但为什么浏览器DOM在经过10年的努力后仍然如此缓慢？ [英] But why's the browser DOM still so slow after 10 years of effort?
问题描述
网络浏览器DOM自90年代末以来一直存在，但它仍然是性能/速度最大的限制之一。
我们拥有来自Google，Mozilla，Microsoft，Opera，W3C以及其他各方面为我们所有人提供网络技术的世界上最聪明的人物，所以显然这不是一个简单的哦，我们没有优化它的问题。
我的问题是网络浏览器的一部分专门处理这个问题，为什么我有这么难的时间让它运行得更快？
我的问题不是请求
有趣的是，似乎不同的浏览器似乎都有不同生成DOM时的挑战为什么在这里存在这种差异？
当您在DOM中更改某些内容时，可能会有无数的副作用与重新计算布局，样式表等。
这不是唯一的原因：当您设置 element.innerHTML = x 你不再处理普通的在这里存储一个值变量，而是在设置它们时更新浏览器内部状态的特殊对象。
element.innerHTML = x 的全部含义是巨大的。粗糙的概述：
- 解析
x作为HTML - 请求浏览器扩展获取权限
- 破坏
元素 - 创建的现有子节点子节点
- 重新计算由父子关系定义的样式
- 重新计算页面元素的物理尺寸
- 通知浏览器扩展更改
- 更新JavaScript变量，这些变量是实际DOM节点的句柄
所有这些更新都必须通过一个API来桥接Javascript和HTML引擎。 Javascript的这个日子很快的一个原因是我们将它编译成更快的语言甚至是机器代码，因为这些值的行为被明确定义，所以出现了大量的优化。通过DOM API工作时，可以使用 none 。其他地方的加速使DOM落后。
The web browser DOM has been around since the late '90s, but it remains one of the largest constraints in performance/speed.
We have some of the world's most brilliant minds from Google, Mozilla, Microsoft, Opera, W3C, and various other organizations working on web technologies for all of us, so obviously this isn't a simple "Oh, we didn't optimize it" issue.
My question is if i were to work on the the part of a web browser that deals specifically with this, why would I have such a hard time making it run faster?
My question is not asking what makes it slow, it's asking why hasn't it become faster?
This seems to be against the grain of what's going on elsewhere, such as JS engines with performance near that of C++ code.
Example of quick script:
for (var i=0;i<=10000;i++){
someString = "foo";
}
Example of slow because of DOM:
for (var i=0;i<=10000;i++){
element.innerHTML = "foo";
}
Some details as per request:
After bench marking, it looks like it's not an unsolvable slow issue, but often the wrong tool is used, and the tool used depends on what you're doing cross-browser.
It looks like the DOM efficiency varies greatly between browsers, but my original presumption that the dom is slow and unsolvable seems to be wrong.
I ran tests against Chrome, FF4, and IE 5-9, you can see the operations per second in this chart:
Chrome is lightning fast when you use the DOM API, but vastly slower using the .innerHTML operator (by a magnitude 1000-fold slower), however, FF is worse than Chrome in some areas (for instance, the append test is much slower than Chrome), but the InnerHTML test runs much faster than chrome.
IE seems to actually be getting worse at using DOM append and better at innerHTML as you progress through versions since 5.5 (ie, 73ops/sec in IE8 now at 51 ops/sec in IE9).
I have the test page over here:
http://jsperf.com/browser-dom-speed-tests2
What's interesting is that it seems different browsers seem to all be having different challenges when generating the DOM. Why is there such disparity here?
When you change something in the DOM it can have myriad side-effects to do with recalculating layouts, style sheets etc.
This isn't the only reason: when you set element.innerHTML=x you are no longer dealing with ordinary "store a value here" variables, but with special objects which update a load of internal state in the browser when you set them.
The full implications of element.innerHTML=x are enormous. Rough overview:
- parse
xas HTML - ask browser extensions for permission
- destroy existing child nodes of
element - create child nodes
- recompute styles which are defined in terms of parent-child relationships
- recompute physical dimensions of page elements
- notify browser extensions of the change
- update Javascript variables which are handles to real DOM nodes
All these updates have to go through an API which bridges Javascript and the HTML engine. One reason that Javascript is so fast these days is that we compile it to some faster language or even machine code, masses of optimisations happen because the behaviour of the values is well-defined. When working through the DOM API, none of this is possible. Speedups elsewhere have left the DOM behind.
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