Demonstration of the MPS hang bug

mpshang.py

"""This script should be run on a machine with at least 2 GPUs and an MPS server running. You can launch an MPS daemon with
```
nvidia-cuda-mps-control -d
```
The script first uses `test_cuda` to verify a CUDA context can be created on each GPU. It then spawns two workers; a 
'good' worker and a 'bad' worker. The workers collaborate through Pytorch's DataDistributedParallel module to calculate
the gradient for a trivial computation. The 'good' worker carries out both the forward and backward pass, while the 
bad worker carries out the forward pass and then exits. This seems to lock up the MPS server, and any subsequent 
attempts to create CUDA contexts fail by hanging eternally.

```
PyTorch version: 1.4.0
Is debug build: No
CUDA used to build PyTorch: 10.1

OS: Ubuntu 18.04.3 LTS
GCC version: (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
CMake version: Could not collect

Python version: 3.7
Is CUDA available: Yes
CUDA runtime version: 10.1.243
GPU models and configuration: 
GPU 0: GeForce RTX 2080 Ti
GPU 1: GeForce RTX 2080 Ti

Nvidia driver version: 440.26
cuDNN version: Could not collect

Versions of relevant libraries:
[pip] numpy==1.18.1
[pip] torch==1.4.0
[pip] torchfile==0.1.0
[pip] torchvision==0.5.0
[conda] blas                      1.0                         mkl  
[conda] mkl                       2019.4                      243  
[conda] mkl-service               2.3.0            py37he904b0f_0  
[conda] mkl_fft                   1.0.15           py37ha843d7b_0  
[conda] mkl_random                1.1.0            py37hd6b4f25_0  
[conda] pytorch                   1.4.0           py3.7_cuda10.1.243_cudnn7.6.3_0    pytorch
[conda] torchfile                 0.1.0                    pypi_0    pypi
[conda] torchvision               0.5.0                py37_cu101    pytorch
```
"""
import os
import torch
import time
from torch.nn.parallel import DistributedDataParallel as DDP
from torch import distributed as dist
from torch import multiprocessing as mp
from torch import nn

def _test_cuda(device):
    import torch
    torch.tensor([0]).to(device)

def test_cuda():
    """Tests whether creating a CUDA context hangs on any device by creating a subprocess for each 
    device and having them call the minimal `_test_cuda` function. If a subprocess doesn't 
    terminate in 10s, the corresponding device has hung."""

    procs = []
    for device in range(torch.cuda.device_count()):
        proc = mp.Process(target=_test_cuda, args=(device,))
        proc.start()
        procs.append(proc)
    
    for _ in range(10):
        status = ['checking' if p.is_alive() else 'ready' for p in procs]
        if all(s == 'ready' for s in status):
            print(f"CUDA ready")
            return
        time.sleep(1)
    else:
        raise ValueError(f"One of the CUDA checks overrun: {', '.join(status)}")

def _setup(rank):
    """Create a trivial network and batch on each device, preparing to go forward/backward with
    them via DDP"""

    os.environ['MASTER_ADDR'] = '127.0.0.1'
    os.environ['MASTER_PORT'] = '29500'
    torch.cuda.set_device(rank)
    dist.init_process_group("nccl", rank=rank, world_size=2)

    net = DDP(nn.Linear(1, 1).cuda(), device_ids=[rank])
    batch = torch.zeros(1).cuda()

    return net, batch

def bad_worker():
    """This worker goes forward and then exits, leaving the good worker hanging"""
    print('Bad worker starting')
    net, batch = _setup(0)
    print('Bad worker set up')

    loss = net(batch).sum()
    print('Bad worker finished')

def good_worker():
    """This worker goes forward and then tries to go backwards. It'll be left hanging by the bad worker"""
    print('Good worker starting')
    net, batch = _setup(1)
    print('Good worker set up')

    loss = net(batch).sum()
    loss.backward()
    print('Good worker finished')

def run():
    test_cuda()

    bad = mp.Process(target=bad_worker)
    good = mp.Process(target=good_worker)

    bad.start()
    good.start()
    
    bad.join()
    print('Bad joined')
    good.join()
    print('Good joined')

    test_cuda()

if __name__ == '__main__':
    mp.set_start_method('forkserver')
    run()

Issue thread is here

问题线程在这里

thanks~~~~