author: wiltzius@

Intro

Rendering performance is a huge topic of great interest to the Chrome project and web developers. As the web platform is essentially an application development framework, ensuring it has the capability to handle input and put new pixels on the screen at a speed that keeps up with the display’s refresh rate is critical.

“Performance” is an oft-cited problem area for web developers. We spend a lot of time investigating and (hopefully) improving performance of the platform, often guided by specific bad examples. This is the pursuit and elimination of jank.

But what does jank mean? What are people complaining about when they say a web page “feels slow”? This document attempts to categorize the various symptoms that all get put under the “jank” category, for the purposes of accurate identification and precise discussion.

To understand the descriptions of potential causes of jank, it's important to understand roughly how rendering works. See The Rendering Critical Path for a primer; these two documents should be read together.

Finally, a note on the videos of examples -- video playback performance can tragically mask subtle performance problems as it frequently isn't rendered at 60Hz. The examples here should be obvious enough that the problem is still visible, but the originals of these videos are attached to the page, which you can download for slightly more accurate recordings.

Symptom Taxonomy

Incomplete content

Checkerboarding

Some background, for the curious. The checkerboarding problem is a very fundamental one for a document viewer: do you preserve responsiveness to user input or do you only show fully correct content? Chrome has developed a lot of (expensive) machinery to combat this problem, preferencing responsiveness. Checkerboarding occurs mainly during fast scrolling, when the page cannot be rasterized quickly enough to put up a new viewport’s worth of content every frame. When this happens Chrome will continue to scroll in an attempt to preserve the physical metaphor of page under the user’s finger... but in place of the missing content there will just be a blank space. On desktop platforms Chrome will draw a greyish checkerboard pattern; on Android (where this is more common) it draws the background color of the page.

To try to avoid having to checkerboard Chrome will pre-rasterize content around the viewport and keep it as bitmaps in memory. This policy trades CPU and memory resource consumption for a greater likelihood that the page content will be ready if/when the user starts scrolling. It’s essentially a giant buffer to guard against how slow software rasterization can be.

This approach has several limitations. For one, it’s a bit of a resource hog. It also isn’t under the control of the app developer at all, so if for instance they would prefer to jank (not produce a frame) instead of checkerboard when the scroll hits an unrasterized region of the page, they’re out of luck. It’s also ineffective if parts of the page that are pre-rasterized get invalidated by JavaScript -- now that pre-rasterized content is stale. It’s still useful to have around, since at least Chrome can display the pre-rasterized content until the new content is ready (this is the whole pending tree vs. active tree architecture; see the compositing design doc for an explanation), but it isn’t a replacement for fast rasterization. This is one of the (many) reasons motivating the move to GL-based rasterization (Ganesh), and when using Ganesh Chrome pre-paints only a very small buffer around the screen.

Checkerboarding special case: low resolution tiles

YouTube Video

Brief: Parts of the page displaying, but displaying at low resolution instead of high resolution.

This is a “feature” of impl-side painting, designed mainly to gracefully handle pinch/zoom scenarios. Chrome will quickly rasterize large sections of the pending tree (i.e. the content that’s queued up to go on screen) at low resolution first, with the aim of at least showing something. The cause here is identical to the cause of checkerboarding; this is essentially an intermediate state between having the content fully rasterized and not rasterized at all (checkerboard).

Checkerboarding common cause: URL bar jank

Brief: Full-screen invalidations caused by the URL bar’s showing / hiding on scroll.

There are times when Chrome doesn’t have a chance to rasterize enough content before a scroll gets to a region of the page (e.g. on very long pages), but for the most part impl-side painting handles static content well. As mentioned in the checkerboarding section, however, it copes poorly when the page is frequently invalidated by JavaScript. The resize event fired when the URL bar appears and disappears (per the current Chrome for Android omnibox UX) is a tragically common, totally unnecessary cause of invalidation on many mobile websites. Chrome itself has to do a re-layout to account for the additional pixels that just got added or subtracted from the total document height, but this is an optimized layout pass that shouldn’t (theoretically) cause full-document invalidations. Unfortunately many pages listen for the resize event, assume getting it means the page has actually changed dimensions dramatically as in the case of e.g. a rotation, and then modify their entire document’s style in response. This typically ends up looking exactly the same, but invalides everything as a result.

Partial content during network load

Brief: When loading a web page from the network Chrome will attempt to render the page as quickly as possible, even if it doesn’t have all the required resources yet. This is a long-standing feature of the platform to let you read content as quickly as possible, and not technically jank, but is included for completeness.

Unfortunately, it also looks significantly less polished than the style of many native applications, which show nothing (or a splash screen) until a basic UI shell is ready, and fill that shell in with data atomically when the data is ready. It’s possible to emulate this behavior on the web platform, and it looks much more polished to do so, but that’s unfortunately not a very common practice at this point.

Framerate

Low sustained framerate during any kind of animation (“janky animations”)

There’s typically a consistent bottleneck in the system when this is the case, and about:tracing is designed for diagnosing exactly this case. If encountered, a standard trace (no need for Frame Viewer) of the low-framerate animation should show what operations are the long pole for the animation.

It’s worth noting that there are two types of animation: those handled entirely by the compositor thread (so-called “accelerated” animations) and those that need to synchronously call into Blink or v8. The only type of accelerated animations are CSS animations (and CSS transitions and Web Animation) on opacity, transform, and certain CSS filters; plus scrolling and pinch/zoom. Everything else goes through Blink. Accelerated animations will never be bottlenecked on anything in the RendererMain thread, but non-accelerated animations can be.

Delayed beginning to any animation

This is commonly caused by accelerated animations that are set up but require rasterization before beginning. E.g. creating a new layer and setting a CSS animation on its transform -- the animation will not start until the new layer is done being painted. The timeline delay here is designed to prevent animations beginning immediately only to drop a number of frames. The result is typically a much smoother-looking interaction overall, but it can be surprising.

Low framerate during to swipe input (throughput)

Unlike the delay at the beginning of an animation, which can be stretched out tens of milliseconds without the user really noticing, the only acceptable frame time for a running animation is the vsync interval (e.g. 16ms). Touch input can only be programmatically handled by JavaScript on the RendererMain thread, which means that all visual touch-input-based effects are technically non-accelerated animations. Note that they can still avoid painting by using an accelerated animation property, but they’ll subject to jank from all other activity on the main thread because the JavaScript input events can be blocked.

Isolated long pauses (“jank”) during JS-driven animations (incl touch)

Sometimes an animation is mostly fine, but stutters at one point. This is often harder to isolate, but about:tracing and the Dev Tools timeline are invaluable for figuring out what went wrong. Special cases of this include the delay at the beginning of animations. All non-accelerated animations are subject to jank of this type from any other activity on the Blink main thread (e.g. JavaScript timers executing).

Latency

Long input latency (rather than low framerate)

Some examples of high latency are covered in the framerate examples, for instance delays at the beginning of an animation that’s kicked off in response to input. Generally if the entire pipeline takes long enough that it doesn’t fit into frame budget, we categorize it as a framerate rather than a latency issue. However, there are some edge cases where latency can be increased by seemingly unrelated issues.

High scrolling latency or delays beginning scrolling when the document has touch event handlers

There’s a delicate balance between honoring the contract with touch event handlers the page has registered and staying responsive. In particular, becuase JavaScript touch event handlers are stuck on the main thread with everything else, completely unrelated activity such as XHR parsing or style recalculation or JavaScript timers that run on the main thread will all block touch event handlers from running (none of these tasks currently yield, and as of this writing Blink’s entire event loop is a FIFO queue with no prioritization, although that’s changing with the advent of the Blink scheduler). The result is that if style recalculation runs for 100ms right when the user is dragging the page around, the scroll won’t move for 100ms until style recalc is finished, the touch event gets run and preventDefault doesn’t get called during it.

Note that browser behavior here is wildly under-specified. Chrome’s behavior has evolved over time, and changed significantly recently with the advent of asynchronous touch move processing. One of the more egregious behavior hacks here is that Chrome typically gave touch events a ~200ms deadline to run, and if the event wasn’t processed by then it would get dropped and the page would scroll anyway. This was designed to preserve basic responsiveness even in the face of adversarial content.

Scroll-linked effects out of sync

This is somewhat complicated, but boils down to two different modes of operation in Chrome’s multi-stage rendering pipeline. In low-latency mode each stage of the pipeline will complete fast enough that all of the stages complete within 16ms. If any of the stages run long, Chrome will fall out of low latency mode into high latency mode. At that point there will be an extra frame of latency in the entire rendering pipeline, which effectively increases frame budget by allowing thread parallelism but at the cost of latency.

Note that technically there is currently a low/high latency mode for both the main thread > compositor thread step and the compositor thread > browser UI thread step. The problem described here is specific to the main thread > compositor thread step.

The synchronization issue is that scrolling is a compositor-thread operation whereas the scroll-linked effects are necessarily main thread operations. This means that in high latency mode the main thread may be 1 frame behind the compositor thread, which means that the scroll position that the compositor is using to position the page and the scroll position Blink/JavaScript is currently aware of will be 1 step out of sync. Other than keeping the page in low latency mode by never blowing frame budget, there’s currently nothing a page can do about this.

It’s also worth noting, though, that input latency is rarely a visible problem outside of these scroll-linked effects (like pull to refresh, or poorly implemented parallax, etc). The platform’s biggest problem remains that it’s incredibly difficult to maintain a high framerate.