performance-optimization（性能优化）

Performance Optimization

Overview

Measure first, optimize second. This guide covers profiling techniques and optimization strategies.

Profiling First

The Golden Rule

1. Don't optimize prematurely
2. Measure before optimizing
3. Optimize the biggest bottleneck first
4. Measure again to verify improvement

CPU Profiling (Node.js)

// Using Node.js built-in profiler
// Start with: node --prof app.js
// Process: node --prof-process isolate-*.log > profile.txt

// Or use clinic.js
// npm install -g clinic
// clinic doctor -- node app.js
// clinic flame -- node app.js

// Programmatic profiling
import { performance, PerformanceObserver } from 'perf_hooks';

const obs = new PerformanceObserver((items) => {
  items.getEntries().forEach((entry) => {
    console.log(`${entry.name}: ${entry.duration}ms`);
  });
});
obs.observe({ entryTypes: ['measure'] });

performance.mark('start');
await expensiveOperation();
performance.mark('end');
performance.measure('expensive-op', 'start', 'end');

Memory Profiling

// Check memory usage
console.log(process.memoryUsage());
// {
//   rss: 30000000,      // Resident Set Size - total memory
//   heapTotal: 7000000, // V8 heap total
//   heapUsed: 5000000,  // V8 heap used
//   external: 800000    // C++ objects bound to JS
// }

// Take heap snapshot
const v8 = require('v8');
const fs = require('fs');

const snapshotFile = `heap-${Date.now()}.heapsnapshot`;
const snapshot = v8.writeHeapSnapshot(snapshotFile);
// Open in Chrome DevTools Memory tab

Database Optimization

Query Optimization

-- ❌ Slow: SELECT * and no index
SELECT * FROM orders WHERE user_id = 123;

-- ✅ Better: Select only needed columns, with index
CREATE INDEX idx_orders_user_id ON orders(user_id);
SELECT id, total, status FROM orders WHERE user_id = 123;

-- ❌ N+1 query problem
-- For each user, query their orders (100 users = 101 queries)
SELECT * FROM users;
SELECT * FROM orders WHERE user_id = 1;
SELECT * FROM orders WHERE user_id = 2;
...

-- ✅ Single query with JOIN
SELECT u.*, o.* FROM users u
LEFT JOIN orders o ON u.id = o.user_id;

-- Or batch loading
SELECT * FROM orders WHERE user_id IN (1, 2, 3, ...);

Explain Analyze

EXPLAIN ANALYZE
SELECT u.name, COUNT(o.id) as order_count
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
WHERE u.created_at > '2024-01-01'
GROUP BY u.id
ORDER BY order_count DESC
LIMIT 10;

-- Output shows:
-- - Execution plan (Seq Scan vs Index Scan)
-- - Estimated vs actual rows
-- - Time per operation
-- - Total execution time

Index Strategies

-- Single column index
CREATE INDEX idx_users_email ON users(email);

-- Composite index (order matters!)
-- Good for: WHERE status = 'active' AND created_at > '2024-01-01'
CREATE INDEX idx_orders_status_created ON orders(status, created_at);

-- Partial index (smaller, faster for filtered queries)
CREATE INDEX idx_active_users ON users(email)
WHERE status = 'active';

-- Covering index (includes all needed columns)
CREATE INDEX idx_orders_user_covering ON orders(user_id)
INCLUDE (total, status, created_at);

-- When NOT to index:
-- - Small tables
-- - Columns with low cardinality (e.g., boolean)
-- - Frequently updated columns
-- - Write-heavy tables

Caching

Cache Strategies

// Cache-aside pattern
async function getUser(id: string): Promise<User> {
  // Try cache first
  const cached = await cache.get(`user:${id}`);
  if (cached) return JSON.parse(cached);

  // Cache miss - fetch from DB
  const user = await db.users.findById(id);

  // Store in cache
  if (user) {
    await cache.set(`user:${id}`, JSON.stringify(user), 'EX', 3600);
  }

  return user;
}

// Write-through pattern
async function updateUser(id: string, data: Partial<User>): Promise<User> {
  // Update DB
  const user = await db.users.update(id, data);

  // Update cache immediately
  await cache.set(`user:${id}`, JSON.stringify(user), 'EX', 3600);

  return user;
}

Cache Invalidation

// Event-based invalidation
eventBus.on('user:updated', async (userId: string) => {
  await cache.del(`user:${userId}`);
  await cache.del(`user:${userId}:orders`);
});

// Tag-based invalidation
async function invalidateUserRelated(userId: string) {
  const keys = await cache.keys(`*:user:${userId}:*`);
  if (keys.length > 0) {
    await cache.del(...keys);
  }
}

// Version-based cache busting
const CACHE_VERSION = 'v2';
const cacheKey = `${CACHE_VERSION}:user:${id}`;

Memoization

// Simple memoization
function memoize<T extends (...args: any[]) => any>(fn: T): T {
  const cache = new Map();

  return ((...args: Parameters<T>) => {
    const key = JSON.stringify(args);
    if (cache.has(key)) return cache.get(key);

    const result = fn(...args);
    cache.set(key, result);
    return result;
  }) as T;
}

// With TTL
function memoizeWithTTL<T extends (...args: any[]) => any>(
  fn: T,
  ttlMs: number
): T {
  const cache = new Map<string, { value: any; expires: number }>();

  return ((...args: Parameters<T>) => {
    const key = JSON.stringify(args);
    const cached = cache.get(key);

    if (cached && cached.expires > Date.now()) {
      return cached.value;
    }

    const result = fn(...args);
    cache.set(key, { value: result, expires: Date.now() + ttlMs });
    return result;
  }) as T;
}

Frontend Performance

Core Web Vitals

Code Splitting

// React lazy loading
import { lazy, Suspense } from 'react';

const Dashboard = lazy(() => import('./Dashboard'));
const Settings = lazy(() => import('./Settings'));

function App() {
  return (
    <Suspense fallback={<Loading />}>
      <Routes>
        <Route path="/dashboard" element={<Dashboard />} />
        <Route path="/settings" element={<Settings />} />
      </Routes>
    </Suspense>
  );
}

// Dynamic import for heavy libraries
async function processImage(file: File) {
  const sharp = await import('sharp');
  return sharp(file).resize(800).toBuffer();
}

Image Optimization

<!-- Responsive images -->
<img
  src="image-800.jpg"
  srcset="
    image-400.jpg 400w,
    image-800.jpg 800w,
    image-1200.jpg 1200w
  "
  sizes="(max-width: 600px) 400px, 800px"
  loading="lazy"
  alt="Description"
/>

<!-- Modern formats with fallback -->
<picture>
  <source srcset="image.avif" type="image/avif" />
  <source srcset="image.webp" type="image/webp" />
  <img src="image.jpg" alt="Description" />
</picture>

Bundle Optimization

// webpack.config.js
module.exports = {
  optimization: {
    splitChunks: {
      chunks: 'all',
      cacheGroups: {
        vendor: {
          test: /[\\/]node_modules[\\/]/,
          name: 'vendors',
          chunks: 'all',
        },
        // Separate large libraries
        react: {
          test: /[\\/]node_modules[\\/](react|react-dom)[\\/]/,
          name: 'react',
          chunks: 'all',
        },
      },
    },
  },
};

Backend Performance

Connection Pooling

// Database connection pool
import { Pool } from 'pg';

const pool = new Pool({
  max: 20,                    // Maximum connections
  idleTimeoutMillis: 30000,   // Close idle connections after 30s
  connectionTimeoutMillis: 2000, // Fail if can't connect in 2s
});

// Use pool, not individual connections
async function getUser(id: string) {
  const result = await pool.query('SELECT * FROM users WHERE id = $1', [id]);
  return result.rows[0];
}

// HTTP connection keep-alive
import http from 'http';

const agent = new http.Agent({
  keepAlive: true,
  maxSockets: 50,
});

fetch('https://api.example.com', { agent });

Async Processing

// Move slow operations out of request path
app.post('/api/orders', async (req, res) => {
  // Quick: Create order
  const order = await db.orders.create(req.body);

  // Don't wait: Queue async tasks
  await queue.add('send-confirmation-email', { orderId: order.id });
  await queue.add('update-inventory', { items: order.items });
  await queue.add('notify-warehouse', { orderId: order.id });

  // Respond immediately
  res.status(201).json(order);
});

// Process queue separately
queue.process('send-confirmation-email', async (job) => {
  const order = await db.orders.findById(job.data.orderId);
  await emailService.sendOrderConfirmation(order);
});

Response Compression

import compression from 'compression';
import express from 'express';

const app = express();

// Compress responses > 1KB
app.use(compression({
  threshold: 1024,
  filter: (req, res) => {
    if (req.headers['x-no-compression']) {
      return false;
    }
    return compression.filter(req, res);
  }
}));

Algorithm Optimization

Time Complexity

// O(n²) → O(n) with Set
// Find duplicates
function hasDuplicates(arr: number[]): boolean {
  // ❌ O(n²)
  for (let i = 0; i < arr.length; i++) {
    for (let j = i + 1; j < arr.length; j++) {
      if (arr[i] === arr[j]) return true;
    }
  }
  return false;

  // ✅ O(n)
  const seen = new Set<number>();
  for (const num of arr) {
    if (seen.has(num)) return true;
    seen.add(num);
  }
  return false;
}

// O(n) → O(1) with Map
// Lookup by key
class UserCache {
  // ❌ O(n) lookup
  private users: User[] = [];

  find(id: string) {
    return this.users.find(u => u.id === id);
  }

  // ✅ O(1) lookup
  private usersMap = new Map<string, User>();

  findFast(id: string) {
    return this.usersMap.get(id);
  }
}

Space vs Time Tradeoff

// Trade memory for speed: Precompute
class TaxCalculator {
  private taxRates = new Map<string, number>();

  constructor() {
    // Precompute all tax rates
    for (const state of US_STATES) {
      this.taxRates.set(state, this.computeTaxRate(state));
    }
  }

  // O(1) instead of O(complex calculation)
  getTaxRate(state: string): number {
    return this.taxRates.get(state) ?? 0;
  }
}

Monitoring Performance

// Application metrics
import { Counter, Histogram } from 'prom-client';

const httpRequestDuration = new Histogram({
  name: 'http_request_duration_seconds',
  help: 'Duration of HTTP requests',
  labelNames: ['method', 'route', 'status'],
  buckets: [0.1, 0.5, 1, 2, 5]
});

app.use((req, res, next) => {
  const start = Date.now();

  res.on('finish', () => {
    const duration = (Date.now() - start) / 1000;
    httpRequestDuration
      .labels(req.method, req.route?.path || 'unknown', res.statusCode.toString())
      .observe(duration);
  });

  next();
});

Related Skills

• [[database]] - Database optimization details
• [[frontend]] - Frontend performance
• [[monitoring-observability]] - Performance monitoring