global-sort: reduce number of SST ingested into TiKV

[D3Hunter]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

currently, global sort always split data into ranges of 96M, but TiKV already increased the default region size to 256M now, so we will ingest 3 SST files per region.
for import-into see

tidb/pkg/disttask/importinto/planner.go

Lines 550 to 551 in bf21e59

	int64(config.SplitRegionSize),
	int64(config.SplitRegionKeys),

, for add-index see

tidb/pkg/ddl/backfilling_dist_scheduler.go

Line 675 in bf21e59

int64(config.SplitRegionSize), int64(config.SplitRegionKeys),

and the ingest performance of TiKV slows down when the number of SST files grows too much, say more than 300K

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

v8.1.0

[D3Hunter]

local sort, including lightning, add-index local sort, import-into local sort, all have similar issue

[lance6716]

tidb/pkg/disttask/importinto/planner.go

Lines 550 to 551 in bf21e59

	int64(config.SplitRegionSize),
	int64(config.SplitRegionKeys),

This is the size of ingest job. And the reason why it's different with region size is to increase the concurency. In historical feature development TiKV can set ~10GB per region (dynamic region feature), and lightning must choose a smaller value to increase concurrency. At that time I chose the traditional default value 96MB. So if now it's a bottleneck of import performance, the value can be adjusted again. The value should not be too large to keep a proper overall concurrency.

[D3Hunter]

This is the size of ingest job

actually, the ingest job data size is determined by min(this range size, region split size)

see the link PR too

[kennedy8312]

Related to issue/feature: tikv/tikv#17309
Related to PR: tikv/tikv#17311