diff --git a/docs/source/getting_started/debugging.rst b/docs/source/getting_started/debugging.rst new file mode 100644 index 00000000..6199b726 --- /dev/null +++ b/docs/source/getting_started/debugging.rst @@ -0,0 +1,36 @@ +.. _debugging: + +Debugging Tips +=============== + +Debugging hang/crash issues +--------------------------- + +When an vLLM instance hangs or crashes, it is very difficult to debug the issue. Here are some tips to help debug the issue: + +- Set the environment variable ``export VLLM_LOGGING_LEVEL=DEBUG`` to turn on more logging. +- Set the environment variable ``export CUDA_LAUNCH_BLOCKING=1`` to know exactly which CUDA kernel is causing the trouble. +- Set the environment variable ``export NCCL_DEBUG=TRACE`` to turn on more logging for NCCL. +- Set the environment variable ``export VLLM_TRACE_FUNCTION=1`` . All the function calls in vLLM will be recorded. Inspect these log files, and tell which function crashes or hangs. **Note: it will generate a lot of logs and slow down the system. Only use it for debugging purposes.** + +With more logging, hopefully you can find the root cause of the issue. + +Here are some common issues that can cause hangs: + +- The network setup is incorrect. The vLLM instance cannot get the correct IP address. You can find the log such as ``DEBUG 06-10 21:32:17 parallel_state.py:88] world_size=8 rank=0 local_rank=0 distributed_init_method=tcp://xxx.xxx.xxx.xxx:54641 backend=nccl``. The IP address should be the correct one. If not, override the IP address by setting the environment variable ``export VLLM_HOST_IP=your_ip_address``. +- Hardware/driver setup is incorrect. GPU communication cannot be established. You can run a sanity check script below to see if the GPU communication is working correctly. + +.. code-block:: python + + # save it as `test.py`` , and run it with `NCCL_DEBUG=TRACE torchrun --nproc-per-node=8 test.py` + # adjust `--nproc-per-node` to the number of GPUs you want to use. + import torch + import torch.distributed as dist + dist.init_process_group(backend="nccl") + data = torch.FloatTensor([1,] * 128).to(f"cuda:{dist.get_rank()}") + dist.all_reduce(data, op=dist.ReduceOp.SUM) + torch.cuda.synchronize() + value = data.mean().item() + assert value == dist.get_world_size() + +If the problem persists, feel free to open an `issue `_ on GitHub, with a detailed description of the issue, your environment, and the logs. diff --git a/docs/source/index.rst b/docs/source/index.rst index 0ebd1fb7..0ff0ea1d 100644 --- a/docs/source/index.rst +++ b/docs/source/index.rst @@ -66,6 +66,7 @@ Documentation getting_started/neuron-installation getting_started/cpu-installation getting_started/quickstart + getting_started/debugging getting_started/examples/examples_index .. toctree::