lightning might oom when mem/cpu ratio is low

[D3Hunter]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

• each row of the target table is large, say 20k, and there's many int values cannot be stored correctly, so cause ErrDataOutOfRange warning when encoding
• lightning might oom when memory is not that enough during sorting, say cpu/mem is 1:2, and we're using default region-concurrency

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

current master

[D3Hunter]

will not pick to older versions, user can use newer version directly