I’m starting to type this up as EdgeConf draws to a close. I spoke on the performance panel, with Shane O’Sullivan, Rowan Beentje and Pavel Feldman, moderated by Matt Delaney, and tried to bring a platform perspective to the affair. I found the panel very interesting, and it reminded me how little I know about the high-level of web development. Similarly, I think it also highlighted how little consideration there usually is for the platform when developing for the web. On the whole, I think that’s a good thing (platform details shouldn’t be important, and they have a habit of changing), but a little platform knowledge can help in structuring things in a way that will be fast today, and as long as it isn’t too much of a departure from your design, it doesn’t hurt to think about it. At one point in the panel, I listed a few things that are particularly slow from a platform perspective today. While none of these were intractable problems, they may not be fixed in the near future and feedback indicated that they aren’t all common knowledge. So what follows are a few things to avoid, and a few things to do that will help make your pages scroll smoothly on both desktop and mobile. I’m going to try not to write lies, but I hope if I get anything slightly or totally wrong, that people will correct me in the comments and I can update the post accordingly

Avoid reflow

When I mentioned this at the conference, I prefaced it with a quick explanation of how rendering a web page works. It’s probably worth reiterating this. After network and such have happened and the DOM tree has been created, this tree gets translated into what we call the frame tree. This tree is similar to the DOM tree, but it’s structured in a way that better represents how the page will be drawn. This tree is then iterated over and the size and position of these frames are calculated. The act of calculating these positions and sizes is referred to as reflow. Once reflow is done, we translate the frame tree into a display list (other engines may skip this step, but it’s unimportant), then we draw the display list into layers. Where possible, we keep layers around and only redraw parts that have changed/newly become visible.

Really, reflow is actually quite fast, or at least it can be, but it often forces things to be redrawn (and drawing is often slow). Reflow happens when the size or position of things changes in such a way that dependent positions and sizes of elements need to be recalculated. Reflow usually isn’t something that will happen to the entire page at once, but incautious structuring of the page can result in this. There are quite a few things you can do to help avoid large reflows; set widths and heights to absolute values where possible, don’t reposition or resize things, don’t unnecessarily change the style of things. Obviously these things can’t always be avoided, but it’s worth thinking if there are other ways to achieve the result you want that don’t force reflow. If positions of things must be changed, consider using a CSS translate transform, for example – transforms don’t usually cause reflow.

If you absolutely have to do something that will trigger reflow, it’s important to be careful how you access properties in JavaScript. Reflow will be delayed as long as possible, so that if multiple things happen in quick succession that would cause reflow, only a single reflow actually needs to happen. If you access a property that relies on the frame tree being up to date though, this will force reflow. Practically, it’s worth trying to batch DOM changes and style changes, and to make sure that any property reads happen outside of these blocks. Interleaving reads and writes can end up forcing multiple reflows per page-draw, and the cost of reflow can add up quickly.

Avoid drawing

This sounds silly, but you should really only make the browser do as little drawing as is absolutely necessary. Most of the time, drawing will happen on reflow, when new content appears on the screen and when style changes. Some practical advice to avoid this would be to avoid making DOM changes near the root of the tree, avoid changing the size of things and avoid changing text (text drawing is especially slow). While repositioning doesn’t always force redrawing, you can ensure this by repositioning using CSS translate transforms instead of top/left/bottom/right style properties. Especially avoid causing redraws to happen while the user is scrolling. Browsers try their hardest to keep up the refresh rate while scrolling, but there are limits on memory bandwidth (especially on mobile), so every little helps.

Thinking of things that are slow to draw, radial gradients are very slow. This is really just a bug that we should fix, but if you must use CSS radial gradients, try not to change them, or put them in the background of elements that frequently change.

Avoid unnecessary layers

One of the reasons scrolling can be fast at all on mobile is that we reduce the page to a series of layers, and we keep redrawing on these layers down to a minimum. When we need to redraw the page, we just paste these layers that have already been drawn. While the GPU is pretty great at this, there are limits. Specifically, there is a limit to the amount of pixels that can be drawn on the screen in a certain time (fill-rate) – when you draw to the same pixel multiple times, this is called overdraw, and counts towards the fill-rate. Having lots of overlapping layers often causes lots of overdraw, and can cause a frame’s maximum fill-rate to be exceeded.

This is all well and good, but how does one avoid layers at a high level? It’s worth being vaguely aware of what causes stacking contexts to be created. While layers usually don’t exactly correspond to stacking contexts, trying to reduce stacking contexts will often end up reducing the number of resulting layers, and is a reasonable exercise. Even simpler, anything with position: fixed, background-attachment: fixed or any kind of CSS transformed element will likely end up with its own layer, and anything with its own layer will likely force a layer for anything below it and anything above it. So if it’s not necessary, avoid those if possible.

What can we do at the platform level to mitigate this? Firefox already culls areas of a layer that are made inaccessible by occluding layers (at least to some extent), but this won’t work if any of the layers end up with transforms, or aren’t opaque. We could be smarter with culling for opaque, transformed layers, and we could likely do a better job of determining when a layer is opaque. I’m pretty sure we could be smarter about the culling we already do too.

Avoid blending

Another thing that slows down drawing is blending. This is when the visual result of an operation relies on what’s already there. This requires the GPU (or CPU) to read what’s already there and perform a calculation on the result, which is of course slower than just writing directly to the buffer. Blending also doesn’t interact well with deferred rendering GPUs, which are popular on mobile.

This alone isn’t so bad, but combining it with text rendering is not so great. If you have text that isn’t on a static, opaque background, that text will be rendered twice (on desktop at least). First we render it on white, then on black, and we use those two buffers to maintain sub-pixel anti-aliasing as the background varies. This is much slower than normal, and also uses up more memory. On mobile, we store opaque layers in 16-bit colour, but translucent layers are stored in 32-bit colour, doubling the memory requirement of a non-opaque layer.

We could be smarter about this. At the very least, we could use multi-texturing and store non-opaque layers as a 16-bit colour + 8-bit alpha, cutting the memory requirement by a quarter and likely making it faster to draw. Even then, this will still be more expensive than just drawing an opaque layer, so when possible, make sure any text is on top of a static, opaque background when possible.

Avoid overflow scrolling

The way we make scrolling fast on mobile, and I believe the way it’s fast in other browsers too, is that we render a much larger area than is visible on the screen and we do that asynchronously to the user scrolling. This works as the relationship between time and size of drawing is not linear (on the whole, the more you draw, the cheaper it is per pixel). We only do this for the content document, however (not strictly true, I think there are situations where whole-page scrollable elements that aren’t the body can take advantage of this, but it’s best not to rely on that). This means that any element that isn’t the body that is scrollable can’t take advantage of this, and will redraw synchronously with scrolling. For small, simple elements, this doesn’t tend to be a problem, but if your entire page is in an br that covers most or all of the viewport, scrolling performance will likely suffer.

On desktop, currently, drawing is synchronous and we don’t buffer area around the page like on mobile, so this advice doesn’t apply there. But on mobile, do your best to avoid using brs or having elements that have overflow that aren’t the body. If you’re using overflow to achieve a two-panel layout, or something like this, consider using position:fixed and margins instead. If both panels must scroll, consider making the largest panel the body and using overflow scrolling in the smaller one.

I hope we’ll do something clever to fix this sometime, it’s been at the back of my mind for quite a while, but I don’t think scrolling on sub-elements of the page can ever really be as good as the body without considerable memory cost.

Take advantage of the platform

This post sounds all doom and gloom, but I’m purposefully highlighting what we aren’t yet good at. There are a lot of things we are good at (or reasonable, at least), and having a fast page need not necessarily be viewed as lots of things to avoid, so much as lots of things to do.

Although computing power continues to increase, the trend now is to bolt on more cores and more hardware threads, and the speed increase of individual cores tends to be more modest. This affects how we improve performance at the application level. Performance increases, more often than not, are about being smarter about when we do work, and to do things concurrently, more than just finding faster algorithms and micro-optimisation.

This relates to the asynchronous scrolling mentioned above, where we do the same amount of work, but at a more opportune time, and in a way that better takes advantage of the resources available. There are other optimisations that are similar with regards to video decoding/drawing, CSS animations/transitions and WebGL buffer swapping. A frequently occurring question at EdgeConf was whether it would be sensible to add ‘hints’, or expose more internals to web developers so that they can instrument pages to provide the best performance. On the whole, hints are a bad idea, as they expose platform details that are liable to change or be obsoleted, but I think a lot of control is already given by current standards.

On a practical level, take advantage of CSS animations and transitions instead of doing JavaScript property animation, take advantage of requestAnimationFrame instead of setTimeout, and if you find you need even more control, why not drop down to raw GL via WebGL, or use Canvas?

I hope some of this is useful to someone. I’ll try to write similar posts if I find out more, or there are significant platform changes in the future. I deliberately haven’t mentioned profiling tools, as there are people far more qualified to write about them than I am. That said, there’s a wiki page about the built-in Firefox profiler, some nice documentation on Opera’s debugging tools and Chrome’s tools look really great too.

Lucas Rocha and I gave a talk at FOSDEM over the weekend on Firefox for Android. It went ok, I think we could have rehearsed it a bit better, but it was generally well-received and surprisingly well-attended! I’m sure Lucas will have the slides up soon too. If you were unfortunate enough not to have attended FOSDEM, and doubly unfortunate that you missed our talk (guffaw), we’ll be reiterating it with a bit more detail in the London Mozilla space on February 22nd. We’ll do our best to answer any questions you have about Firefox for Android, but also anything Mozilla-related. If you’re interested in FirefoxOS, there may be a couple of phones knocking about too. Do come along, we’re looking forward to seeing you

p.s. I’ll be talking on a performance panel at EdgeConf this Saturday. Though it’s fully booked, I think tickets occasionally become available again, so might be worth keeping an eye on. They’ll be much cleverer people than me knocking about, but I’ll be doing my best to answer your platform performance related questions.

So back from layout into graphics again! For the last few weeks, I’ve been working with Benoit Girard on getting progressive tile rendering finished and turned on by default in Firefox for Android. The results so far are very promising! First, a bit of background (feel free to skip to the end if you just want the results).

You may be aware that we use a multi-threaded application model for Firefox for Android. The UI runs in one thread and Gecko, which does the downloading and rendering of the page, runs in another. This is a bit of a simplification, but for all intents and purposes, that’s how it works. We do this so that we can maintain interactive performance – something of paramount important with a touch-screen. We render a larger area than you see on the screen, so that when you scroll, we can respond immediately without having to wait for Gecko to render more. We try to tell Gecko to render the most relevant area next and we hope that it returns in time so that the appearance is seamless.

There are two problems with this as it stands, though. If the work takes too long, you’ll be staring at a blank area (well, this isn’t quite true either, we do a low-resolution render of the entire page and use that as a backing in this worst-case scenario – but that often doesn’t work quite right and is a performance issue in and of itself…) The second problem is that if a page is made up of many layers, or updates large parts of itself as you scroll, uploading that work to the graphics unit can take a significant amount of time. During this time, the page will appear to ‘hang’, as unfortunately, you can’t upload data to the GPU and continue to use it to draw things (this isn’t true in every single case, but again, for our purposes, it is).

Progressive rendering tries to spread this load by breaking up that work into several smaller tiles, and processing them one-by-one, where appropriate. This helps us mitigate those pauses that may happen for particularly complex/animated pages. Alongside this work, we also add the ability for a render to be cancelled. This is good for the situation that a page takes so long to render that by the time it’s finished, what it rendered is no longer useful. Currently, because a render is done all at once, if it takes too long, we can waste precious cycles on irrelevant data. As well as splitting up this work, and allowing it to be cancelled, we also try to do it in the most intelligent order – render areas that the user can see that were previously blank first, and if that area intersects with more than one tile, make sure to do it in the order that maintains visual coherence the best.

A cherry on the top (which is still very much work-in-progress, but I hope to complete it soon), is that splitting this work up into tiles makes it easy to apply nice transitions to make the pathological cases not look so bad. With that said, how’s about some video evidence? Here’s an almost-Nightly (an extra patch or two that haven’t quite hit central), with the screenshot layer disabled so you can see what can happen in a pathological case:

And here’s the same code, with progressive tile rendering turned on and a work-in-progress fading patch applied.

This page is a particularly slow page to render due to the large radial gradient in the background (another issue which will eventually be fixed), so it helps to highlight how this work can help. For a fast-to-render page that we have no problems with, this work doesn’t have such an obvious effect (though scrolling will still be smoother). I hope the results speak for themselves

]]>

One of the perks of being a Virgin Media customer (beyond getting my name wrong and constant up-sell harassment) is that I got cheap, early-access Eurogamer Expo tickets! This was my first Eurogamer Expo, though I’m no stranager to ECTS or ATEI/EAG. The setup was quite good – perhaps a bit smaller than I expected, but nice to see a games show that’s actually aimed at gamers. I was always amused at the hoops you had to jump through to get tickets for ECTS and ATEI; more so when you actually visit the events and realise the majority of people there are gamers who have jumped through those same hoops. Good to see that the games industry, finally, after several years, got wise.

There was a fair amount on show. Lots of soon and not-so-soon to be released games, the WiiU, a surprising and pleasing amount of indie content and various bits and bobs. The WiiU was certainly the main attraction, but was managed terribly and was extremely disappointing. While most of the company reps were great and very helpful, a couple of Nintendo’s were oddly aggressive and patronising. I don’t think anyone at Eurogamer needs to be told how to play WiiU mini-games, or have buttons on their controllers pressed for them. The decision to dedicate three entire kiosks in the WiiU section to a video panorama viewer was baffling too. It’s almost as if no one at Nintendo has picked up a smart-phone in the last 5 years or so – this isn’t astounding stuff. Wonderful 101 seemed quite fun, but not as fun as I was expecting. The rest of the WiiU content was very disappointing. Pikmin 3 looking bland and boring was especially upsetting. It’s ironic that playing on the console has secured my decision not to buy it on release. I could easily write about how disappointing the WiiU was for a lot longer, but I just don’t care enough.

What was pleasantly surprising was how good Sony’s presence and content was. Reps were polite and helpful, not getting in the way where they weren’t needed and turning up when they were. Much like a good waiter. They had plenty of kiosks and space, and queues were minimal (not due to lack of interest, mind). Playstation All-stars Battle Royale, though clearly a Smash Bros. rip-off, is actually a very good one. We spent quite a while on it, and it was very enjoyable (possibly more so than Smash Bros. Brawl, but it doesn’t even approach the heights of Melee). The cross-play was especially impressive too, mirroring almost the exact same game frame-for-frame with only minor graphical omissions. Stand-out game of the show had to be When Vikings Attack, though. Incredibly simple concept, but perfect execution and impressive cross-play again. The only disappointment was that it doesn’t have a confirmed release date, but Clever Beans say it will be on PSN before the end of the year. This is definitely day-one purchase material.

Carmageddon definitely deserves a mention. It’s just as much fun as it was all those years ago, and the tablet/smartphone port has been handled perfectly. A shame that there was no demo or footage for the Carmageddon Reincarnation project, but hopefully it made a few more people aware. Also worth mentioning was God of War: Ascension, which although is more of the same, it’s a brilliant same that it’s more of. The multiplayer worked surprisingly well too, though a LAN setup is always going to be more fun than online. There were a few things that I’d have liked to have tried, but queues prevented me – nothing I would deem queue-worthy though. Hitman looked quite impressive, but the whole misogyny thing has put me off. Same goes for Tomb Raider. Dishonoured looked interesting, but not so interesting to queue for. Halo 4 looked like more of the same, though the considerable graphical upgrade certainly doesn’t hurt. Dead or Alive 5 was quite fun, and pleasing to see that they’ve returned to the mechanics of Dead or Alive 2 (clearly the series high). Disappointing amount of guys picking bikini-clad women to fight and leaving the camera aimed at crotch/chest areas; we evened the score a bit by playing as ridiculous-looking guys and aiming at the groin. Yes, I am 12. Disappointed to see that they’ve not included Zack’s weird sports-bra costume. The indie games arcade section is probably worth mentioning in that almost everything in it was terrible and just trading on a quirky look with zero gameplay to back it up. I conclude that there’s still plenty of room for ideas and innovation in the British indie games community.

All in all, a pretty fun event. Slightly disappointing that the industry still hasn’t moved on from the whole booth-babe thing, but it’s definitely far less prevalent than it used to be, so that leaves me with some hope. The graphical standard of console games is astounding, especially given there hasn’t been a hardware refresh in over 5 years. I’ll definitely be returning next year.

]]>