Is there any real life examples of achieved speedup?
Yes, most pages have significant speedups during the initial restyle especially. Wikipedia pages tend to get 3x or so improved style recalc time on typical systems with Intel quad core CPUs, for example.
Of course, styling is only one part of the whole, so your overall speedups are limited by the rest of the rendering pipeline. That's Amdahl's Law for you. But that's no reason to not parallelize at all; it just means that we have lots more work to do once this is done :)
Do you have absolute numbers? If it's 100 vs. 300 ms, that'd be huge. If it's 1 vs. 3 ms, I don't really care.
Stylo actually achieves nearly linear speedup if you just measure styling. It's really impressive.
Note that Stylo is still faster than the old Gecko system even in sequential mode.
Though, I have to admit I am also a little torn on this. Yes, browsing is typically done "during compile" or some other task. However, I have also begun doing most of that work remotely so that I can save battery on my laptop. To that end, it is now less of a concern on preserving cores for my tasks that actually need it.
And I seriously question whether my browser needs this to perform well. I am not completely closed to the idea, but I am highly skeptical.
Almost every modern device running a browser has multiple cores, and that trend is almost certainly going to increase – so it definitely feels like allowing a core part of the web platform to expand across cores will be beneficial.
It is also a bloat race. Web pages are getting increasingly complicated. With very little benefit to end users. I'd wager a growing number of the cycles and network requests are going to tracking, nowdays. Not to mention the ui language doing more and more contortions to give us a page that could be much more succinctly described.
None of this is to say I want it stopped. I just have a feeling of concern that this is leading to an ever increasing march to faster and faster machines to do basic work.
In the modern era, a program cannot take more than its fair share of CPU time - otherwise, a runaway program could easily render your computer nearly unusable. (Linux, macOS and Windows all use preemptive multitasking.)
The way to tell your operating system what you desire prioritized is, on *nix systems, 'nice'.
I'm not necessarily against all of this, but I'm also not eagerly embracing more crap to slow down my machine for no apparent reason.
I could almost guarantee you this is not the bottleneck of any modern setup.
Now, if the browser becomes more and more consuming, hitting my browser could get closer and closer to kicking off another giant compile.
And, as others, you are also assuming I was not pegging out my machine doing something by choice.
[0] https://youtu.be/7q9vIMXSTzc?t=35m (2015)
I want it to be true. I expect that someone should be able to show this with data. I've never seen it done, though.
Using the various CPU* options, you can turn on CPUAccounting, pin a given process and its children/threads to a range of CPUs, place CPUQuotas and so forth. There's a lot of power and granularity there.
I know, anecdotally, that devops/sysadmin folk use this to also audit and test energy consumption of processes over time. (Certain popular PID 1 programs have a run tool that allows you to easily, dynamically change and audit process resource usage.)
My typical use case, for instance, is auditing and managing various Emacs' processes lifetimes while running potentially racy elisp code.
It would be nice if everyone pushing some of these would collect some data for their claims, though. Especially if any of them have better setups (read: more than the single machine I have).