Unexpected pending task by io limiter

[flowbehappy]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

TiFlash runs on ebs (2 nodes, 8vc, 287MB/s). Workload is ch-benchmark. Run AP queries after data import.

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

3. What did you see instead (Required)

Although the IO throughput is small, the fgread task is pending.

4. What is your TiFlash version? (Required)

v6.2.0