# Performance

Latency, throughput, resource use under representative load. Most teams
have monitoring; fewer have benchmarks in CI they trust.

## What goes wrong

- **Latency regressions on the hot path.** A change increases p99 by
  20 ms and nobody notices until customers do.
- **Throughput collapse under load.** The system handles 1× fine and
  falls over at 3×. Not visible at single-user load.
- **Resource leaks.** Memory, file descriptors, connection pools.
  Linear creep over hours or days; invisible in short tests.
- **Accidental algorithmic complexity.** $O(n^2)$ where $O(n \log n)$ was
  intended. Innocuous in tests with $n = 10$, catastrophic at $n = 10^6$.
- **Coordination overhead at scale.** Lock contention, cache misses,
  garbage-collection pauses. Manifest only above some throughput.
- **Tail latency.** p50 looks fine; p99.9 is a disaster. The mean is
  the wrong statistic for user-perceived latency.

## Superlinear cost at scale

The everyday version is the database **N+1 query**: one query per row instead of
one for the whole set, so cost climbs with the data. An empirical big-O
regression gate catches it before production scale does, as does a profile of a
realistic dataset. Tools that assert on the query count are in
[algorithmic complexity testing](https://quality.stereobooster.com/algorithmic-complexity.md).

## What success looks like

- **Tail-latency percentiles are named and tracked.** Specific
  percentiles tied to user impact (p99, p99.9), not the mean.
- **SLOs and error budgets are explicit.** *"p99 < 200 ms on the
  search endpoint, error budget X minutes per quarter."*
- **Regressions are caught in CI, not in production.** A
  microbenchmark or pre-merge load test fails before merge.
- **The benchmark setup is reproducible.** Same hardware, same
  data, same warm-up; numbers don't move by 30% between runs.

## Referenced by

- [Quality dimensions](https://quality.stereobooster.com/quality-dimensions.md) · Quality dimensions
- [Choosing methods](https://quality.stereobooster.com/choosing.md) · Overview

## Acronyms

- SLO — service level objective