eventservice: optimize scanwindow

[asddongmen]

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[gemini-code-assist]

Code Review

This pull request replaces the existing scan interval adjustment logic with a new adaptiveScanWindowController that utilizes Exponential Moving Averages (EMAs) and a pressure score for more stable memory pressure management. The update includes comprehensive simulation tests and enhanced Prometheus metrics for monitoring controller decisions. Review feedback highlights a non-monotonic discontinuity in the emergency brake calculation, potential over-throttling caused by latching peak usage values, the presence of magic numbers, and the use of a redundant maxFloat64 helper that should be replaced by the built-in max function.

[gemini-code-assist]

The logic in scanWindowEmergencyBrakeInterval has two significant issues:

1. Discontinuity: There is a sharp jump at the boundary of 6*defaultScanInterval (30s). For an input of 30s, it returns 15s (current/2), but for 30.1s, it returns ~7.5s (current/4). This non-monotonic behavior can cause unstable oscillations in the scan interval.
2. Unreachable Minimum: The function floors at defaultScanInterval (5s) for any current <= 30s. This makes the minScanInterval (1s) constant effectively unreachable during emergency pressure (98%+ usage) if the interval has already been reduced to a moderate level. If the goal is to allow the interval to drop to 1s under extreme congestion, the floor should be minScanInterval in both branches.

[gemini-code-assist]

Using usage.max in shouldReduceForHighPressureLocked may lead to excessive interval reductions. Since usage.max tracks the peak usage over the last 30 seconds, a single transient spike will keep triggering reductions every 10 seconds (the cooldown) for the entire duration the spike remains in the window, even if the current usage (usage.last) and EMAs indicate that pressure has subsided. Consider relying primarily on EMAs or the current report value to ensure the response is truly adaptive to the present state.

[gemini-code-assist]

The magic numbers 0.03 and 0.02 used as offsets for the low pressure check should be defined as named constants to improve code clarity and maintainability.

[gemini-code-assist]

The maxFloat64 helper function is redundant as Go 1.21+ provides a built-in max function. The codebase already uses the built-in max in other parts of this file (e.g., lines 450, 464), so this helper should be removed and its call sites (lines 679, 681, 687) updated to use the built-in function for consistency.