I’m on vacation this week, but the show must go on! So I pre-recorded a shorter episode of The Joy of Coding last Friday.
In this episode1, I focused on a tool I wrote that I alluded to in the last episode, which is a soundboard to use during Joy of Coding episodes.
I demo the tool, and then I explain how it works. After I finished the episode, I pushed to repository to GitHub, and you can check that out right here.
So I’ll see you next week with a full length episode! Take care!
Which, several times, I mistakenly refer to as the 15th episode, and not the 16th. Whoops. ↩
In Episode 15, we kept working on the same bug as the last two episodes – proxying the printing dialog on OS X to the parent process from the content process. At the end of Episode 14, we’d finished the serialization bits, and put in the infrastructure for deserialization. In this episode, we did the rest of the deserialization work.
And then we attempted to print a test page. And it worked!
We did it!
Then, we cleaned up the patches and posted them up for review. I had a lot of questions about my Objective-C++ stuff, specifically with regards to memory management (it seems as if some things in Objective-C++ are memory managed, and it’s not immediately obvious what that applies to). So I’ve requested review, and I hope to hear back from someone more experienced soon!
I also plugged a new show that’s starting up! If you’re a designer, and want to see how a designer at Mozilla does their work, you’ll love The Design Hour, by Ricardo Vazquez. His design chops are formidable, and he shows you exactly how he operates. It’s great!
Finally, I failed to mention that I’m on holiday next week, so I can’t stream live. I have, however, pre-recorded a shorter Episode 16, which should air at the right time slot next week. The show must go on!
In this episode, I kept working on the same bug as last week – proxying the print dialog from the content process on OS X. We actually finished the serialization bit, and started doing deserialization!
Hopefully, next episode we can polish off the deserialization and we’l be done. Fingers crossed!
Note that this episode was about 2 hours and 10 minutes, but the standard-definition recording up on Air Mozilla only plays for about 13 minutes and 5 seconds. Not too sure what’s going on there – we’ve filed a bug with the people who’ve encoded it. Hopefully, we’ll have the full episode up for standard-definition soon.
Had to deal with some network issues during this video – sorry if people were getting dropped frames during the live show! I have personally checked this recording, and almost all frames are there.
The only frames that are missing are the ones where I scramble around to connect to the wired network, which was boring anyhow.
In this episode, I worked on proxying the print dialog from the content process on OS X. It was a wild ride, and I learned quite a bit about Cocoa stuff. It was also a throwback to my very first episode, where I essentially did the same thing for Linux!
We’ll probably polish this off in the next episode, or in the episode after.
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?
This is still unclear. And I don’t think it’s a single thing – many things might be causing this problem. Anything that blocks up the main thread of the content process, like slow JavaScript running on a web-site, can cause the spinner.
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.
