ddl: make local sort generate only one subtask for each instance

[tangenta]

What problem does this PR solve?

Issue Number: ref #48795

Problem Summary:

Previously, the subtask count is unreasonable:

tidb/pkg/ddl/backfilling_dist_scheduler.go

Lines 325 to 327 in 35a7c9e

	// Make subtask large enough to reduce the overhead of local/global flush.
	avgTasksPerDisk := int(int64(variable.DDLDiskQuota.Load()) / int64(config.SplitRegionSize))
	regionBatch = min(avgTasksPerDisk, avgTasksPerInstance)

It is inaccurate to use 96M to calculate the size of index data in a region. As a result, the number of subtasks will become very large, which is quite inefficient.

What changed and how does it work?

Make local sort generate only one subtask for each instance.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  1.194TiB table, 8 worker count:
  Before this PR: 1 hour 1 min 29 sec
  
  After this PR: 26 min 10 sec
  
• 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 @tangenta. Thanks for your PR.

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[codecov]

Codecov Report

Merging #50925 (1b278bf) into master (9aabf72) will increase coverage by 2.1319%.
Report is 20 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #50925        +/-   ##
================================================
+ Coverage   70.5281%   72.6600%   +2.1319%     
================================================
  Files          1466       1466                
  Lines        434112     437986      +3874     
================================================
+ Hits         306171     318241     +12070     
+ Misses       108699      99707      -8992     
- Partials      19242      20038       +796     

Flag	Coverage Δ
integration	49.0020% <100.0000%> (?)
unit	70.3055% <100.0000%> (-0.0345%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	54.1120% <ø> (ø)
parser	∅ <ø> (∅)
br	51.0174% <ø> (+5.2488%)	⬆️

[tangenta]

/retest

[tiprow]

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: wjhuang2016, zimulala

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-02 07:08:00.364432174 +0000 UTC m=+1722121.928729879: ☑️ agreed by zimulala.
• 2024-02-05 06:17:36.568690306 +0000 UTC m=+164782.135460197: ☑️ agreed by wjhuang2016.