Firefox Front-End Performance Update #13

It’s been just a little over two weeks since my last update, so let’s see where we are!

A number of our projects are centered around trying to improve start-up time. Start-up can mean a lot of things, so we’re focused specifically on cold start-up on the Windows 10 2018 reference device when the machine is at rest.

If you want to improve something, the first thing to do is measure it. There are lots of ways to measure start-up time, and one of the ways we’ve been starting to measure is by doing frame recording analysis. This is when we capture display output from a testing device, and then analyze the videos.

This animated GIF shows eight videos. The four on the left are Firefox Nightly, and the four on the right are Google Chrome (71.0.3578.98). The videos are aligned so that both browsers are started at the same time.

The four on the left are Firefox Nightly, and the four on the right are Google Chrome (71.0.3578.98)

Some immediate observations:

  • Firefox Nightly is consistently faster to reach first paint (that’s the big white window)
  • Firefox Nightly is consistently faster to draw its toolbar and browser UI
  • Google Chrome is faster at painting its initial content

This last bullet is where the team will be focusing its efforts – we want to have the initial content painted and settled much sooner than we currently do.

Document Splitting Foundations (In-Progress by Doug Thayer)

After some pretty significant refactorings to work better with APZ, Doug posted a new stack of patches late last week which will sit upon the already large stack of patches that have already landed. There are still a number of reviews pending on the main stack, but this work appears to be getting pretty close to conclusion, as the patches are in the final review and polish stage.

After this, once retained display lists are enabled in the parent process, and an API is introduced to WebRender to generate frames for multiple documents in a single transaction, we can start thinking about enabling document splitting by default.

Warm-up Service (In-Progress by Doug Thayer)

A Heartbeat survey went out a week or so back to get some user feedback about a service that would speed up the launching of Firefox at the cost of adding some boot time to Windows. The responses we’ve gotten back have been quite varied, but can be generally bucketed into three (unsurprising) groups:

  • Users who say they do not want to make this trade
  • Users who say they would love to make this trade
  • Users who don’t care at all about this trade

Each group is sufficiently large to warrant further exploration. Our next step is to build a version of this service that we can turn on and off with a pref and test either in a lab and/or out in the wild with a SHIELD study.

Startup Cache Telemetry (In-Progress by Doug Thayer)

We do a number of things to try to improve real and perceived start-up time. One of those things is to cache things that we calculate at runtime during start-up to the disk, so that for subsequent start-ups, we don’t have to do those calculations again.

There are a number of mechanisms that use this technique, and Doug is currently adding some Telemetry to see how they’re behaving in the wild. We want to measure cache hits and misses, so that we know how healthy our cache system is out in the wild. If we get signals back that our start-up caches are missing more than we expect, this will highlight an important area for us to focus on.

Smoother Tab Animations (In-Progress by Felipe Gomes)

UX has gotten back to us with valuable feedback on the current implementation, and Felipe is going through it and trying to find the simplest way forward to address their concerns.

Having been available (though disabled by default) on Nightly, we’ve discovered one bug where the tab strip can become unresponsive to mouse events. Felipe is currently working on this.

Lazy Hidden Window (In-Progress by Felipe Gomes)

Under the hood, Firefox’s front-end has a notion of a “hidden window”. This mysterious hidden window was originally introduced long long ago1 for MacOS, where it’s possible to close all windows yet keep the application running.

Since then, it’s been (ab)used for Linux and Windows as well, as a safe-ish place to do various operations that require a window (since that window will always be around, and not go away until shutdown).

That window opens pretty early during start-up, and Felipe found an old patch that was written, and then abandoned to make its construction lazier. Felipe thinks we can still make this idea work, and has noted that in our internal benchmarks, this shaves off a few percentage points on our start-up tests

Activity Stream seems to depend on the hidden window early enough that we think we’re going to have to find an alternative there, but once we do, we should get a bit of a win on start-up time.

Browser Adjustment Project (In-Progress by Gijs Kruitbosch)

Gijs updated the patch so that the adjustment causes the main thread to skip every other VSync rather than swithing us to 30fps globally2.

We passed the patch off to Denis Palmeiro, who has a sophisticated set-up that allows him to measure a pageload benchmark using frame recording. Unfortunately, the results we got back suggested that the new approach regressed visual page load time significantly in the majority of cases.

We’re in the midst of using the same testing rig to test the original global 30fps patch to get a sense of the magnitude of any improvements we could get here. Denis is also graciously measuring the newer patch to see if it has any positive benefits towards power consumption.

Better about:newtab Preloading (In-Progress by Gijs Kruitbosch)

By default, users see about:newtab / a.k.a Activity Stream when they open new tabs. One of the perceived performance optimizations we’ve done for many years now is to preload the next about:newtab in the background so that the next time that the user opens a tab, the about:newtab is all ready to roll.

This is a perceived performance optimization where we’re moving work around rather than doing less work.

Right now, we preload a tab almost immediately after the first tab is opened in a window. That means that the first opened tab is never preloaded, but the second one is. This is for historical reasons, but we think we can do better.

Gijs is working on making it so that we choose a better time to preload the tab – namely, when we’ve found an idle pocket of time where the user doesn’t appear to be doing anything. This should also mean that the first new tab that gets opened might also be preloaded, assuming that enough idle time was made available to trigger the preload. And if there wasn’t any idle time, that’s also good news – we never got in the users way by preloading when it’s clear they were busy doing something else

Experiments with the Process Priority Manager (In-Progress by Mike Conley)

The Process Priority Manager has been enabled on Nightly for a few weeks now. Except for a (now fixed) issue where audio playing in background tabs would drop samples periodically, it’s been all quiet for regression reports.

The next step is to file a bug to run an experiment on Beta to see how this work impacts page load time.

Enable the separate Activity Stream content process by default (Stalled by Mike Conley)

This work is temporarily stalled while I work on other things, so there’s not too much to report here.

Grab bag of notable performance work


  1. Check out that commit date – 2003! 

  2. The idea here being that we can then continue to composite scrolling and video at 60fps, but main thread paints will only be updated at 30fps 

Firefox Front-End Performance Update #12

Well, here I am again – apologizing about a late update. Lots of stuff has been going on performance-wise in the Firefox code-base, and I’ll just be covering a small section of it here.

You might also notice that I changed the title of the blog series from “Firefox Performance Update” to “Firefox Front-end Performance Update”, to reflect that the things the Firefox Front-end Performance team is doing to keep Firefox speedy (though I’ll still add a grab-bag of other performance related work at the end).

So what are we waiting for? What’s been going on?

Migrate consumers to the new Places Observer system (Paused by Doug Thayer)

Doug was working on this later in 2018, and successfully ported a good chunk of our bookmarks code to use the new batched Places Observer system. There’s still a long-tail of other call sites that need to be updated to the new system, but Doug has shifted focus from this to other things in the meantime.

Document Splitting (In-Progress by Doug Thayer)

With WebRender becoming an ever-closer reality to our general user population, Doug has been focusing on “Document Splitting”, which makes WebRender more efficient by splitting updates that occur in the browser UI from updates that occur in the content area.

This has been a pretty long-haul task, but Doug has been plugging away, and landed a significant chunk of the infrastructure for this. At this time, Doug is working with kats to make Document Splitting integrate nicely with Async-Pan-Zooming (APZ).

The current plan is for Document Splitting to land disabled by default, since it’s blocked by parent-process retained display lists (which still have a few bugs to shake out).

Warm-up Service (In-Progress by Doug Thayer)

Doug is investigating the practicalities of having a service run during Windows start-up to preload various files that Firefox will need when started.

Doug’s prototype shows that this can save us something like 1 second of net start-up time, at least on the reference hardware.

We’re still researching this at multiple levels, and haven’t yet determined if this is a thing that we’d eventually want to ship. Stay tuned.

Smoother Tab Animations (In-Progress by Felipe Gomes)

After much ado, simplification, and review back-and-forth, the initial set of new tab animations have landed in Nightly. You can enable them by setting browser.tabs.newanimations to true in about:config and then restarting the browser. These new animations run entirely on the compositor, instead of painting at each refresh driver tick, so they should be smoother than the current animations that we ship.

There are still some cases that need new animations, and Felipe is waiting on UX for those.

Overhauling about:performance (V1 Completed by Florian Quèze)

The new about:performance shipped late last year, and now shows both energy as well as memory usage of your tabs and add-ons.

The current iteration allows you to close the tabs that are hogging your resources. Current plans should allow users to pause JavaScript execution in busy background tabs as well.

Browser Adjustment Project (In-Progress by Gijs Kruitbosch)

Gijs has landed some patches in Nightly (which have recently uplifted to Beta, and are only enabled on early Betas), which lowers the default frame rate of Firefox from 60fps to 30fps on devices that are considered “low-end”1.

This has been on Nightly for a while, but as our Nightly population tends to skew to more powerful hardware, we expect not a lot of users have experienced the impact there.

At least one user has noticed the lowered frame rate on Beta, and this has highlighted that our CPU sampling code doesn’t take dynamic changes to clock speed into account.

While the lowered frame rate seemed to have a positive impact on page load time in the lab on our “low-end” reference hardware, we’re having a much harder time measuring any appreciable improvement in CI. We have scheduled an experiment to see if improvements are detectable via our Telemetry system on Beta.

We need to be prepared that this particular adjustment will either not have the desired page load improvement, or will result in a poorer quality of experience that is not worth any page load improvement. If that’s the case, we still have a few ideas to try, including:

  • Lowering the refresh driver tick, rather than the global frame rate. This would mean things like scrolling and videos would still render at 60fps, but painting the UI and web content would occur at a lower frequency.
  • Use the hardware vsync again (switching to 30fps turns hardware vsync off), but just paint every other time. This is to test whether or not software vsync results in worse page load times than hardware vsync.

Avoiding spurious about:blank loads in the parent process (Completed by Gijs Kruitbosch)

Gijs short-circuited a bunch of places where we were needlessly creating about:blank documents that we were just going to throw away (see this bug and dependencies). There are still a long tail of cases where we still do this in some cases, but they’re not the common cases, and we’ve decided to apply effort for other initiatives in the meantime.

Experiments with the Process Priority Manager (In-Progress by Mike Conley)

This was originally Doug Thayer’s project, but I’ve taken it on while Doug focuses on the epic mountain that is WebRender Document Splitting.

If you recall, the goal of this project is to lower the process priority for tabs that are only sitting in the background. This means that if you have tabs in the background that are attempting to use system resources (running JavaScript for example), those tabs will have less priority at the operating system level than tabs that are in the foreground. This should make it harder for background tabs to cause foreground tabs to be starved of processing resources.

After clearing a few final blockers, we enabled the Process Priority Manager by default last week. We also filed a bug to keep background tabs at a higher priority if they’re playing audio and video, and the fix for that just landed in Nightly today.

So if you’re on Windows on Nightly, and you’re curious about this, you can observe the behaviour by opening up the Windows Task Manager, switching to the “Details” tab, and watching the “Base priority” reading on your firefox.exe processes as you switch tabs.

Cheaper tabs in titlebar (Completed by Mike Conley)

After an epic round of review (thanks, Dao!), the patches to move our tabs-in-titlebar logic out of JS and into CSS landed late last year.

Along with simplifying our code, and hammering out at least one pretty nasty layout bug, this also had the benefit of reducing the number of synchronous reflows caused when opening new windows to zero.

This project is done!

Enable the separate Activity Stream content process by default (In-Progress by Mike Conley

There’s one known bug remaining that’s preventing us from letting the privileged content process from being enabled by default.

Thankfully, the cause is understood, and a fix is being worked on. Unfortunately, this is one of those bugs where the proper solution involves refactoring a bit of old crufty stuff, so it’s taking longer than I’d like.

Still, if all goes well, this bug should be closed out soon, and we can see about letting the privileged content process ride the trains.

Grab bag of notable performance work

This is an informal list of things that I’ve seen land in the tree lately that I believe will have a positive performance impact for our users. Have you seen something that you’d like to nominate for a future list? Submit the bug here!

Also, keep in mind that some of these landed months ago and already shipped to release. That’s what I get for taking so long to write a blog post.


  1. For now, “low-end” means a machine with 2 or fewer cores, and a clock speed of 1.8Ghz or slower 

Firefox Performance Update #11

Wow, it’s been a while1 since I posted one of these. We haven’t been resting on our laurels though – a bunch of work has been going on, and I want to highlight some of the big pieces that I’ve seen go by.

But first…

This Performance Update is brought to you by: getBoundsWithoutFlushing

For privileged JavaScript running in the browser, you have access to an interface called nsIDOMWindowUtils. These days, instead of doing a bunch of XPCOM gymnastics to get to that interface, you can access it via window.windowUtils. windowUtils exposes a handy function called getBoundsWithoutFlushing, and it delivers exactly what it says on the tin: you can pass it an element, and it’ll give you the most recently calculated bounds for the element without causing a style or layout flush.

That’s great! However, use with caution – because we’re getting information without flushing, the bounds information might be stale. For example, if you have an element that’s 50×50 pixels, and then were to apply some style in JavaScript that makes the element 500 pixels wide, using getBoundsWithoutFlushing immediately after seeing the style would still return the 50×50 pixel box. The information will only be brought up to date after the next flush, which either will occur from the refresh driver (good!) or some other code causing a layout flush (maybe bad!).

If you want a refresher on style and layout flushes, I highly recommend reading this document that the front-end team put together.

And now, without further ado, here’s what the Firefox Front-end Performance team’s been up to lately!

ClientStorage (Completed by Doug Thayer)

This is a big one if you’re on macOS. Doug’s work here allows us to communicate more efficiently with the GPU on Mac hardware, which should result in smoother animations, and hopefully less CPU (and power!) bandwidth being hogged with memory-copying operations.

This was so effective that it closed out the remaining performance bug on macOS that was preventing tab warming from landing! This means that tab warming should be shipping to our release channel on macOS in Firefox 63!

Experiments with the Background Process Priority Manager (In-Progress by Doug Thayer)

This project attempts to take advantage of our multi-process architecture by reducing the priority of processes that have no tabs being displayed in the foreground to the user2. This is the first time, at least to my knowledge, that we’ve attempted to fiddle with process priority on Firefox Desktop3, which means there are a bunch of unknowns for us to sort through.

Doug and I have been testing lowered background tab process priority for a few weeks, and have already identified one bug that has been recently fixed. Once that bug is available in Nightly builds, we’ll keep tinkering with it to see if any other bugs surface, and then we’ll considering testing it out on our Nightly audience.

If you want to experiment with it right now, you can go to about:config, and create a new bool pref called dom.ipc.processPriorityManager.enabled, and set it to true. Please be warned, this is still very much in the early stages, so you might see some odd behaviour.

Migrate consumers to the new Places Observer system (In-Progress by Doug Thayer)

Doug overhauled the Places Observer Notification APIs a month or so back, allowing consumers to take advantage of batches of notifications4. Doug is now in the process of converting a number of callsites within our Bookmarking code to take advantage of this. Once he puts out a few test failures and lands these patches, operations on large numbers of bookmarks should be handled more efficiently.

Document Splitting (In-Progress by Doug Thayer)

With our Graphics team getting closer and closer to making WebRender a reality, we’ve been looking at ways we can make our front-end code work more efficiently with it.

Disclaimer: I’m not 100% up-to-speed on the various nuances to this project, so I might get a few things wrong below. If someone from the Graphics team reads this and has some corrections or clarifications, please send them my way.

Document splitting will allow Gecko and WebRender to draw updates to the browser UI independently from web content. Historically, we’ve done something like this with layers and layer invalidation, but with WebRender, we have one giant display list that gets shipped over to the GPU thread to render for the whole window.

With document splitting, we’ll have independent structures for (at least for starters) the UI and web content. We suspect this will allow us to render more efficiently – especially when there’s a lot going on in web content (or there’s a lot going on in the UI!).

Make the RemotePageManager lazy (Completed by Felipe Gomes)

Felipe made it so that the RemotePageManager module isn’t loaded until necessary, and that saved us a handy 3.5% on content process start-up time, and 1% on base content memory used by JavaScript!

Smoother Tab Animations (In-Progress by Felipe Gomes)

The Photon UI shipped in Firefox 57 to great fanfare, and all of us front-end folk were pretty psyched about it. Unfortunately, as is always the case, there was some work that had to be cut for time.

Felipe is picking up some work that we cut that re-works how we do tab animations5. Our current animations involve growing and shrinking tabs, and for each frame of that animation, we calculate the change in style and layout and paint the change on the main thread.

The flashing occurs when we paint. Notice how the tabs flash as they open and close.

The new animations were designed from the ground up to take advantage of compositor-accelerated CSS6.

Felipe has some early try builds that he’s posted with the new animations, and we’re pretty excited to see where it goes. Or, if you don’t want to try a build, you can check out this video. Or this video (it’s the previous video in slow motion). Or this video for a variation!

Overhauling about:performance (In-Progress by Florian Quèze)

Florian and Tarek Ziade have proven out the platform work to support the new about:performance, and are now trying to bang out the final bits to make the new about:performance something we actually want to ship. They’re working with our UX team to figure out exactly what that looks like, but we’re hoping ultimately to give the user the most informed picture possible on what is eating up their CPU cycles.

You can try the new about:performance today in Nightly by setting dom.performance.enable_scheduler_timing to true, then restarting the browser, and then visiting about:performance.

Browser Adjustment Project (In-Progress by Gijs Kruitbosch)

Informed by the Firefox Hardware Report, Gijs has been fitted out with some new hardware that we think helps to encapsulate what we consider to be “average” consumer hardware, and “weak” consumer hardware. Gijs has been focusing on prior art by other browser engines, as well as operating systems to see how best we can stand on the shoulders of friends and not re-invent the wheel.

Again, this is still an early-days research project, so no code’s been written yet, but we hope to have a clearer picture on how best to proceed soon.

Avoiding spurious about:blank loads in the parent process (In-Progress by Gijs Kruitbosch)

This work should allow us to avoid some unnecessary work when we create new windows and tabs. This has involved changing a very large number of tests, and doing a bunch of plumbing to get Firefox ready for this change. The dependency tree on the bug gives you a bit of the picture.

Thankfully, I think we’re approaching the home stretch on this one. Hopefully, this should buy us some precious milliseconds when starting up and opening new windows.

Enable the separate Activity Stream content process by default (In-Progress by Mike Conley and Jay Lim)

Enabling the separate Activity Stream content process will allow users to take advantage of the script caching work that my intern Jay Lim did a few months back, which should let us render about:newtab more quickly.

Unfortunately, turning this separate content process on by default has been plagued with problems – most recently, a shutdown leak when running our automated tests. Thankfully, we’ve recently made a breakthrough on the leak, and we’re working on eliminating the cause.

Cheaper tabs in titlebar (In-Progress by Mike Conley)

We run a bit of JavaScript when rendering the browser UI to figure out how exactly to lay out the tabs in the titlebar.

Unfortunately, this JavaScript involves synchronous style and layout flushes, and ultimately we’re doing calculations that’s best left solely to the layout engine.

I’m working on swapping out the JavaScript for raw CSS. Running the benchmarks locally, this saved anywhere from 16-20ms on the window opening Talos benchmark. That might not sound like a lot, but from a performance engineer’s perspective, that’s a pretty solid gain.

Grab bag of Notable Performance Work

And without further ado, here’s a bunch of miscellaneous work that’s gone into the browser recently that has helped make it faster and better! Kudos to all the folk who landed these things! A bunch of these fixes are going out in Firefox 63 and Firefox 64.


  1. Like… 2 months. Oof. The blog guilt is overwhelming! 

  2. Across all windows, if you were worried about that. 

  3. It looks like we did something like this for Firefox OS though, since I believe the infrastructure we’re using to do this comes from that project. 

  4. Up until now, changes were handled one at a time. 

  5. Check out the “new tab motions” attachment in this bug for some videos by our talented designer epang! 

  6. Here’s a great post from some of our friends at Google about this sort of work. 

Firefox Performance Update #10

Hey folks – another Performance Update coming at you! It’s been a few weeks since I posted one of these, mostly due to travel, holidays and the Mozilla SF All-Hands. However, we certainly haven’t been idle during that time. Much work has been done Performance-wise, and there’s a lot to tell. So strap in! But first…

This Performance Update is brought to you by: promiseDocumentFlushed

promiseDocumentFlushed is a utility that’s available for browser engineers in chrome documents on the window global. The goal of promiseDocumentFlushed is to help avoid synchronous layout flushes in our JavaScript code by scheduling work to only occur after the next “natural” layout flush occurs1.

promiseDocumentFlushed takes a function and returns a Promise. The function it takes will run the next time a natural layout flush and paint has finished occurring. At this point, the DOM should not be “dirty”, and size and position queries should be very cheap to calculate. It is critically important for the callback to not modify the DOM. I’ve filed bugs to make modifying the DOM inside that callback enter some kind of failure state, but it hasn’t been resolved yet.

The return value of the callback is what promiseDocumentFlushed’s returned Promise resolves with. Once the Promise resolves, it is then safe to modify the DOM.

This mechanism means that if, for some reason, you need to gather information about the size or position of things in the DOM, you can do it without forcing a synchronous layout flush – however, a paint will occur before that information is given to you. So be on the look-out for flicker, since that’s the trade-off here.

And now, here’s a list of the projects that the team has been working on lately:

ClientStorage (In-Progress by Doug Thayer)

The ClientStorage project should allow Firefox to communicate with the GPU more efficiently on macOS, which should hopefully reduce jank on the compositor thread2. This is right on the verge of landing3, and we’re very excited to see how this impacts our macOS users!

Init WindowsJumpLists off-main-thread (Completed by Doug Thayer)

The JumpList is a Windows-only feature – essentially an application-specific context menu that opens when you right-click on the application in the task bar. Adding entries to this context menu involves talking to Windows, and unfortunately, the way we were originally doing this involved writing to the disk on the main thread. Thankfully, the API is thread-safe, so Doug was able to move the operation onto a background thread. This is good, because arewesmoothyet was reporting the Windows JumpList code as one of the primary causes of main-thread hangs caused by our front-end code.

Reduce painting while scrolling panels on macOS (Completed by Doug Thayer)

Matt Woodrow noticed that the recently added All Tabs list was performing quite poorly when scrolling it on macOS. After turning on paint-flashing for our browser UI, he noticed that we were re-painting the entire menu every time it scrolled. After some investigation, Matt realized that this was because our Graphics code was skipping some optimizations due to the rounded corners of the panels on macOS. We briefly considered removing the rounded corners on macOS, but then Doug found a more general fix, and now we only re-paint the minimum necessary to scroll the menu, and it’s much smoother!

Make the RemotePageManager lazy (In-Progress by Felipe Gomes)

The RemotePageManager is the way that the parent process communicates with a whitelist of privileged about: pages running in the content process. The RemotePageManager hooks itself in pretty early in a content process’s lifetime, but it’s really only necessary if and when one of those whitelisted about: pages loads. Felipe is working on using some of our new lazy script machinery to load RemotePageManager at the very last moment.

Overhauling about:performance (In-Progress by Florian Quèze)

Florian is working on improving about:performance, with the hopes of making it more useful for browser engineers and users for diagnosing performance problems in Firefox. Here’s a screenshot of what he has so far:

A screenshot of the nascent about:performance showing how much CPU tabs are consuming.

Apparently, mining cryptocurrency takes a lot of CPU!

Thanks to the work of Tarek Ziade, we now have a reliable mechanism for getting information on which tabs are consuming CPU cycles. For example, in the above screenshot, we can see that the coinhive tab that Firefox has open is consuming a bunch of CPU in some workers (mining cryptocurrency). Florian has also been clearing out some of the older code that was supporting about:performance, including the subprocess memory table. This table was useful for our browser engineers when developing and tuning the multi-process project, but we think we can replace it now with something more actionable and relevant to our users. In the meantime, since gathering the memory data causes jank on the main thread, he’s removed the table and the supporting infrastructure. The about:performance work hasn’t landed in the tree yet, but Florian is aiming to get it reviewed and landed (preffed off) soon.

Browser Adjustment Project (In-Progress by Gijs Kruitbosch)

This is a research project to find ways that Firefox can classify the hardware it’s running on, which should make it easier for the browser to make informed decisions on how to deal with things like CPU scheduling, thread and process priority, graphics and UI optimizations, and memory reclamation strategies. This project is still in its early days, but Gijs has already identified prior art and research that we can build upon, and is looking at lightweight ways we can assign grades to a user’s CPU, disk, and graphics hardware. Then the plan is to try hooking that up to the toolkit.cosmeticAnimations pref, to test disabling those animations on weaker hardware. He’s also exploring ways in which the user can override these measurements in the event that they want to bypass the defaults that we choose for each environment.

Avoiding spurious about:blank loads in the parent process (In-Progress by Gijs Kruitbosch)

When we open new browser windows, the initial browser tab inside them runs in the parent process and loads about:blank. Soon after, we do a process flip to load a page in the content process. However, that initial about:blank still has cost, and we think we can avoid it. There’s a test failure that Gijs is grappling with, but after much thorough detective work deep in the complex ball of code that supports our window opening infrastructure, he’s figured out a path forward. We expect this project to be wrapped up soon, which should hopefully make window opening cheaper and also produce less flicker.

Load Activity Stream scripts from ScriptPreloader (Completed by Jay Lim)

Jay has recently made it possible for Activity Stream to load its start-up scripts from the ScriptPreloader. From his local measurements on his MBP, this saves a sizeable chunk of time (around 20-30ms if I recall) on the time to load and render Activity Stream! This optimization is not available, however, unless the separate Activity Stream content process is enabled.

Enable the separate Activity Stream content process by default (In-Progress by Jay Lim)

This project not only ensures that Activity Stream content activity doesn’t impact other tabs (and vice versa), but also allows Firefox to take advantage of the ScriptPreloader to load Activity Stream faster. This does, however, mean an extra process flip when moving from about:home, about:newtab or about:welcome to a new page and back again. Because of this, Jay is having to modify some of our tests to accommodate that, as well as part of our Session Restore code to avoid unnecessary loading indicators when moving between processes.

Defer calculating Activity Stream state until idle (In-Progress by Jay Lim)

When Firefox starts up, one of the first things it prepares to do is show you the Activity Stream page, since that’s the default home and new tab page. Jay thinks we might be able to save the state of Activity Stream at shutdown, and load it again quickly during startup within the content process, and then defer the calculations necessary to produce a more recent state until after the parent process has become idle. We’re unsure yet what this will buy us in terms of start-up speed, but Jay is hacking together a prototype to see. I’m eager to find out!

Grab bag of Notable Performance Work

Thank you Jay Lim!

As I draw this update to a close, I want to give a shout-out to my intern and colleague Jay Lim, whose internship is ending in a few short days. Jay took to performance work like a duck in water, and his energy, ideas and work were greatly appreciated! Thank you so much, Jay!


  1. By “natural”, I mean a layout flush triggered by the refresh driver, and not by some JavaScript requesting size or position information on a dirty DOM 

  2. And when it comes to smoothness and responsiveness, jank on the compositor thread is deadly 

  3. it landed and bounced once due to a crash test failure, but Doug has just gotten a fix for it approved 

Firefox Performance Update #9

Hello, Internet! Here we are with yet another Firefox Performance Update for your consumption. Hold onto your hats – we’re going in!

But first a word from our sponsor: ScriptPreloader!

A lot of the Firefox front-end is written using JavaScript. With the possible exception of system add-ons that update outside of the normal release cycle, these scripts tend to be the same until you update.

About a year ago, Mozilla developer Kris Maglione had an idea: let’s try to optimize browser start time by noticing which scripts are being loaded during start-up, and then converting those scripts into a binary representation1 that we can cache on disk. That way, next time we start up, we can just grab the cached binaries off of the disk, skip the parsing step and start executing the JavaScript right away.

Long-time Mozillians might know that we already do some aggressive caching to improve start time for things like XUL, XBL, manifests and other things that are read at start-up. I think we actually were already caching JavaScript files too – but I don’t think we were storing them pre-parsed. And the old caching stuff was definitely not caching scripts that were loading in content processes (since content processes didn’t exist when the old caching stuff was designed).

At any rate, my understanding is that the ScriptPreloader pays attention to script loads between main process start and the point where the first browser window fires the “browser-delayed-startup-finished” observer notification (after the window paints and does post-painting script loading). At that point, the ScriptPreloader examines the list of scripts that the parent and content processes have loaded, and2 writes their pre-parsed bytecode representation to disk.

After that cache is written, the next time the main process or content processes start up, the cache is checked for the binary data. If it exists, this means that we can skip the parsing step. The ScriptPreloader goes one step further and starts to “decode”3 that binary format off of the main thread, even before those scripts are requested. Then, when the scripts are finally requested, they’re very much ready to execute right away.

When the ScriptPreloader landed, we saw some really nice wins in our start-up performance!

I’m now working on a series of patches in this bug that will widen the window of time where we note scripts that we can cache. This will hopefully improve the speed of privileged scripts that run up until the idle point of the first browser window.

And now for some Performance Project updates!

Early first blank paint (lead by Florian Quèze)

User Research has hired a contractor to perform a study to validate our hypothesis that the early first blank paint perceived performance optimization will make Firefox seem like it’s starting faster. More data to come out of that soon!

Faster content process start-up time (lead by Felipe Gomes)

The patches that Felipe wrote a few weeks back have landed and have had a positive impact! The proof is in the pudding – let’s look at some graphs:

The cpstartup impact. Those two clusters are test runs “before” and “after” Felipe’s patches landed, respectively.

The above graph shows a nice drop in the cpstartup Talos test. The cpstartup test measures the time it takes to boot up the content process and have it be ready to show you web pages.

This is a screen capture of a Base Content JS improvement in the AreWeSlimYet test. This graph measures the amount of memory that content processes consume via JavaScript not long after starting up.

In the graph above, we can see that the patches also helped reduce the memory that content processes use by default, by making more scripts only load when they’re needed.

It’s always nice to see our work have an impact in our graphs. Great work, Felipe! Keep it up!

LRU cache for tab layers (lead by Doug Thayer)

The patch to introduce the LRU cache landed last week, and was enabled for a few days so we could collect some data on its performance impact.

The good news is that it appears that this has had a significant and positive impact on tab switch times – tab switch times went down, and the number of Nightly instances reporting tab switch spinners went down by about 10%. Great work, Doug!

A number of bugs were filed against the original bug due to some glitchy edge-cases that we don’t handle well just yet.

We also detected a ~8% resident memory regression in our automated testing suites. This was expected (keeping layers around isn’t free!) and gave us a sense of how much memory we might consume were we to enable this by default.

The experiment is concluded for now, and we’re going to disable the cache for a bit while we think about ways to improve the implementation.

ClientStorageTextureSource for macOS (lead by Doug Thayer)

This project should allow us to be more efficient when uploading layers to the compositor on macOS. Doug has solved the crashing issues he was getting in automation(yay!), and is now attempting to figure out some Talos regressions on the MotionMark test suite. Deeper profiling is likely required to untangle what’s happening there.

Swapping DataURLs for Blobs in Activity Stream (lead by Jay Lim)

Jay’s patch to swap out DataURLs for Blobs for Activity Stream images has passed a first round of review from Mardak! He’s now waiting for a second review from k88hudson, and then hopefully this can land and give us a bit of a memory win. Having done some analysis, we expect this buy back quite a bit of memory that was being contained within those long DataURL strings.

Caching Activity Stream JS in the JS Bytecode Cache (lead by Jay Lim)

After examining the JavaScript Bytecode Cache that’s used for Web Content, Jay has determined that it’s really not the right mechanism for caching the Activity Steam scripts.

However, that ScriptPreloader that I was talking about earlier sounds like a much more reasonable candidate. Jay is now doing a deep dive on the ScriptPreloader to see whether or not the Activity Stream scripts are already being cached – and if not, why not.

Tab warming (lead by Mike Conley)

No news is good news here. Tab warming continues to ride and no new bugs have been filed. The work to reduce the number of paints when warming tabs has stalled a bit while I dealt with a rather strange cpstartup Talos regression. Ultimately, I think I can get rid of the second paint when warming by keeping background tabs display port suppressed4, and then only triggering the display port unsuppression after a tab switch. This will happily take advantage of a painting mechanism that Doug Thayer put in as part of the LRU cache experiment.

Firefox’s Most Wanted: Performance Wins (lead by YOU!)

Before we go into the grab-bag list of performance-related fixes – have you seen any patches landing that should positively impact Firefox’s performance? Let me know about it so I can include it in the list, and give appropriate shout-outs to all of the great work going on! That link again!

Grab-bag time

And now, without further ado, a list of performance work that took place in the tree:

(🌟 indicates a volunteer contributor)

Thanks, folks!


  1. XDR, I think? 

  2. My understanding breaks down here a little 

  3. I assume that’s a type of de-serialization 

  4. This is an optimization that we do that shrinks the painted area to just the region that’s visible to the browser. We normally paint a bit outside the viewable area so that it’s ready when a user starts scrolling