[Bugfix] Fix MQLLMEngine hanging

[robertgshaw2-neuralmagic]

SUMMARY:

• MQLLMEngineClient can hang if the MQLLMEngine crashes during LLMEngine.__init__. Previously, we checked if the process is_alive, but if an exception is raised in the MQLLMEngine the process can sometimes still report is_alive=True.
• To get around this, we use a shared variable, and wrap the MQLLMEngine loop in a try...catch. We update the shared variable if an exception occurs and also log the exception. This ensures that the error will always be logged and the client can then check the shared variable and cleanly shut down

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[youkaichao]

add type annotation for the engine_alive variable?

[youkaichao]

thanks for the fix! I'm not familiar with this code though, would be better to get reviews from @njhill

[youkaichao]

if an exception is raised in the MQLLMEngine the process can sometimes still report is_alive=True.

why is that the case?

[robertgshaw2-neuralmagic]

if an exception is raised in the MQLLMEngine the process can sometimes still report is_alive=True.

why is that the case?

I spent an hour or so reading around on the internet, but I could not find anything conclusive. This did solve the issue I was seeing with the hanging though.

@russellb - do you have any experience with this?

[russellb]

if an exception is raised in the MQLLMEngine the process can sometimes still report is_alive=True.

why is that the case?

I spent an hour or so reading around on the internet, but I could not find anything conclusive. This did solve the issue I was seeing with the hanging though.

@russellb - do you have any experience with this?

not off the top of my head, but I'm happy to take a look today!

[robertgshaw2-neuralmagic]

going to merge to fix the bug while we look into why is_alive is reporting true in the background

[russellb]

going to merge to fix the bug while we look into why is_alive is reporting true in the background

did you have an easy way to reproduce it? I just tried to reproduce it by forcing LLMEngine.init to fail, but that didn't do it.

[robertgshaw2-neuralmagic]

going to merge to fix the bug while we look into why is_alive is reporting true in the background

did you have an easy way to reproduce it? I just tried to reproduce it by forcing LLMEngine.init to fail, but that didn't do it.

I cannot quite determine the conditions in which LLMEngine.__init__ raises exception but the process does not die. There was a previously head of main that had this issue from Sunday. I will send a githash later.

[robertgshaw2-neuralmagic]

This git hash 18bd7587b78b3b9868fea29d59ae8c3600c3e5a5 hangs on:

VLLM_USE_V1=1 vllm serve Qwen/Qwen2-0.5B-Instruct