Google Chrome's Multi-process Architecture

Unlike most current web browsers, Google Chrome uses many operating system processes to keep web sites separate from each other and from the rest of your computer. In this blog post, I'll explain why using a multi-process architecture can be a big win for browsers on today's web. I'll also talk about which parts of the browser belong in each process and in which situations Google Chrome creates new processes.

1. Why use multiple processes in a browser?

In the days when most current browsers were designed, web pages were simple and had little or no active code in them. It made sense for the browser to render all the pages you visited in the same process, to keep resource usage low.

Today, however, we've seen a major shift towards active web content, ranging from pages with lots of JavaScript and Flash to full-blown "web apps" like Gmail. Large parts of these apps run inside the browser, just like normal applications run on an operating system. Just like an operating system, the browser must keep these apps separate from each other.

On top of this, the parts of the browser that render HTML, JavaScript, and CSS have become extraordinarily complex over time. These rendering engines frequently have bugs as they continue to evolve, and some of these bugs may cause the rendering engine to occasionally crash. Also, rendering engines routinely face untrusted and even malicious code from the web, which may try to exploit these bugs to install malware on your computer.

In this world, browsers that put everything in one process face real challenges for robustness, responsiveness, and security. If one web app causes a crash in the rendering engine, it will take the rest of the browser with it, including any other web apps that are open. Web apps often have to compete with each other for CPU time on a single thread, sometimes causing the entire browser to become unresponsive. Security is also a concern, because a web page that exploits a vulnerability in the rendering engine can often take over your entire computer.

It doesn't have to be this way, though. Web apps are designed to be run independently of each other in your browser, and they could be run in parallel. They don't need much access to your disk or devices, either. The security policy used throughout the web ensures this, so that you can visit most web pages without worrying about your data or your computer's safety. This means that it's possible to more completely isolate web apps from each other in the browser without breaking them. The same is true of browser plug-ins like Flash, which are loosely coupled with the browser and can be separated from it without much trouble.

Google Chrome takes advantage of these properties and puts web apps and plug-ins in separate processes from the browser itself. This means that a rendering engine crash in one web app won't affect the browser or other web apps. It means the OS can run web apps in parallel to increase their responsiveness, and it means the browser itself won't lock up if a particular web app or plug-in stops responding. It also means we can run the rendering engine processes in a restrictive sandbox that helps limit the damage if an exploit does occur.

Interestingly, using multiple processes means Google Chrome can have its own Task Manager (shown below), which you can get to by right clicking on the browser's title bar. This Task Manager lets you track resource usage for each web app and plug-in, rather than for the entire browser. It also lets you kill any web apps or plug-ins that have stopped responding, without having to restart the entire browser.

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