Firefox 66 has been released, Firefox 67 is out on the beta channel, and Firefox 68 is cooking for the folks on the Nightly channel! These trains don’t stop!

With that, let’s take a quick peek at what the Firefox Front-end Performance team has been doing these past few weeks…

Volunteer Contributor Highlight: Nikki!

I first wanted to call out some great work from Nikki, who’s a volunteer contributor. Nikki fixed a bug where we’d stall the parent process horribly if ever hovering a link with a really really long URL (like a base64 encoded Data URL). Stalling the parent process is the worst, because it makes everything else seem slow as a result.

Thank you for your work, Nikki!

Document Splitting Foundations for WebRender (In-Progress by Doug Thayer)

An impressive set of patches were recently queued to land, which should bring document splitting to WebRender, but in a disabled state. The gfx.webrender.split-render-roots pref is what controls it, but I don’t think we can reap the full benefits of document splitting until we get retained display lists enabled in the parent process for the UI. I believe, at that point, we can start enabling document splitting, which means that updating the browser UI area will not involve sending updates to the content area for WebRender.

In other WebRender news, it looks like it should be enabled by default for some of our users on the release channel in Firefox 67, due to be released in mid-May!

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

Doug has written the bits of code that tie a Firefox preference to an HKLM registry key, which can be read by the warm-up service at start-up. The next step is to add a mode to the Firefox executable that loads its core DLLs and then exits, and then have the warm-up service call into that mode if enabled.

Once this is done, we should be in a state where we can user test this feature.

Startup Cache Telemetry (In-Progress by Doug Thayer)

Two things of note here:

1. With the probes having now uplifted to Beta, data will slowly trickle in these next few days that will show us how the Firefox startup cache is behaving in the wild for users that aren’t receiving two updates a day (like our Nightly users). This important, because oftentimes, those updates cause some or all of the startup cache to be invalidated. We’re eager to see how the startup caches are behaving in the wild on Beta.
2. One of the tests that was landed for the startup cache Telemetry appears to have caught an issue with how the QuantumBar code works with it – this is useful, because up until now, we’ve had very little automated testing to ensure that the startup cache is working as expected.

Smoother Tab Animations (Paused by Felipe Gomes)

UX, Product and Engineering have been having discussions about how the new tab animations work, and one thing has been decided upon: we want our User Research team to run some studies to see how tab animations are perceived before we fully commit to changing one of the fundamental interactions in the browser. So, at this time, Felipe is pausing his efforts here until User Research comes back with some information on guidance.

Browser Adjustment Project (Concluded by Gijs Kruitbosch)

We originally set out to see whether or not we could do something for users running weaker hardware to improve their browsing experience. Our initial hypothesis was that by lowering the frame rate of the browser on weaker hardware, we could improve the overall page load time.

This hypothesis was bolstered by measurements done in late 2018, where it appeared that by manually lowering the frame rate on a weaker reference laptop, we could improve our internal page load benchmarks by a significant degree. This measurement was reproduced by Denis Palmeiro on Vicky Chin’s team, and so Gijs started implementing a runtime detection mechanism to do that lowering of the frame rate for machines with 2 or fewer cores where each core’s clockspeed was 1.8Ghz or slower¹.

However, since then, we’ve been unable to reproduce the same positive effect on page load time. Neither has Denis. We suspect that recent work on the RefreshDriver, which changes how often the RefreshDriver runs during the page load window, is effectively getting the same kind of win².

We did one final experiment to see whether or not lowering the frame rate would improve battery life, and it appeared to, but not to a very high degree. We might revisit that route were we tasked with trying to improve power usage in Firefox.

So, to reduce code complexity, Gijs landed patches to remove the low-end hardware switches and frame rate lowering code today. This experiment and project is now concluded. It’s not a satisfying end with a slum dunk perf win, but you can’t win them all.

Better about:newtab Preloading (Completed by Gijs Kruitbosch)

The patch to preload about:newtab in an idle callback has landed and stuck! This means that we don’t preload about:newtab immediately after opening a new tab (which is good for responsiveness right around the time when you’re likely to want to do something), and also means that we have the possibility of preloading the first new tab in new windows! Great job, Gijs!

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

I had a meeting today with Saptarshi, one of our illustrious Data Scientists, to talk about the upcoming experiment. One of the things he led me to conclude was that this experiment is going to have a lot of confounds, and it will be difficult to conclude things from.

Part of the reason for that is because there are often times when a background tab won’t actually have its content process priority lowered. The potential reasons for this are:

1. The tab is running in a content process which is also hosting a tab that is running in the foreground of either the same or some other browser window.
2. The tab is playing audio or video.

Because of this, we can’t actually do things like measure how page load is being impacted by this feature because we don’t have a great sense of how many tabs have their content process priorities lowered. That’s just not a thing we collect with Telemetry. It’s theoretically possible, either due to how many windows or videos or tabs our Beta users have open, that very few of them will ever actually have their content process priorities lowered, and then the data we’d draw from Telemetry would be useless.

I’m working with Saptarshi now to try to find ways of either altering the process priority manager or adding new probes to reduce the number of potential confounds.

Grab bag of other performance improvements

• A member of the Wikimedia Performance team noticed that Firefox 66 dramatically improved it’s page load time on their Facebook benchmark. This is likely due to the hard work of Randell Jesup, who made setTimeout lower priority during page load.
• Nathan Froyd made it so that we do less main-thread IO on Windows when checking for the existence of filesystem directories

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1. These criteria for what makes “weak hardware” was mostly plucked from the air, but we had to start somewhere. ↩

2. But for all users, not just users on weaker hardware. ↩

We’re only a few weeks away from Firefox 67 merging from the Nightly channel to Beta, and since my last update, a number of things have landed.

It’s the end of a long week for me, so I apologize for the brevity here. Let’s check it out!

Document Splitting Foundations for WebRender (In-Progress by Doug Thayer)

dthayer is still trucking along here – he’s ironed out a number of glitches, and kats is giving feedback on some APZ-related changes. dthayer is also working on a WebRender API endpoint for generating frames for multiple documents in a single transaction, which should help reduce the window of opportunity for nasty synchronization bugs.

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

dthayer is pressing ahead with this experiment to warm up a number of critical files for Firefox shortly after the OS boots. He is working on a prototype that can be controlled via a pref that we’ll be able to test on users in a lab-setting (and perhaps in the wild as a SHIELD experiment).

Startup Cache Telemetry (In-Progress by Doug Thayer)

dthayer landed this Telemetry early in the week, and data has started to trickle in. After a few more days, it should be easier for us to make inferences on how the startup caches are operating out in the wild for our Nightly users.

Smoother Tab Animations (In-Progress by Felipe Gomes)

UX, Product and Engineering are currently hashing out the remainder of the work here. Felipe is also aiming to have the non-responsive tab strip bug fixed soon.

Lazier Hidden Window (Completed by Felipe Gomes)

After a few rounds of landings and backouts, this appears to have stuck! The hidden window is now created after the main window has finished painting, and this has resulted in a nice ts_paint (startup paint) win on our Talos benchmark!

There’s still potential for more improvements on the hidden window, but that’s been split out to a separate project / bug.

Browser Adjustment Project (In-Progress by Gijs Kruitbosch)

This project appears to be reaching its conclusion, but with rather unsatisfying results. Denis Palmeiro from Vicky Chin’s team has done a bunch of testing of both the original set of patches that Gijs landed to lower the global frame rate (painting and compositing) from 60fps to 30fps for low-end machines, as well as the new patches that decrease the frequency of main-thread painting (but not compositing) to 30fps. Unfortunately, this has not yielded the page load wins that we wanted¹. We’re still waiting to see if there’s a least a power-usage win here worth pursuing, but we’re almost ready the pull the plug on this one.

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

Gijs has a set of patches that should make this possible, which will mean (in theory) that we’ll present a ready-to-roll about:newtab when users request one more often than not.

Unfortunately, there’s a small snag with a test failure in automation, but Gijs is on the case.

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

The Process Priority Manager has been enabled in Nightly for a number of weeks now, and no new bugs have been filed against it. I filed a bug earlier this week to run a pref-flip experiment on Beta after the Process Priority Manager patches are uplifted later this month. Our hope is that this has a neutral or positive impact on both page load time and user retention!

Make the PageStyleChild load lazily (Completed by Mike Conley)

There’s an infrequently used feature in Firefox that allows users to switch between different CSS stylesheets that a page might offer. I’ve made the component that scans the document for alternative stylesheets much lazier, and also made it skip non web-pages, which means (at the very least) less code running when loading about:home and about:newtab

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1. This was unexpected – we ran an experiment late in 2018 where we noticed that lowering the frame rate manually via the layout.frame_rate pref had a positive impact on page load time… unfortunately, this effect is no longer being observed. This might be due to other refresh driver work that has occurred in the meantime. ↩