This patch (currently WIP) alters the way we determine whether jank is user-visible or not.
Instead of measuring the total time spent doing JS, we now use an
indicator provided by the vsync driver: how long it takes to deliver
the signal from the vsync timer to the main thread. This lets us find
out more accurately if there is user-visible jank. In the future, this
will also let us add an observer to find out whether the process
itself is janky, regardless of JS.
To decrease the number of apparent false positives, we classify jank
alerts as visible or invisible. We use the following heuristic:
- if the process is currently animating something, any jank alert is visible;
- if the process has just handled a user input, any jank alert is visible;
- if some user input is handled during the current iteration, any jank alert is visible;
- otherwise, jank alerts are not visible.
This patch introduces a new API to the nsPerformanceStatsService to register observers for slow performance. This API has several advantages:
- as it doesn't require polling, it also doesn't need to wake up the parent process every 15 seconds for the AddonWatcher;
- as it doesn't require polling, it doesn't need to wake up the child processes every time we wish to obtain data on slow performance;
- as it provides immediate data on performance alerts, it makes it possible to get rid of the complex and expensive post-processing performed by JS to merge data from all processes and attempt to extract performance alerts.
The old API is still available.
