ES6 Proxy 性能之我见
作者:互联网
本文翻译自https://thecodebarbarian.com/thoughts-on-es6-proxies-performance
Proxy是ES6的一个强力功能,它通过为 get/set
一个属性 设置"陷阱"(函数处理器)让我们可以拦截对于对象属性的操作。比如:
const obj = {}; const proxy = new Proxy(obj, { get: () => { console.log('hi'); } }); obj.a; // "hi"
Proxy被称赞为现在已经被废弃的Object.observe()
属性的取代者。
然而不幸的是,Proxy有一个致命缺陷:性能。
更打击人的是,Object.observe()
就是因为性能被废弃的,而以我(原作者)对V8的理解,对于JIT(Just in Time,准时制)来说,Object.observe()
比Proxy容易优化多了。
Proxy到底有多慢?
我(原作者)在node v6.9.0中用benchmark简单试了一下:
var Benchmark = require('benchmark'); var suite = new Benchmark.Suite; var obj = {}; var _obj = {}; var proxy = new Proxy(_obj, { set: (obj, prop, value) => { _obj[prop] = value; } }); var defineProp = {}; Object.defineProperty(defineProp, 'prop', { configurable: false, set: v => defineProp._v = v }); // 译者注: vanilla js 指的就是原生js suite. add('vanilla', function() { obj.prop = 5; }). add('proxy', function() { proxy.prop = 5; }). add('defineProperty', function() { defineProp.prop = 5; }). on('cycle', function(event) { console.log(String(event.target)); }). on('complete', function() { console.log('Fastest is ' + this.filter('fastest').map('name')); }). run();
结果如下:
$ node proxy.js vanilla x 74,288,023 ops/sec ±0.78% (86 runs sampled) proxy x 3,625,152 ops/sec ±2.51% (86 runs sampled) defineProperty x 74,815,513 ops/sec ±0.80% (85 runs sampled) Fastest is defineProperty,vanilla $
从这个简单的benchmark中我们可以看到,Proxy的set
比直接赋值和defineProperty慢非常多(译者注:ops/sec,每秒进行的操作数,越大越快)。
为防大家好奇,我(原作者)又在node 4.2.1测试了一下Object.observe()
:
$ node proxy.js vanilla x 78,615,272 ops/sec ±1.55% (84 runs sampled) defineProperty x 79,882,188 ops/sec ±1.31% (85 runs sampled) Object.observe() x 5,234,672 ops/sec ±0.86% (89 runs sampled) Fastest is defineProperty,vanilla
有些文章可能让你觉得只要Proxy不用get/set
而是只设置getOwnPropertyDescriptor()
的话,就比其他的快,于是我(原作者)又试了试:
var _obj = {}; var propertyDescriptor = { configurable: true, set: v => { _obj.prop = v; } }; var proxy = new Proxy(_obj, { getOwnPropertyDescriptor: (target, prop) => propertyDescriptor });
不幸的是,反而更慢了:
$ node proxy.js vanilla x 73,695,484 ops/sec ±1.04% (88 runs sampled) proxy x 2,026,006 ops/sec ±0.74% (90 runs sampled) defineProperty x 74,137,733 ops/sec ±1.25% (88 runs sampled) Fastest is defineProperty,vanilla $
用Proxy包裹一个函数并调用同样比原生的包裹函数并调用慢非常多:
var Benchmark = require('benchmark'); var suite = new Benchmark.Suite; var fn = () => 5; var proxy = new Proxy(function() {}, { apply: (target, context, args) => fn.apply(context, args) }); var wrap = () => fn(); // add tests suite. add('vanilla', function() { fn(); }). add('proxy', function() { proxy(); }). add('wrap', function() { wrap(); }). on('cycle', function(event) { console.log(String(event.target)); }). on('complete', function() { console.log('Fastest is ' + this.filter('fastest').map('name')); }). run();
$ node proxy2.js vanilla x 78,426,813 ops/sec ±0.93% (88 runs sampled) proxy x 5,244,789 ops/sec ±2.17% (87 runs sampled) wrap x 75,350,773 ops/sec ±0.85% (85 runs sampled) Fastest is vanilla
无用的提升Proxy性能的方法
目前最有影响力的提升Proxy性能的方法是让被修改的属性的configurable设为false:
var _obj = {}; Object.defineProperty(_obj, 'prop', { configurable: false }); var propertyDescriptor = { configurable: false, enumerable: true, set: v => { _obj.prop = v; } }; var proxy = new Proxy(_obj, { getOwnPropertyDescriptor: (target, prop) => propertyDescriptor });
(译者注:这段代码有些问题,enumerable
在configurable
为false时是无效的)
$ node proxy.js vanilla x 74,622,163 ops/sec ±0.95% (85 runs sampled) proxy x 4,649,544 ops/sec ±0.47% (85 runs sampled) defineProperty x 77,048,878 ops/sec ±0.60% (88 runs sampled) Fastest is defineProperty $
要是这样写set/get
,还不如直接用 Object.defineProperty()
。
这样写的话,你就不得不设置每个你要在Proxy中用到的属性不可配置(not configurable)。
不然的话,V8就会报错:
var _obj = {}; Object.freeze(_obj); var propertyDescriptor = { configurable: false, enumerable: true, set: v => { _obj.prop = v; } }; var proxy = new Proxy(_obj, { getOwnPropertyDescriptor: (target, prop) => propertyDescriptor }); // Throws: // "TypeError: 'getOwnPropertyDescriptor' on proxy: trap returned // descriptor for property 'prop' that is incompatible with the // existing property in the proxy target" // 拦截'prop'属性返回的descriptor和target(原对象)已经存在的属性不匹配 proxy.prop = 5;
Proxy 也要不行了么?
Proxy比 Object.defineProperty()
有不少优点:
- Proxy 可以嵌套,而
Object.defineProperty()
的getter/setter
就不能嵌套,这样你就不需要知道提前知道你要拦截的所有属性 - 可以拦截数组变化
但它性能太差了。
性能有多大影响呢?
以Promise和回调为例:
var Benchmark = require('benchmark'); var suite = new Benchmark.Suite; var handleCb = cb => cb(null); // add tests suite. add('new function', function() { handleCb(function(error, res) {}); }). add('new promise', function() { return new Promise((resolve, reject) => {}); }). add('promise resolve', function() { Promise.resolve().then(() => {}); }). on('cycle', function(event) { console.log(String(event.target)); }). on('complete', function() { console.log('Fastest is ' + this.filter('fastest').map('name')); }). run();
$ node promise.js new function x 26,282,805 ops/sec ±0.74% (90 runs sampled) new promise x 1,953,037 ops/sec ±1.02% (86 runs sampled) promise resolve x 194,173 ops/sec ±13.80% (61 runs sampled) Fastest is new function $
Promise也慢了非常多,
但是 bluebird声称为Promise提供"非常好的性能",测试一下:
$ node promise.js new function x 26,986,342 ops/sec ±0.48% (89 runs sampled) new promise x 11,157,758 ops/sec ±1.05% (87 runs sampled) promise resolve x 671,079 ops/sec ±27.01% (18 runs sampled) Fastest is new function
虽然快了很多,但仍然比回调慢不少。
所以我们要因此放弃Promise么?
并不是这样的,很多公司仍然选择了使用Promise。我(原作者)虽然不是很确定,但是Uber好像就在使用Promise。
结论
Proxy很慢,但是在你因其性能而放弃它之前,记得同样性能很差的Promise在最近几年中被快速采用。
如果你想使用代理,很可能你不会感觉到性能的影响,除非你发现自己为了性能的原因改变了Promise库(或者完全避开了它们)。
更新
2019.01, 在node v11.3.0中: Promise已经变得足够好, Proxy还是那样
vanilla x 833,244,386 ops/sec ±0.76% (89 runs sampled) proxy x 28,590,800 ops/sec ±0.72% (88 runs sampled) wrap x 824,349,552 ops/sec ±0.87% (86 runs sampled) Fastest is vanilla,wrap new function x 834,121,566 ops/sec ±0.82% (89 runs sampled) new promise x 819,789,350 ops/sec ±0.76% (87 runs sampled) promise resolve x 1,212,009 ops/sec ±40.98% (30 runs sampled) Fastest is new function
标签:function,ES6,runs,sampled,ops,之我见,sec,Proxy,var 来源: https://blog.51cto.com/u_15166492/2708118