sessionctx,infoschema: change @@tidb_schema_cache_size lower bound to 64MB

[tiancaiamao]

What problem does this PR solve?

Issue Number: ref #50959

Problem Summary:

What changed and how does it work?

I've tested the relationship between schema cache size and active tables been keep in-memory.

• 95208 tables => 103MB cache size to make them all in memory
• 151641 tables => 163MB
• 200062 tables => 216MB

On average, 1M schema cache size can cache 926 active tables.
So 64MB schema cache size can keep nearly 60K active tables in memory.
128MB schema cache size can keep nearly 120K active tables in memory.

The default schema cache size is still 512MB which is large enough for 475K active tables.
This change makes it easier to cover cache miss cases.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Test different table numbers and check the corresponding @@tidb_schema_cache_size can make them all in-memory.
(This can be verified by check the metrics with active workload visiting a varity of tables)

Test with 1M tables with workload visiting tables [0-500000] in 0-2min, tables [500000-1000000) in 2-4min, and than [0-500000] again in 4-6min ... change the table range every 2min and watch the cache miss behaviour:

• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[tiprow]

Hi @tiancaiamao. Thanks for your PR.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.2698%. Comparing base (a997f95) to head (9096ae8).
Report is 22 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #56316        +/-   ##
================================================
+ Coverage   73.3850%   76.2698%   +2.8847%     
================================================
  Files          1605       1641        +36     
  Lines        445291     462689     +17398     
================================================
+ Hits         326777     352892     +26115     
+ Misses        98457      88268     -10189     
- Partials      20057      21529      +1472     

Flag	Coverage Δ
integration	52.3873% <100.0000%> (?)
unit	72.7635% <100.0000%> (+0.2494%)	⬆️

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Components	Coverage Δ
dumpling	52.9478% <ø> (ø)
parser	∅ <ø> (∅)
br	60.7653% <ø> (+14.7343%)	⬆️

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-09-25 13:35:30.324121874 +0000 UTC m=+1659400.064545810: ☑️ agreed by lance6716.
• 2024-09-25 13:45:14.779369941 +0000 UTC m=+1659984.519793879: ☑️ agreed by CbcWestwolf.

[tiancaiamao]

/test unit-test

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CbcWestwolf, lance6716, wjhuang2016

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Needs approval from an approver in each of these files:

• OWNERS [CbcWestwolf,lance6716,wjhuang2016]
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[tiancaiamao]

/retest

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[tiancaiamao]

/retest

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/retest

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[tiancaiamao]

/ok-to-test