Add clear documentation around the impact of debugging flag

[annapendleton]

Purpose

Add clear documentation around the flag that caused issues for starting up gemma-3-1b model with vllm, ref: #17386

(Optional) Documentation Update

Make clear that this flag is not to be used in any other way except for debugging.

[github-actions]

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[gemini-code-assist]

Hello @annapendleton, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

Summary of Changes

Hello team, gemini-code-assist here to provide a summary of this pull request. This PR focuses on improving the documentation within the vllm project. Specifically, it adds important clarification regarding the VLLM_TRACE_FUNCTION=1 debugging flag. The intent, as stated in the PR description and title, is to make it clear that this flag is intended solely for debugging and should not be used otherwise, as it can cause significant delays during startup. This change directly addresses an issue encountered when starting models like gemma-3-1b with vLLM.

Highlights

• Documentation Update: Added a crucial note to the troubleshooting documentation (docs/usage/troubleshooting.md) explaining the impact and intended use of the VLLM_TRACE_FUNCTION=1 environment variable.
• Debugging Flag Clarification: Explicitly warned users that enabling VLLM_TRACE_FUNCTION=1 will cause significant delays in startup time and should only be used when absolutely necessary for debugging.

Changelog

• docs/usage/troubleshooting.md
  • Added a warning and usage restriction to the description of the VLLM_TRACE_FUNCTION=1 debugging flag (line 43).

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[gemini-code-assist]

Code Review

This pull request effectively addresses the need for clearer documentation regarding the VLLM_TRACE_FUNCTION=1 flag. The added warning is concise, well-placed, and clearly communicates the potential performance impact and intended use case of this debugging flag. This will be very helpful for users troubleshooting issues and prevent accidental misuse that could lead to significant startup delays, as referenced in the linked issue. Great job on this targeted and impactful documentation improvement!

Merge Readiness

The documentation change in this pull request is clear, accurate, and directly addresses the stated purpose. From a code review perspective, the changes look good and improve the quality of the troubleshooting guide. While I cannot approve the pull request myself, I believe it is ready for merging after any other required reviews and approvals are completed.

[gemini-code-assist]

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