‘Just Right’ Multi-Process Architecture Helps Firefox 54 Claim Memory Usage Crown

Firefox 54 launched with a more advanced multi-process architecture than the one we saw implemented in Firefox 48 last year. The improved architecture raises the number of processes enabled by default from two to five, which Mozilla argues is a “just right” compromise between low memory usage on one side and performance and security on the other.

Firefox’s Electrolysis Architecture

Mozilla started work on the multi-process Electrolysis architecture more than eight years ago after it saw the advantages it can bring to a browser. Chrome implemented its multi-process architecture from day one, which could be seen as foresight by Google’s engineers, who saw that this type of architecture would be needed to protect users in the future. It was also a benefit of making a new browser from scratch.

Ryan Pollock, Mozilla’s Senior Product Marketing Manager, admitted that the organization wasn’t willing to switch too aggressively to Electrolysis because it knew it would break the add-ons that made Firefox so popular in its early days. Mozilla may have changed its mind when it saw that advanced add-ons that could modify the browser’s core weren’t needed, as proven by the popularity of Chrome’s simpler extension model.

The organization recently announced that it has embraced the “WebExtensions” API, which will allow for Chrome-like extensions to work in Firefox with little changes. Mozilla also hopes that this will lead to the creation of a new “Browser Extension” specification that will be adopted by all browser vendors.

Switching to a simpler extension model allows Firefox to enable multiple processes and also isolate them in sandboxes. Mozilla previously enabled only two processes, one for the UI and one for content, last year, in Firefox 48. This ensured that the browser wouldn’t hang as much due to web pages affecting the performance of the browser interface. It also brought partial sandboxing by keeping the content isolated from the browser (as much as possible).

Mozilla is now taking it to the next level by implementing one process for the browser interface and four for content. Why four? The organization believes that this is the “just right” amount of processes to have for the majority of users, and also in terms of optimizing memory usage.

The more sandboxed the content is, the less sharing happens between multiple tabs. With a rise in the number of used browser tabs, the memory usage also increases significantly, as “redundant” code is added to each process and sandbox. This is why after four content processes are filled with browser tabs or extensions, Firefox will begin to include multiple tabs in the same process.

Mozilla’s Memory Usage Benchmarks

Mozilla ran its own memory usage benchmarks, which showed significant memory usage reduction compared to Chrome:

Windows 10 — Chrome used 1.77X memory as Firefox (64-bit), and 2.44X as Firefox (32-bit)macOS — Chrome used 1.36X memory as Firefox (64-bit)Linux — Chrome used 1.42X memory as Firefox (64-bit)

Firefox 54 is not only more efficient than Chrome, but also all the other major browsers, too:

This drastic reduction in memory usage may not come without significant drawbacks, at least in terms of security. The end results will remain to be seen in future Pwn2Own contests, as well as through the availability of exploits found against Firefox in the wild. If Firefox’s five processes can be 99% as effective in stopping attacks as Chrome’s “one process per tab” architecture is, then Mozilla may have made the right compromise here.

The good news is that the architecture doesn’t seem to be stuck on five processes-only, because Firefox users can also customize the number of processes to be used in the browser. You can enter about:config in your address bar and then change the number in dom.ipc.processCount.

If you know how many tabs you typically keep open and how many extensions you’ve installed, then you can set that number as the number of processes to be enabled. Pollock said that this feature will be available in the browser’s main settings in a future release, which should make it a little more accessible for all users.

Revamping Firefox For Performance And Security

Over the past couple of years or so, Mozilla seems to have accelerated its plans to revamp Firefox, particularly in terms of performance and security. These actions came not a moment too soon, because Firefox has already lost significant market share in recent years because of Chrome, which many have considered a faster and more secure browser.

It’s yet to be seen whether or not Firefox’s “just right” multi-process architecture can bring the browser close enough to Chrome’s security and performance for the differences to be negligible. However, Mozilla has only just begun to improve Firefox’s performance and security.

"Project Quantum" will be implemented soon in Firefox, which will replace core parts of the rendering engine with code written in Rust, a fast memory-safe language that promises to do away with memory corruption bugs. The experimental browser rendering engine, called Servo, that Mozilla has written in Rust, can take advantage of all threads in a CPU to process the elements of a page.

The benefits of the Rust language coupled with the modern architectures that Mozilla plans to implement in its browser may allow Firefox to eventually surpass Chrome (its biggest rival) in both performance and security in the not too distant future. Once achieved, these advantages may not be easily matched by Chrome, unless Google also decides to rewrite parts of its browser in Rust or a similar fast and memory-safe programming language.