在V8中使用数组（性能问题）

本文介绍了在V8中使用数组（性能问题）的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 我尝试了下一个代码（它在Google Chrome和nodejs中显示类似的结果）： $ p $ var c = new Array（200000 ）; console.time（ WTF）; for（var i = 0; i wtf：27839.499ms undefined 我也运行了下一个测试： p> var t = []; console.time（ WTF）; for（var i = 0; i wtf：449.948ms undefined var t = []; console.time（ WTF）; for（var i = 0; i wtf：406.710ms undefined 但是在Firefox中，第一个变体： >>> var t = new Array（200000）; console.time（ WTF）; ... {t.push（Math.random（））;} console.timeEnd（'wtf'）; wtf：602ms 在V8中会发生什么？ $ pre> var t = new Array（99999）; console.time（ WTF）; for（var i = 0; i wtf：220.936ms undefined var t = new Array（100000）; t [99999] = 1; console.time（ WTF）; for（var i = 0; i wtf：1731.641ms undefined var t = new Array（100001）; console.time（ WTF）; for（var i = 0; i wtf：1703.336ms undefined var t = new Array（180000）; console.time（ WTF）; for（var i = 0; i wtf：1725.107ms undefined var t = new Array（181000）; console.time（ WTF）; for（var i = 0; i wtf：27587.669ms undefined 解决方案如果预分配，不要使用 .push ，因为您将创建一个由散列表支持的稀疏数组。 您可以预先分配最多99999个元素的稀疏数组由一个C数组支持，之后它是一个哈希表。 第二个数组是从0开始以连续的方式添加元素的，所以它将由一个真正的C数组，而不是一个哈希表。 如此大概： 0到Length-1，没有孔，那么它可以用一个快速C数组表示。如果你的数组中有的空洞，那么它将用一个非常慢的散列表来表示。例外是，如果你预先分配一个数组的大小< 100000，那么你可以在数组中有空洞，仍然得到一个C数组的支持： pre $ var $ a = new Array（N ）; //如果N a [50000] =稀疏; var b = [] //或新数组（N），N> = 100000 // B将由散列表b [50000] =疏; //b.push(\"Sparse），与上面的大致相同，如果你使用新的Array与N> 0 I tried next code (it shows similar results in Google Chrome and nodejs):var t = new Array(200000); console.time('wtf'); for (var i = 0; i < 200000; ++i) {t.push(Math.random());} console.timeEnd('wtf');wtf: 27839.499msundefinedI also runned next tests:var t = []; console.time('wtf'); for (var i = 0; i < 400000; ++i) {t.push(Math.random());} console.timeEnd('wtf');wtf: 449.948msundefinedvar t = []; console.time('wtf'); for (var i = 0; i < 400000; ++i) {t.push(undefined);} console.timeEnd('wtf');wtf: 406.710msundefinedBut in Firefox all looks fine with the first variant:>>> var t = new Array(200000); console.time('wtf'); ...{t.push(Math.random());} console.timeEnd('wtf');wtf: 602msWhat happens in V8?UPD* magically decreasing performance *var t = new Array(99999); console.time('wtf'); for (var i = 0; i < 200000; ++i) {t.push(Math.random());} console.timeEnd('wtf');wtf: 220.936msundefinedvar t = new Array(100000); t[99999] = 1; console.time('wtf'); for (var i = 0; i < 200000; ++i) {t.push(Math.random());} console.timeEnd('wtf');wtf: 1731.641msundefinedvar t = new Array(100001); console.time('wtf'); for (var i = 0; i < 200000; ++i) {t.push(Math.random());} console.timeEnd('wtf');wtf: 1703.336msundefinedvar t = new Array(180000); console.time('wtf'); for (var i = 0; i < 200000; ++i) {t.push(Math.random());} console.timeEnd('wtf');wtf: 1725.107msundefinedvar t = new Array(181000); console.time('wtf'); for (var i = 0; i < 200000; ++i) {t.push(Math.random());} console.timeEnd('wtf');wtf: 27587.669msundefined 解决方案 If you preallocate, do not use .push because you will create a sparse array backed by a hashtable. You can preallocate sparse arrays up to 99999 elements which will be backed by a C array, after that it's a hashtable.With the second array you are adding elements in a contiguous way starting from 0, so it will be backed by a real C array, not a hash table.So roughly:If your array indices go nicely from 0 to Length-1, with no holes, then it can be represented by a fast C array. If you haveholes in your array, then it will be represented by a much slower hash table. The exception is that if you preallocate an arrayof size < 100000, then you can have holes in the array and still get backed by a C array:var a = new Array(N); //If N < 100000, this will not make the array a hashtable:a[50000] = "sparse";var b = [] //Or new Array(N), with N >= 100000//B will be backed by hash tableb[50000] = "Sparse";//b.push("Sparse"), roughly same as above if you used new Array with N > 0 这篇关于在V8中使用数组（性能问题）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！