.destroy_process_group()'.
• Using Different Process Groups: Initialize the process group with a different rank or world size for each run.
• Restarting the Computer: Restarting your computer ensures that all processes are properly closed, preventing potential conflicts.

By addressing these points and ensuring proper initialization of the default process group, you can successfully implement PyTorch distributed training.