Wow! I’ve been a way from this blog for too long. I also haven’t posted any new episodes for The Joy of Coding. I also haven’t been keeping up with my Things I’ve Learned posts.
Time to get back in the saddle. First thing’s first, here are 6 episodes of The Joy of Coding that have aired. Unfortunately, I haven’t put together summaries for any of them, but I’ve put their agendas near the videos so that might give some clues.
That big spinner means that the graphics part of Gecko hasn’t given us a frame yet to paint for this browser tab. That means we have nothing yet to show for the tab you’ve selected.
In the single-process Firefox that we ship today, this graphics operation of preparing a frame is something that Firefox will block on, so the tab will just not switch until the frame is ready. In fact, I’m pretty sure the whole browser will become unresponsive until the frame is ready.
With Electrolysis / multi-process Firefox, things are a bit different. The main browser process tells the content process, “Hey, I want to show the content associated with the tab that the user just selected”, and the content process computes what should be shown, and when the frame is ready, the parent process hears about it and the switch is complete. During that waiting time, the rest of the browser is still responsive – we do not block on it.
So there’s this window of time where the tab switch has been requested, and when the frame is ready.
During that window of time, we keep showing the currently selected tab. If, however, 300ms passes, and we still haven’t gotten a frame to paint, that’s when we show the big spinner.
So that’s what the big spinner means – we waited 300ms, and we still have no frame to draw to the screen.
How bad is it?
I suspect it varies. I see the spinner a lot less on my Windows machine than on my MacBook, so I suspect that performance is somehow worse on OS X than on Windows. But that’s purely subjective. We’ve recently landed some Telemetry probes to try to get a better sense of how often the spinner is showing up, and how laggy our tab switching really is. Hopefully we’ll get some useful data out of that, and as we work to improve tab switch times, we’ll see improvement in our Telemetry numbers as well.
Where is the badness coming from?
I also seem to see the spinner when I have “many” tabs open (~30), and have a build going on in the background (so my machine is under heavy load).
One thing I’ve noticed is that there’s this function in the graphics layer, “ClientTiledLayerBuffer::ValidateTile”, that takes much, much longer in the content process than in the single-process case. I’ve filed a bug on that, and I’ll ask folks from the Graphics Team this week.
How you can help
UPDATE (June 1, 2015): Getting profiles from Windows is currently broken because the symbol server appears to be busted. Any profiles from Windows machines will be useless until this bug is fixed. Alternatively, set profiler.symbolicationUrl to http://symbolapi.mocotoolsstaging.net in about:config.
If you’d like to help me find more potential causes, Profiles are very useful! NOTE – I don’t mean “user profiles”, as in, your bookmarks / customizations / history, etc, in the profile folder. I don’t mean this thing. I mean a performance profile.
A performance profile is a read-out of everything that Firefox / Gecko is doing over a particular span of time. When the profiler is running, Firefox / Gecko will record where the process is in the stack every 1ms or so. It’ll also record information about how long since it’s serviced the event loop, which helps us find jank.
To help, grab the Gecko Profiler add-on, make sure it’s enabled, and then dump a profile when you see the big spinner of doom. The interesting part will be between two markers, “AsyncTabSwitch:Start” and “AsyncTabSwitch:Finish”. There are also markers for when the parent process displays the spinner – “AsyncTabSwitch:SpinnerShown” and “AsyncTabSwitch:SpinnerHidden”. The interesting stuff, I believe, will be in the “Content” section of the profile between those markers. Here are more comprehensive instructions on using the Gecko Profiler add-on.
The fourth episode is up! Richard Milewski and I found the right settings to get OBS working properly on my machine, so this weeks episode is super-readable! If you’ve been annoyed with the poor resolution for past episodes, rejoice!
In this fourth episode, I solve a few things – I clean up a busted rebase, I figure out how I’d accidentally broken Linux printing, I think through a patch to make sure it does what I need it to do, and I review some code!
The third episode is up! My machine was a little sluggish this time, since I had OBS chugging in the background attempting to do a hi-res screen recording simultaneously.
Richard Milewski and I are going to try an experiment where I try to stream with OBS next week, which should result in a much higher-resolution stream. We’re also thinking about having recording occur on a separate machine, so that it doesn’t bog me down while I’m working. Hopefully we’ll have that set up for next week.
So this third episode was pretty interesting. Probably the most interesting part was when I discovered in the last quarter that I’d accidentally shipped a regression in Firefox 36. Luckily, I’ve got a patch that fixes the problem that has been approved for uplift to Aurora and Beta. A point release is also planned for 36, so I’ve got approval to get the fix in there too. \o/
Here are the notes for the bug I was working on. The review feedback from karlt is in this bug, since I kinda screwed up where I posted the review request with MozReview.
The second episode is up! We seem to have solved the resolution problem this time around – big thanks to Richard Milewski for his work there. This time, however, my microphone levels were just a bit low for the first half-hour. That’s my bad – I’ll make sure my gain is at the right level next time before I air.