*: add global variable tidb_schema_version_cache_limit to control infoschema cache size

[crazycs520]

What problem does this PR solve?

Issue Number: close #46524

Problem Summary: add global variable tidb_schema_version_cache_limit to control infoschema cache size and add/refine metric.

What is changed and how it works?

To fix #46524, we can simply enlarge the info-scheme cache capacity from 16 to 64. However, in scenarios with a large number of tables, this increases the risk of TiDB OOM.

So this PR just add global variable tidb_schema_version_cache_limit to control infoschema cache size and refine metric in grafana to better diagnose this issue later. If you meet issue in #46524 with same root cause, you can enlarge the global variable tidb_schema_version_cache_limit to fix it.

Check List

Tests

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

1. Deploy a TiDB cluster.
2. run stale read load.

sysbench --config-file=sysbench.conf mussel_minimal.lua --threads=20 --read_staleness=-5 run

3. run DDL load by loadgen. Following load will use 1 thread to continuously create tables, from t1 to t100000.

loadgen bench --sql 'create table t#seq-val (id int key, b int)' --thread=1 --valmin=1 --valmax=100000 --db=test2

4. check the metrics.

• 17:12: Start running stale-read load, and the query QPS is 18000.
• 17:14: Start running DDL load, then the query QPS is dropping to 400. From the Load Schema OPS metrics, you can see many load snapshot schema OPS, this is what causes QPS to drop.
• 17:18: set @@global. tidb_schema_version_cache_limit=64;, then the query QPS return to 18000.

Side effects

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

Documentation

• N/A

Release note

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

None

[tiprow]

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[hawkingrei]

/ok-to-test

[hawkingrei]

@crazycs520 Can you add the monitoring screenshots to PR description？

[codecov]

Codecov Report

Merging #46558 (c8f5b20) into master (fa0c5ce) will decrease coverage by 0.6223%.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #46558        +/-   ##
================================================
- Coverage   73.3027%   72.6804%   -0.6224%     
================================================
  Files          1322       1343        +21     
  Lines        396235     402539      +6304     
================================================
+ Hits         290451     292567      +2116     
- Misses        87261      91413      +4152     
- Partials      18523      18559        +36     

Flag	Coverage Δ
integration	27.7664% <54.5454%> (?)
unit	73.3000% <100.0000%> (-0.0027%)	⬇️

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Components	Coverage Δ
dumpling	54.0444% <ø> (ø)
parser	84.9609% <ø> (ø)
br	48.2621% <ø> (-4.3561%)	⬇️

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[crazycs520]

@xhebox @tangenta PTAL

[crazycs520]

@xhebox @zimulala @cfzjywxk PTAL

[cfzjywxk]

Need to discuss with @easonn7 first as there's session variable changes.

[zimulala]

LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-01 04:51:26.658617258 +0000 UTC m=+2075451.207633230: ☑️ agreed by xhebox.
• 2023-09-05 06:52:46.594496967 +0000 UTC m=+2428331.143512953: ☑️ agreed by zimulala.

[crazycs520]

/test unit-test

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[easonn7]

/approve

In the future, we need to enhance memory tracing to avoid OOM issues.

[crazycs520]

/test all

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, easonn7, xhebox, zimulala

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[cfzjywxk]

/merge

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[crazycs520]

/test all

[hawkingrei]

/retest

[crazycs520]

/retest

[crazycs520]

/ok-to-merge

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/test build

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