[debug] how to step into python code in EngineCore process

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1

Hi, I used to step into python code via pdb/pudb for debug purpose, but it does not work with vllm.

vllm/vllm/v1/engine/core_client.py at main · vllm-project/vllm · GitHub creates EngineCore process and re-direct the IO, it makes pdb/pudb not work.

import pudb
pudb.set_trace()

Except the print method which is not effective, could you share how to step into the python code of EngineCore process, I’d debug within it, thanks.

2

You cannot directly use pdb or pudb to step into the EngineCore process in vLLM, because EngineCore runs in a separate process and its IO is redirected, which breaks interactive debuggers like pdb/pudb. This limitation is acknowledged in the vLLM community, and there is no officially supported way to attach pdb/pudb to the EngineCore process as of now. The only workaround mentioned is to use print statements, but this is not ideal for complex debugging tasks. For multi-process debugging, some users suggest using tools like forked-pdb, but this is not documented or officially supported by vLLM (github issue 9602).

Would you like more detail on alternative debugging strategies or how to use forked-pdb in this context?

Sources:

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