Revise the Configuration for SimpleLRUCache

[Yisaer]

Feature Request

Is your feature request related to a problem? Please describe:

In my view, some configurations for SimpleLRUCache is misleading and some are over-designed.

For max-memory, the description in our document misleads and may let user to think max-memory is only for managing SimpleLRUCache memory usage.

However, in our config.example.toml, max-memory is described as the memory quota for a single tidb-server. And in #16777 , and we also let max-memory to control the memory quota as a tidb-server. The difference between the usage and description for max-memory may make user confusion.

#17298 also pointed out this.

Also, as GlobalLRUMemUsageTracker introduced in #16777 , I think the memory-guard-ratio is no longer needed and Capacity is enough for the elimination strategy.

Describe the feature you'd like:

To remove the misleading and unnecessary configuration, I think we can

1. introduce a new configuration like server-memory-quota for the memory quota for a single tidb-server. config: Revise SimpleLRUCache Configuration  #17532
2. Fix the annotation in tidb.toml.example and tell the readers that max-memory is only for Plan Cache in tidb.toml.example in all release branches(v2.1, v3.0, v3.1, v4.0)
3. marked max-memory after v4.0

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[zz-jason]

Hi @dbjoa, from the description above, we are going to abandon memory-guard-ratio in the config file, what's your opinion?

[dbjoa]

Hi @zz-jason,
I agree with you if we can control the memory usage of plan cache as we done so far.

[SunRunAway]

4. remain capacity strategy and removed the memory ratio strategy in SimpleLRUCache.

IMO, the memory ratio strategy should be kept (but disabled by default) for compatibility of the upgraded server.

[Yisaer]

1. remain capacity strategy and removed the memory ratio strategy in SimpleLRUCache.

IMO, the memory ratio strategy should be kept (but disabled by default) for compatibility of the upgraded server.

It's ok for me.

[fzhedu]

We should set different memory pool: server memory = plan memory + executor memory + cache memory + schema memory + statistics memory+ others, similar to MySQL, Oracle and others.

[Yisaer]

After discuss with @SunRunAway , we decide still remain memory-guard-ration and use server-mem-quota as its quota instead of max-memory