Figuring out and controlling what tasks run before first paint

Kris Maglione kmaglione at mozilla.com
Wed Aug 9 01:28:28 UTC 2017


On Tue, Aug 08, 2017 at 06:09:05PM -0700, Robert Strong wrote:
>One thing that comes to mind is how some code registers app specific
>observers so the code runs after the UI is displayed.
>...
>
>Perhaps having a single category for after UI has been displayed that
>components can specify in their manifests so they are initialized at that
>time similar to how profile-after-change is typically used. This way they
>wouldn't have to initialize just to register an observer so the work
>happens after theUI is displayed.

The running after first paint part is probably fairly easy. For 
most of the cases I'm thinking of, a promise that resolves at 
the right time, in the middle of a promise chain, should be 
enough.

It's the figuring out what's causing things to run, and ensuring 
that things get run before first paint that's more of a problem. 
We do a lot of async things at startup, like file reads and 
off-thread parses. A lot of those things wind up split up into 
multiple operations, and the event loop is so saturated during 
startup if they miss a chance to run, they often wind up running 
after first paint. If we do something like spin the event loop 
to make sure they run in time, extra things run before paint.

So perhaps it would be a fairly simple problem if we ensured 
that everything that runs during startup explicitly defers 
things that don't need to run before paint. But at this point, 
it's so hard to get a hold on what's being scheduled and why 
that I don't even know where to start.

>On Tue, Aug 8, 2017 at 5:42 PM, Kris Maglione <kmaglione at mozilla.com> wrote:
>
>> One of my biggest frustrations in profiling startup performance has been
>> the fact that exactly which code runs during before or after first paint
>> changes based on arbitrary timing factors. If I make a 5ms improvement to
>> one section of code, a 100ms chunk of code winds up running after first
>> paint rather than before. If I make a 5ms improvement to another section of
>> code, a 150ms chunk of code winds up running *before* first paint rather
>> than after. This also shows up in the ts_paint timings on talos, where we
>> have a fairly consistent cluster of high times, a fairly consistent cluster
>> of low times, and very little in-between.
>>
>> Presumably, if we're OK with these chunks *ever* running after first
>> paint, then they should always run after first paint. And vice versa.
>>
>> I've made various attempts to get a handle on this, but never with much
>> success. The last time, I got as far as fixing the broken TaskTracer build
>> before I finally gave up trying to find a useful way to analyze the data.
>> What I'd really like is a handle on what tasks are run, when, who schedule
>> them (and when), and what code they run.
>>
>> After that, I'd ideally like to find a way to run async tasks during
>> startup so that I'm guaranteed which parts run before first paint and which
>> run after.
>>
>> Has anyone else made any progress on this front? Are there any other tools
>> that I'm overlooking? Is there a sensible path forward?
>>
>> Thanks.


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