[Bug]: distributed_executor_backend=mp does not work with GPTQ tp>1

[robertgshaw2-redhat]

Your current environment

The output of `python collect_env.py`

🐛 Describe the bug

distributed_executor_backed="mp" is now enabled by default for vLLM. However, this feature is currently incompatible with some GPTQ quantization for tp>1 due to the order in which torch is initialized. We get the classic RuntimeError: Cannot re-initialize CUDA in forked subprocess. To use CUDA with multiprocessing, you must use the 'spawn' start method

Setting distributed_backend_executor="ray" works for GPTQ

The following fails:

from vllm import LLM

MODEL_NAME="TheBloke/TinyLlama-1.1B-Chat-v1.0-GPTQ"
TENSOR_PARALLEL_SIZE=2

model = LLM(MODEL_NAME, enforce_eager=True, tensor_parallel_size=TENSOR_PARALLEL_SIZE, distributed_executor_backend="mp")
print(model.generate("The best thing about the internet is")[0].outputs[0].text)

with:

(vllm-upstream) rshaw@beaker:~/vllm$ python3 run.py 
INFO 06-30 14:26:18 gptq_marlin.py:140] The model is convertible to gptq_marlin during runtime. Using gptq_marlin kernel.
INFO 06-30 14:26:18 llm_engine.py:169] Initializing an LLM engine (v0.5.0.post1) with config: model='TheBloke/TinyLlama-1.1B-Chat-v1.0-GPTQ', speculative_config=None, tokenizer='TheBloke/TinyLlama-1.1B-Chat-v1.0-GPTQ', skip_tokenizer_init=False, tokenizer_mode=auto, revision=None, rope_scaling=None, rope_theta=None, tokenizer_revision=None, trust_remote_code=False, dtype=torch.bfloat16, max_seq_len=2048, download_dir=None, load_format=LoadFormat.AUTO, tensor_parallel_size=2, disable_custom_all_reduce=False, quantization=gptq_marlin, enforce_eager=True, kv_cache_dtype=auto, quantization_param_path=None, device_config=cuda, decoding_config=DecodingConfig(guided_decoding_backend='outlines'), observability_config=ObservabilityConfig(otlp_traces_endpoint=None), seed=0, served_model_name=TheBloke/TinyLlama-1.1B-Chat-v1.0-GPTQ)
(VllmWorkerProcess pid=3438507) Process VllmWorkerProcess:
(VllmWorkerProcess pid=3438507) Traceback (most recent call last):
(VllmWorkerProcess pid=3438507)   File "/home/rshaw/.pyenv/versions/3.10.14/lib/python3.10/multiprocessing/process.py", line 314, in _bootstrap
(VllmWorkerProcess pid=3438507)     self.run()
(VllmWorkerProcess pid=3438507)   File "/home/rshaw/.pyenv/versions/3.10.14/lib/python3.10/multiprocessing/process.py", line 108, in run
(VllmWorkerProcess pid=3438507)     self._target(*self._args, **self._kwargs)
(VllmWorkerProcess pid=3438507)   File "/home/rshaw/vllm/vllm/executor/multiproc_worker_utils.py", line 210, in _run_worker_process
(VllmWorkerProcess pid=3438507)     worker = worker_factory()
(VllmWorkerProcess pid=3438507)   File "/home/rshaw/vllm/vllm/executor/gpu_executor.py", line 67, in _create_worker
(VllmWorkerProcess pid=3438507)     wrapper.init_worker(**self._get_worker_kwargs(local_rank, rank,
(VllmWorkerProcess pid=3438507)   File "/home/rshaw/vllm/vllm/worker/worker_base.py", line 311, in init_worker
(VllmWorkerProcess pid=3438507)     self.worker = worker_class(*args, **kwargs)
(VllmWorkerProcess pid=3438507)   File "/home/rshaw/vllm/vllm/worker/worker.py", line 86, in __init__
(VllmWorkerProcess pid=3438507)     self.model_runner: GPUModelRunnerBase = ModelRunnerClass(
(VllmWorkerProcess pid=3438507)   File "/home/rshaw/vllm/vllm/worker/model_runner.py", line 196, in __init__
(VllmWorkerProcess pid=3438507)     self.attn_backend = get_attn_backend(
(VllmWorkerProcess pid=3438507)   File "/home/rshaw/vllm/vllm/attention/selector.py", line 45, in get_attn_backend
(VllmWorkerProcess pid=3438507)     backend = which_attn_to_use(num_heads, head_size, num_kv_heads,
(VllmWorkerProcess pid=3438507)   File "/home/rshaw/vllm/vllm/attention/selector.py", line 151, in which_attn_to_use
(VllmWorkerProcess pid=3438507)     if torch.cuda.get_device_capability()[0] < 8:
(VllmWorkerProcess pid=3438507)   File "/home/rshaw/vllm-upstream/lib/python3.10/site-packages/torch/cuda/__init__.py", line 430, in get_device_capability
(VllmWorkerProcess pid=3438507)     prop = get_device_properties(device)
(VllmWorkerProcess pid=3438507)   File "/home/rshaw/vllm-upstream/lib/python3.10/site-packages/torch/cuda/__init__.py", line 444, in get_device_properties
(VllmWorkerProcess pid=3438507)     _lazy_init()  # will define _get_device_properties
(VllmWorkerProcess pid=3438507)   File "/home/rshaw/vllm-upstream/lib/python3.10/site-packages/torch/cuda/__init__.py", line 279, in _lazy_init
(VllmWorkerProcess pid=3438507)     raise RuntimeError(
(VllmWorkerProcess pid=3438507) RuntimeError: Cannot re-initialize CUDA in forked subprocess. To use CUDA with multiprocessing, you must use the 'spawn' start method
ERROR 06-30 14:26:19 multiproc_worker_utils.py:120] Worker VllmWorkerProcess pid 3438507 died, exit code: 1
INFO 06-30 14:26:19 multiproc_worker_utils.py:123] Killing local vLLM worker processes

Workarounds for now:

• Set distributed_executor_backend="mp"

[DarkLight1337]

It would be great if we can better control when cuda is initialized, not sure whether that's feasible though.

Despite @youkaichao 's fix to the distributed tests, it's quite a headache to ensure that CUDA is not accidentally initialized wrongly during those tests.

[robertgshaw2-redhat]

For sure - Im going to look into this when I get some time. Its specific to GPTQ (does not happen for fp8 quantization). I think the source is that we check cuda_device_capability when deciding if we can convert GPTQ-->Marlin. I think this happens "too early" in the lifecycle

Will look into a workaround when I get some time. I have a couple PRs I want to wrap up before I look into this

[llmpros]

met the same issue - happy to poke around / peek if folks are busy with higher priority tasks

[robertgshaw2-redhat]

Feel free. I think this function is the culprit --- initializes torch:

• vllm/vllm/model_executor/layers/quantization/gptq_marlin.py

  Line 155 in af9ad46

  def is_marlin_compatible(cls, quant_config: Dict[str, Any]):

It is called by this function:

• vllm/vllm/model_executor/layers/quantization/gptq_marlin.py

  Line 125 in af9ad46

  def override_quantization_method(cls, hf_quant_cfg,

Which is called by this function:

• vllm/vllm/config.py

  Line 205 in af9ad46

  quantization_override = method.override_quantization_method(

I think solutions are:

• trying to see if there is a way to get the cuda device capability without calling torch.cuda

[youkaichao]

we can use pynvml to check the compute capability, without calling torch.cuda (which will initialize cuda context).