[PD] Skip tp_size exchange with rank0

[NickLucche]

Follow-up optimization to #18833 that I had previously discussed with @njhill .
Basically we can piggyback tp_size on the kv_connector metadata exchange, saving us one extra handshake with rank0 to get the tp_size.

[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 @NickLucche, 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 implements an optimization in the KV transfer mechanism. It streamlines the handshake process between distributed components by embedding the remote tensor parallel size within the existing metadata exchange initiated by the scheduler, thereby removing a redundant communication step.

Highlights

• Optimization: Removed a dedicated handshake step previously used solely to fetch the remote tensor parallel (TP) size.
• Metadata Update: The remote TP size is now included in the ReqMeta object, which is part of the metadata sent from the scheduler to the worker when initiating a KV transfer request.
• Handshake Logic: Modified the _nixl_handshake method on the worker side to receive the remote TP size as a parameter, eliminating the need to query rank 0 separately for this information.

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 configureGemini 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 optimizes the NIXL connector by eliminating a dedicated handshake with rank0 to fetch tp_size. Instead, tp_size is now piggybacked on the existing KV connector metadata exchange. This is achieved by moving tp_size from NixlAgentMetadata to ReqMeta and updating relevant function signatures and internal logic to use the directly passed tp_size.

The changes appear correct and align with the goal of reducing network handshakes. I've identified a couple of minor areas for improvement related to code clarity and addressing a TODO comment, which should further enhance maintainability.

[njhill]

Thanks @NickLucche, LGTM. I guess the original logic also isn't too bad since it's only done once per agent pair.

cc @wseaton since he's looking at reworking the handshake anyhow.

[NickLucche]

Hey @njhill , I've rebased the PR.
Although it only saves one exchange, it simplifies both code and logic in nixl_connector, so I think it's a safe addition until we settle the http design.
Let me know your thoughts on this.