A high cache hit rate feels like proof of virtue. It is only proof that something was reused. It does not prove that the expensive compile units were among the reused set.
A quick honesty check
Pick ten builds on a busy branch. For each, note:
- Reported cache hit percentage
- Wall-clock time in the heaviest compile stage
- Whether that stage’s inputs actually changed
If hit rate stays high while the heavy stage barely moves, your metric is congratulating the wrong work.
What to record instead
Pair hit rate with heavy-stage duration and a short note on input change. The pair is harder to game and easier to defend in a release meeting.
Workshop use
Our release metric workshop often retires standalone hit-rate slides in favour of this paired reading. Participants grumble; cut week gets quieter.