# [Known Issue] Subrange hotspots can block load-based split and cause TiKV CPU spikes ([tikv#18932](https://github.com/tikv/tikv/issues/18932))

Source note: adapted from GitHub issue [tikv#18932](https://github.com/tikv/tikv/issues/18932).

## Issue

- Link: [tikv#18932](https://github.com/tikv/tikv/issues/18932)
- Summary: with load-based split enabled, hotspots concentrated in only one subrange of a region may still fail the split-balance check, so TiKV cannot find a valid split key and CPU hotspots persist.

## Phenomenon

- TiKV CPU stays high for the hot store or hot region.
- Load-based split is enabled, but hotspot mitigation does not take effect as expected.
- `load base split event` shows repeated failures related to balance validation.

## Root Cause

After TiKV finds a candidate split key for a hot region, it still checks whether the traffic after the split would be balanced. This decision is controlled by `split.split-balance-score`.

- The smaller the value, the stricter the balance requirement.
- The default is `0.25`.
- If traffic is concentrated in only one subrange, the default threshold can be too strict and reject otherwise useful split points.

## Diagnostic Steps

1. Check `tikv-details -> raft admin -> load base split event`.
2. If `no_balance_key` remains high while the related TiKV CPU also remains high, suspect subrange hotspots inside one region.
3. Correlate the hotspot with a specific key range or region if possible.

## Workaround

Increase `split.split-balance-score` appropriately:

```sql
SET CONFIG tikv `split.split-balance-score` = '0.55';
SHOW CONFIG WHERE name LIKE 'split.split-balance-score';
```

## Related Oncall

- v8.5 how to mitigate read unbalance across TiKV `GTOC-7880` / `NAID-11609`