tidb: global memory arbitrator mechanism

[solotzg]

First-time contributors' checklist

• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

Add documentation for the TiDB global memory arbitrator mode, including:

• Add a Memory arbitrator mode section to configure-memory-usage.md, describing the working principles, configuration methods, monitoring metrics, and best practices for the standard and priority modes.
• Add descriptions for the following system variables to system-variables.md:
  • tidb_mem_arbitrator_mode: sets the global memory management mode for a TiDB instance.
  • tidb_mem_arbitrator_query_reserved: sets the amount of memory reserved in advance for a SQL statement before execution.
  • tidb_mem_arbitrator_soft_limit: sets the upper limit of memory allocation for a TiDB instance.
  • tidb_mem_arbitrator_wait_averse: controls the behavior of SQL statements when waiting for memory resources, including the meanings of the values 0 (default, disabled), 1, and nolimit.

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• Other reference link(s): ref memory: global mem resources arbitrator tidb#63073 / design doc: TiDB Global Memory Arbitrator

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-05-22 07:52:34.743548774 +0000 UTC m=+109885.384405587: ☑️ agreed by hfxsd.

[hfxsd]

/retest

[ti-chi-bot]

@hfxsd: Your lgtm message is repeated, so it is ignored.

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[hfxsd]

/approve

[ti-chi-bot]

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