importinto: calculate distsql scan concurency in nextgen (#63341)

[ti-chi-bot]

This is an automated cherry-pick of #63341

What problem does this PR solve?

Issue Number: ref #61702

Problem Summary:

What changed and how does it work?

then we can make the checksum step faster, and don't need user to set it.

Check List

Tests

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

import 100G data locally, see it's 60

[2025/09/02 19:29:36.495 +08:00] [INFO] [import.go:1322] ["set import thread count for nextgen kernel"] [keyspaceName=SYSTEM] [table=sys] ["thread count"=4] ["max node count"=1] ["target node cpu count"=10] ["dist sql scan concurrency"=60] ["total file size"=100GiB]

import 100T, it's 281

[2025/09/02 19:42:52.314 +08:00] [INFO] [import.go:1322] ["set import thread count for nextgen kernel"] [keyspaceName=SYSTEM] [table=sys] ["thread count"=10] ["max node count"=25] ["dist sql scan concurrency"=281] ["target node cpu count"=10] ["total file size"=100TiB]

• 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

[D3Hunter]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: D3Hunter, tangenta

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• pkg/disttask/OWNERS [D3Hunter,tangenta]
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-09-03 11:55:17.843643817 +0000 UTC m=+8061.826741255: ☑️ agreed by D3Hunter.
• 2025-09-04 02:14:00.378348886 +0000 UTC m=+59584.361446324: ☑️ agreed by tangenta.