Monthly Archives: February 2019

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 ↩