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Parallel page rendering with Mozilla Servo

The free-software community may not have excessively many browsers to choose from these days, but there are even fewer choices for developers in search of a web rendering engine—WebKit and Blink dominate the category. Thus, Mozilla's Servo project is interesting for providing another option to consider and, as Lars Bergstrom and Mike Blumenkrantz explained in their LinuxCon Japan talk [PDF], Servo also takes on some new challenges. It is a new engine written with the needs of modern web usage in mind—namely, security and extensive parallelism.

Bergstrom works at Mozilla Research, while Blumenkrantz is from Samsung; Servo is a joint venture of the two organizations. Bergstrom started the session by addressing the fundamental "why" question: why create a new web-rendering engine from scratch then there are several robust engines already (namely WebKit, Blink, and Gecko)? The answer, he said, is that all of the existing engines were designed prior to 2000. The state of the web has changed significantly since then.

One of the biggest such shifts can be seen in the fact that existing browser engines provide parallelism only at an extremely coarse level: separate processes for each tab. In recent years, the main trend in CPU development has been adding more cores, so there are important gains to be made by parallelizing more stages in each page-rendering task. These gains are particularly noticeable on mobile devices, he said, which often run at lower clock speeds and have multiple CPU cores sitting idle. The existing browser engines also tend to be rather monolithic in design, with tightly coupled components that cannot be improved or replaced easily.

The second justification for the project is that the team believes it can improve security by working in the Rust language, taking advantage of Rust's inherent memory safety. Memory handling is the source of most browser security issues, they said, stemming largely from C++'s memory model. Finally, he said, while Gecko used to be maintained as an embeddable component, Mozilla long ago dropped that feature and focused on making Gecko a highly tuned engine for Firefox. Mozilla's commitment, he said, is to making the web better, and that means exploring a rendering engine designed for embedding in other projects.

Bergstrom then spent a few minutes discussing Rust and, in particular, its memory-safety features. Rust is designed to provide memory safety without overhead, he said: no reference counting is required, no garbage collection is needed, and none of the "smart" pointer classes "that litter most C++ codebases" are necessary. Rust's memory safety comes through its type system: every value has an owner, and if ownership changes hands, the previous owner can no longer access the object through references. Thus, there are no race conditions and no need to guard against them with locks.

The Rust toolchain provides static checks that will catch attempts at violating ownership. While Rust's model means there are some types of code you can't write, he said, it uses essentially the same syntax as C++, except that it is safe for concurrency.

Rendering in parallel

Utilizing Rust's concurrency, of course, is the goal of using it in Servo. Bergstrom showed a block diagram of the page-rendering process: a page's HTML, CSS, and JavaScript are first parsed, then a document object model (DOM) constructed. The DOM is handed to the layout engine, where the individual pieces are rendered and displayed. The Servo team investigated where would be the best point in that process to speed matters up with parallelism—and it turned out that the answer was "anywhere."

The reason this is the case is that most modern web pages are composed of multiple independent blocks: <iframe>s and <div>s, for example, many of which are loaded separately. Consider Pinterest and Reddit, he said: the sites load dozens of separate page elements that are independent, and generally of predictable size. Rendering each in a separate thread allows Servo to complete the entire DOM tree in minimal time.

In Servo's rendering architecture, each tab is a "constellation" that gets assigned a number of rendering pipeline threads. The tab's contents are broken down into a DOM tree, then some number of pipelines are started and begin running a work-stealing algorithm to process the nodes in the tree. Each of the pipelines has its own renderer, script engine, and layout engine, although they must (by necessity) share some items that are reused in the page. Bergstrom and Blumenkrantz showed a live demo of Servo keeping an animated image looping smoothly on one side of a page while it continued to dynamically lay out and render the remainder of the page.

The amount of the speed-up experienced when using Servo instead of Gecko depends on how many pipeline threads are started up, and comparisons can be difficult at present because Servo does not yet handle as many HTML and CSS elements as Gecko. But the results are already impressive; the team said that Servo provides a 2x speed increase when rendering the CNN homepage and a 3x speed-up on Reddit. The tricky part is determining which parts of the DOM can be split off to be rendered separately, but even "worst-case scenario" sites like Wikipedia (which is one long page of text) can be parallelized to some degree—sidebars, headers, and image blocks can all be handled independently.

Through testing, the team discovered another benefit that was not obvious at the outset: parallelism results in power savings. Multiple threads working in parallel on a page-rendering job allow the CPU to complete the entire page in the same amount of time while running at a lower frequency.

Making it embeddable

Blumenkrantz then discussed the embeddability of Servo. Project members decided they wanted to avoid the mistakes that other browser-engine projects have made, he said. The most oft-cited complaint about WebKit and Blink is that they have rapidly changing APIs and ABIs. "So you have to either be constantly updating your code or you have to package up the full engine with your application." Consequently, the Servo team wanted to establish a commitment to a reliable API.

They discovered Chromium Embedded Framework (CEF), a small project attempting to make an API-stable engine from Google's Chromium. CEF does not have many users (at least, not many compared to WebKit and Blink), but the users it does have are rather large ones, such as Adobe and Steam. The team decided to commit to supporting the same API as CEF. That would drastically simplify testing, since Servo could be a drop-in replacement for CEF in real workloads and with unit tests.

The implementation was a long and rough process, he said, starting with a massive list of issues in the GitHub repository, and grinding through the list one at a time—using LD_PRELOAD hacks to test individual components. Servo now has full symbol and ABI coverage for CEF's interfaces, he said, although some interfaces are placeholders. The functions are implemented as foreign function interfaces in Rust and all of the structs and memory alignments are identical. The work has now turned toward enforcing identical behavior, he said, like ensuring that two callbacks always fire in the same order in Servo that they do in CEF.

Bergstrom noted that Servo now passes the Acid1 and Acid2 tests and has implemented about 85% of the CSS2 specification. For those tracking feature parity on such matters, he said, "we're creeping right up on Internet Explorer 6." Progress is just a matter of knocking out more and more DOM and CSS features. Over the course of the next quarter, the team is focusing on features that demonstrably break real-world sites—such as the invisible pop-ups that CNN.com uses for trackers. There is still work to be done improving performance, especially in the graphics pipeline, but the team hopes to have an alpha release before the end of 2015. "I wouldn't recommend logging in to your bank with it," he said, "but it should be usable as a basic browser."

Both speakers encouraged interested volunteers to get involved. The entire project is developed in the open at GitHub, they said, and the team prides itself on maintaining a list of "easy bugs" (such as fixing function names) that are actually easy. As the roadmap indicates, while there is no plan to port desktop Firefox over to Servo, individual Servo components could make their way into Gecko. Bergstrom and Blumenkrantz also noted that Servo may have a future with Firefox for Android and Firefox OS; it is already testing on those platforms, via a Java wrapper on Android and over Firefox OS's interprocess communication (IPC) system.

There were a few technical questions from the audience at the end of the session. One attendee asked whether or not all of Servo was written in Rust. The speakers replied that Servo uses Mozilla's existing JavaScript engine, SpiderMonkey, as well as its Azure library for 2D graphics drawing (both of which are C++ projects). Another audience member asked if they had explored other forms of parallelism, such as GPU parallelism. The team had explored it, they replied, but found that as of now, the I/O overhead required to move page contents in and out of the GPU erased all performance gains. They had, however, found some additional performance gains through using SIMD instructions to compute bounding boxes, but that implementation was on hold for now because it would require altering a lot of data structures.

On a final note, Bergstrom and Blumenkrantz answered one audience question about the suitability of Rust for new projects. They responded that the tool support is good—the compiler could use some speed improvements, but it works well with Valgrind, GDB, and other standard tools. For someone used to writing C++ today, Rust is well worth considering. Developers who are already working well in Go or Python may find it a hard adjustment to make, though, since the ownership-based memory model is not as easy to use as garbage collection. In any case, they said, "don't bet the whole team on it. Start it as an experiment first." That seems to be the approach Mozilla is taking, although the signs point to the Servo experiment as having a bright future, at least on some platforms.

[The author would like to thank the Linux Foundation for travel assistance to attend LCJ 2015.]

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