性能问题初步诊断 ​

const mem = process.memoryUsage();
console.log({
  rss: `${Math.round(mem.rss / 1024 / 1024)} MB`,        // 总内存
  heapTotal: `${Math.round(mem.heapTotal / 1024 / 1024)} MB`,  // 堆总量
  heapUsed: `${Math.round(mem.heapUsed / 1024 / 1024)} MB`,    // 堆使用
  external: `${Math.round(mem.external / 1024 / 1024)} MB`     // C++ 对象
});

setInterval(() => {
  const mem = process.memoryUsage();
  console.log(`内存: ${Math.round(mem.heapUsed / 1024 / 1024)} MB`);
}, 5000);

// 可疑的模式
const cache = {};

function process(id, data) {
  cache[id] = data;  // 只增不减
}

// 改进：限制缓存大小
const cache = new Map();
const MAX_SIZE = 1000;

function process(id, data) {
  if (cache.size >= MAX_SIZE) {
    const firstKey = cache.keys().next().value;
    cache.delete(firstKey);
  }
  cache.set(id, data);
}

console.time('数据库查询');
const users = await db.query('SELECT * FROM users');
console.timeEnd('数据库查询');
// 数据库查询: 45.123ms

const start = process.hrtime.bigint();
// 执行操作
const end = process.hrtime.bigint();
console.log(`耗时: ${(end - start) / 1000000n} ms`);

const { performance } = require('perf_hooks');

const start = performance.now();
// 执行操作
const duration = performance.now() - start;
console.log(`耗时: ${duration.toFixed(2)} ms`);

let lastCheck = Date.now();

setInterval(() => {
  const now = Date.now();
  const delay = now - lastCheck - 100;  // 100ms 间隔
  
  if (delay > 50) {  // 超过 50ms 延迟
    console.warn(`事件循环阻塞: ${delay}ms`);
  }
  
  lastCheck = now;
}, 100);

// 同步读取大文件
const content = fs.readFileSync('large-file.txt');  // 阻塞

// 改用异步
const content = await fs.promises.readFile('large-file.txt');

// 复杂计算
for (let i = 0; i < 10000000; i++) {
  // 大量计算阻塞事件循环
}

// 改用 setImmediate 分片
async function processLarge(items) {
  const CHUNK = 1000;
  for (let i = 0; i < items.length; i += CHUNK) {
    const chunk = items.slice(i, i + CHUNK);
    processChunk(chunk);
    await new Promise(r => setImmediate(r));  // 让出控制权
  }
}

// 慢
const files = fs.readdirSync(dir);
for (const file of files) {
  const content = fs.readFileSync(path.join(dir, file));
}

// 快
const files = await fs.promises.readdir(dir);
await Promise.all(files.map(async (file) => {
  const content = await fs.promises.readFile(path.join(dir, file));
}));

// 慢：每次请求都解析
app.get('/config', (req, res) => {
  const config = JSON.parse(fs.readFileSync('config.json'));
  res.json(config);
});

// 快：启动时加载一次
const config = JSON.parse(fs.readFileSync('config.json'));
app.get('/config', (req, res) => {
  res.json(config);
});

// 危险：可能耗尽资源
await Promise.all(urls.map(url => fetch(url)));

// 安全：限制并发
async function fetchWithLimit(urls, limit = 5) {
  const results = [];
  let index = 0;
  
  async function worker() {
    while (index < urls.length) {
      const i = index++;
      results[i] = await fetch(urls[i]);
    }
  }
  
  await Promise.all(Array(limit).fill().map(worker));
  return results;
}