v8.5.5: Add unified.cpu-threshold config

[v01dstar]

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• Other reference link(s): https://github.com/tikv/tikv/pull/18948

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[gemini-code-assist[bot]]

Summary of Changes

Hello @v01dstar, I'm Gemini Code Assist[^1]! 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 enhances TiKV's resource management by adding a new configuration option, unified.cpu-threshold, to the configuration file. This new setting provides more precise control over the CPU resources utilized by the unified read pool, allowing administrators to fine-tune performance and prevent resource contention by setting a maximum CPU usage threshold.

Highlights

• New Configuration Item: unified.cpu-threshold: Introduced a new configuration item cpu-threshold under the unified section for TiKV, specifically for the unified read pool. This setting is new in v8.5.5.
• CPU Utilization Management: The cpu-threshold parameter allows users to define the highest CPU utilization permitted for the unified read pool. It works in conjunction with the busy thread scaling algorithm to prevent excessive CPU consumption, ensuring the read pool operates within defined limits.
• Configuration Details: The documentation for cpu-threshold specifies its default value as 0.0 and its value range as [0.0, 1.0]. A value of 0 disables the CPU constraint, relying solely on thread scaling, while a value greater than 0 enforces CPU threshold checks for scaling up and down.

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