Improve pdRouter with load-aware routing

[googs1025]

Pull Request Description

[Please provide a clear and concise description of your changes here]

• Improve pdRouter with load-aware routing

Related Issues

Resolves: #1600

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[gemini-code-assist]

Summary of Changes

Hello @googs1025, 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 significantly improves the pdRouter's routing intelligence by introducing load-aware strategies for selecting both prefill and decode pods. By considering the current load on available pods, the system can now make more informed routing decisions, leading to better resource distribution, reduced bottlenecks, and an overall more efficient handling of requests.

Highlights

• Load-Aware Prefill Pod Selection: The pdRouter now incorporates logic to detect load imbalance among prefill pods and prioritizes selecting the least-loaded pod for new requests, aiming to distribute workload more evenly.
• Least-Request Decode Pod Selection: The selection mechanism for decode pods has been enhanced to prefer pods with the fewest running requests, improving resource utilization and potentially reducing latency for subsequent requests.
• Logging Enhancements: Detailed logging has been added for both prefill and decode pod selection processes, providing better visibility into routing decisions, including load imbalance detection and fallback scenarios.
• Comment Updates: Minor comment clarifications were made in pkg/cache/cache_init.go for InitWithModelRouterProvider and InitWithProfileCache functions.

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[googs1025]

TODO: add unit test for this part

[gemini-code-assist]

Code Review

This pull request introduces load-aware routing to the pdRouter, which is a great improvement for balancing work between pods. The changes prioritize pod selection based on load metrics before falling back to existing logic. My review identified a critical issue that could lead to a panic due to a potential nil pointer dereference, and a medium-severity issue related to unhandled errors which could make debugging more difficult. I've provided code suggestions to address both of these points.

[gemini-code-assist]

The error returned by utils.SelectRandomPod is being ignored. If no routable decode pods are available, this function will return nil without logging the underlying reason, making debugging harder. It's better to handle the error, for example by logging it, to provide more context when selection fails.

decodePod, err := utils.SelectRandomPod(filteredDecodePods, rand.Intn)
	if err != nil {
		klog.ErrorS(err, "failed to select a random decode pod", "prefill_pod", prefillPod.Name)
	}

[Jeffwan]

/cc @varungup90

[googs1025]

add some unit test in improvment part

[googs1025]

@varungup90 please for final ack 😄 thanks a lot

[Jeffwan]

/cc @varungup90