Two weeks with hlsl-perf: what feels right and what drives me up the wall
Most hlsl-perf articles stop at "how to use it" and never cover "when not to use it". This is an attempt at the second half.
-- The query that broke: a full scan over 20M rows. -- A composite index took P99 from 1.8s down to 42ms. SELECT id, title, created_at FROM posts WHERE community_id = ? AND status = 1 ORDER BY score DESC LIMIT 20;
The first thing was to collapse the variables. We were changing config and upgrading the version at the same time, and afterwards nobody could say which change caused what. We rolled back to moving one variable at a time, re-ran three times, and only then did the curve settle. Tedious, but not skippable.
What genuinely surprised me was the tail. The average looked great while P99 jumped by an order of magnitude past some threshold. The cause was not hlsl-perf itself but our upstream connection reuse — the load test traffic was too clean and hid the long-tail requests.
On trade-offs, my view is this: if nobody on the team owns this area long-term, do not introduce a second mechanism. With two coexistence you first have to work out which one is even in play when things break, and that costs far more than the performance you saved.
We also fixed monitoring along the way: replaced average-based alerts with percentiles and split them per endpoint. False alerts dropped by about seventy percent and the on-call rotation visibly cheered up.