add limit-queue-length param

[nzinov]

No description provided.

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[gemini-code-assist]

Summary of Changes

Hello @nzinov, 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!

This pull request introduces a new mechanism to prevent the vLLM scheduler's waiting queue from becoming overloaded. It adds a configuration option to enable queue length limiting, which, when active, will reject new incoming requests if the number of waiting requests exceeds the number of currently running requests, returning an HTTP 503 error to the client. This helps maintain service stability under high load.

Highlights

• New Configuration Parameter: Introduced limit_queue_length (a boolean flag) to SchedulerConfig in vllm/config.py. When enabled, this parameter activates a mechanism to prevent the scheduler's waiting queue from growing excessively.
• Queue Limiting Logic: Implemented logic within the scheduler's add_request method in vllm/v1/core/sched/scheduler.py. If limit_queue_length is true, new requests are rejected by raising a SchedulerWaitingQueueFullError when the number of waiting requests is greater than or equal to the number of currently running requests.
• New Exception Type: Defined a new exception class, SchedulerWaitingQueueFullError, in vllm/v1/engine/exceptions.py to specifically signal when the scheduler's waiting queue capacity is exceeded.
• API Error Handling: Integrated the new SchedulerWaitingQueueFullError into the OpenAI serving endpoints (vllm/entrypoints/openai/serving_chat.py and vllm/entrypoints/openai/serving_completion.py). When this exception is caught, the API now returns an HTTP 503 (Service Unavailable) response to the client.
• New CLI Argument (Unused by this PR's core logic): Added a --max-waiting-queue-length command-line argument in vllm/engine/arg_utils.py. While added, this argument does not appear to be directly utilized by the limit_queue_length boolean flag's logic introduced in this pull request, which uses a dynamic comparison of waiting vs. running requests.

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point in your pull request via creating an issue comment (i.e. comment on the pull request page) using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in issue comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist is currently in preview and may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments to provide feedback.

You can also get AI-powered code generation, chat, as well as code reviews directly in the IDE at no cost with the Gemini Code Assist IDE Extension.

Footnotes

1. Review the Privacy Notices, Generative AI Prohibited Use Policy, Terms of Service, and learn how to configure Gemini Code Assist in GitHub here. Gemini can make mistakes, so double check it and use code with caution. ↩

[gemini-code-assist]

Code Review

This pull request introduces a feature to limit the request queue length, which is a great addition for managing server load. The core logic seems sound, but there's an issue with the new command-line argument that could cause a runtime error. I've provided a fix for that. I also found a minor inaccuracy in a docstring and suggested a clarification. Once these issues are addressed, the PR should be in good shape.

[gemini-code-assist]

The argument name --max-waiting-queue-length is misleading for a boolean flag. A more appropriate name would be --limit-queue-length to match the configuration parameter limit_queue_length. Also, the code will raise a KeyError because scheduler_kwargs does not contain a key "max_waiting_queue_length". The code should be accessing scheduler_kwargs["limit_queue_length"].

Suggested change

	scheduler_group.add_argument(
	"--max-waiting-queue-length",
	**scheduler_kwargs["max_waiting_queue_length"])
	scheduler_group.add_argument(
	"--limit-queue-length",
	**scheduler_kwargs["limit_queue_length"])

[gemini-code-assist]

The docstring states that requests are rejected when the number of waiting requests exceeds the number of running requests. However, the implementation in vllm/v1/core/sched/scheduler.py uses the condition len(self.waiting) >= len(self.running), which means rejection happens when the number is greater than or equal to. To avoid confusion, update the docstring to accurately reflect the implementation.

    when the number of waiting requests is greater than or equal to the number of running requests.

[simon-mo]

This is one of the desirable feature we want in vLLM btw! Some ways to perform admission control/back pressure to control the queue depth.

[hmellor]

This is being added in #21352