detectron/detectron2/engine/launch.py

# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
import logging
import torch
import torch.distributed as dist
import torch.multiprocessing as mp

from detectron2.utils import comm

__all__ = ["launch"]


def _find_free_port():
    import socket

    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    # Binding to port 0 will cause the OS to find an available port for us
    sock.bind(("", 0))
    port = sock.getsockname()[1]
    sock.close()
    # NOTE: there is still a chance the port could be taken by other processes.
    return port


def launch(main_func, num_gpus_per_machine, num_machines=1, machine_rank=0, dist_url=None, args=()):
    """
    Args:
        main_func: a function that will be called by `main_func(*args)`
        num_machines (int): the total number of machines
        machine_rank (int): the rank of this machine (one per machine)
        dist_url (str): url to connect to for distributed training, including protocol
                       e.g. "tcp://127.0.0.1:8686".
                       Can be set to auto to automatically select a free port on localhost
        args (tuple): arguments passed to main_func
    """
    world_size = num_machines * num_gpus_per_machine
    if world_size > 1:
        # https://github.com/pytorch/pytorch/pull/14391
        # TODO prctl in spawned processes

        if dist_url == "auto":
            assert num_machines == 1, "dist_url=auto cannot work with distributed training."
            port = _find_free_port()
            dist_url = f"tcp://127.0.0.1:{port}"

        mp.spawn(
            _distributed_worker,
            nprocs=num_gpus_per_machine,
            args=(main_func, world_size, num_gpus_per_machine, machine_rank, dist_url, args),
            daemon=False,
        )
    else:
        main_func(*args)


def _distributed_worker(
    local_rank, main_func, world_size, num_gpus_per_machine, machine_rank, dist_url, args
):
    assert torch.cuda.is_available(), "cuda is not available. Please check your installation."
    global_rank = machine_rank * num_gpus_per_machine + local_rank
    try:
        dist.init_process_group(
            backend="NCCL", init_method=dist_url, world_size=world_size, rank=global_rank
        )
    except Exception as e:
        logger = logging.getLogger(__name__)
        logger.error("Process group URL: {}".format(dist_url))
        raise e
    # synchronize is needed here to prevent a possible timeout after calling init_process_group
    # See: https://github.com/facebookresearch/maskrcnn-benchmark/issues/172
    comm.synchronize()

    assert num_gpus_per_machine <= torch.cuda.device_count()
    torch.cuda.set_device(local_rank)

    # Setup the local process group (which contains ranks within the same machine)
    assert comm._LOCAL_PROCESS_GROUP is None
    num_machines = world_size // num_gpus_per_machine
    for i in range(num_machines):
        ranks_on_i = list(range(i * num_gpus_per_machine, (i + 1) * num_gpus_per_machine))
        pg = dist.new_group(ranks_on_i)
        if i == machine_rank:
            comm._LOCAL_PROCESS_GROUP = pg

    main_func(*args)