2nd/10_Wiki/Topics/DevOps_and_Security/High Resolution Time.md

---
id: wiki-2026-0508-high-resolution-time
title: High Resolution Time
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [performance.now, Monotonic Time, HR Time, Hi-Res Timer]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [web-api, performance, timing, security]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: JavaScript/C
  framework: W3C-HRTime/POSIX
---

# High Resolution Time

## 매 한 줄
> **"매 sub-millisecond 정밀도의 매 monotonic clock"**. W3C High Resolution Time spec — `performance.now()` 가 매 `Date.now()` 의 ms 한계 + wall-clock jitter 를 매 해결. 매 Spectre 후 — 모든 brower 가 매 timer 정밀도를 매 100µs ~ 1ms 로 매 reduce + cross-origin isolation 으로 매 5µs 회복.

## 매 핵심

### 매 vs Date.now
- `Date.now()`: wall clock, ms 단위, NTP 으로 점프 가능 (음수 delta!).
- `performance.now()`: monotonic, fractional ms, navigation start 기준.

### 매 timer attack mitigation
- Spectre/Meltdown (2018) → 매 brower 가 timer fuzz/round.
- Default: 100µs ~ 1ms rounding + jitter.
- COOP+COEP (cross-origin isolated) 시 → 5µs 정밀 + `SharedArrayBuffer`.

### 매 응용
1. Animation frame timing (`rAF` callback).
2. Performance profiling (`User Timing API`).
3. WebGL/WebGPU frame budget tracking.
4. Audio scheduling (`AudioContext.currentTime` 동기).

## 💻 패턴

### Basic timing
```javascript
const t0 = performance.now();
heavyWork();
const elapsed = performance.now() - t0;
console.log(`took ${elapsed.toFixed(3)} ms`);  // 12.345 ms
```

### User Timing API (DevTools 표시)
```javascript
performance.mark('render-start');
render();
performance.mark('render-end');
performance.measure('render', 'render-start', 'render-end');
const [m] = performance.getEntriesByName('render');
console.log(m.duration);
// Visible in Chrome DevTools Performance panel.
```

### Frame budget tracker
```javascript
let lastT = performance.now();
function frame(now) {
  const dt = now - lastT;
  lastT = now;
  if (dt > 16.7) console.warn(`slow frame: ${dt.toFixed(1)}ms`);
  requestAnimationFrame(frame);
}
requestAnimationFrame(frame);
```

### Cross-origin isolated (max precision)
```http
# Server response headers
Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp
```
```javascript
if (crossOriginIsolated) {
  // performance.now() granularity ~5µs
  // SharedArrayBuffer 가능
}
```

### POSIX equivalent (C)
```c
#include <time.h>
struct timespec t0, t1;
clock_gettime(CLOCK_MONOTONIC, &t0);
do_work();
clock_gettime(CLOCK_MONOTONIC, &t1);
double elapsed_ns = (t1.tv_sec - t0.tv_sec) * 1e9 + (t1.tv_nsec - t0.tv_nsec);
```

### Rust std (cross-platform)
```rust
use std::time::Instant;
let t0 = Instant::now();
heavy_work();
println!("took {:?}", t0.elapsed());  // sub-ns precision on modern HW
```

### Debounce slow timers
```javascript
// If you measure dt < 0.1ms repeatedly, you're in fuzzed timer mode.
function isolatedPrecisionAvailable() {
  const samples = Array.from({length: 100}, () => {
    const a = performance.now(); const b = performance.now();
    return b - a;
  });
  return samples.some(d => d > 0 && d < 0.05);  // sub-100µs visible
}
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Wall-clock event log | `Date.now()` / `Date.toISOString()` |
| Profile / micro-bench | `performance.now()` + User Timing |
| Frame loop | rAF callback timestamp (매 monotonic) |
| Audio sync | `AudioContext.currentTime` |
| Cross-origin iframe | postMessage with monotonic delta |
| Native (Linux/macOS) | `clock_gettime(CLOCK_MONOTONIC)` |
| Native (Windows) | `QueryPerformanceCounter` |

**기본값**: Duration 측정엔 매 monotonic. 매 Date 는 user-facing timestamp 만.

## 🔗 Graph
- 변형: [[Long Tasks]]
- 응용: [[Core Web Vitals Optimization (INP, LCP, CLS)|Core Web Vitals]]
- Adjacent: [[Spectre]] · [[Cross-Origin Isolation]] · [[SharedArrayBuffer]]

## 🤖 LLM 활용
**언제**: 성능 측정 코드 작성, profiling, frame budget 분석.
**언제 X**: Persistent timestamp / event log 매 wall-clock 필요 시.

## ❌ 안티패턴
- **`Date.now()` for delta**: NTP step 시 음수 / 점프 가능.
- **`new Date()` 매 hot loop**: allocation cost + ms 한계.
- **Assuming sub-ms precision**: COOP/COEP 없으면 매 1ms rounded.
- **Cross-origin worker timing**: postMessage 의 매 ms 단위 transmit overhead.
- **`setTimeout` 으로 정밀 timing**: 매 4ms+ minimum, jittery.

## 🧪 검증 / 중복
- Verified (W3C HR Time Level 3, MDN performance.now, Chrome timer reduction notes).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — performance.now + Spectre mitigation + COOP/COEP 5µs |