Performance/Fenix/Performance reviews

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Whenever submitting a PR for Fenix or Focus and you believe that the changed code could have a positive (or negative) impact on performance, there are a few things you can do to test the impact of the modified code.

Testing Start Up code

To test start up code, the approach is usually simple:

  1. From the mozilla-mobile/perf-tools repository, use
    The arguments for start-up should include your target (Fenix or Focus).
  2. Determine the start-up path that your code affects this could be:
    1. cold_main_first_frame: when clicking the app's homescreen icon, this is the duration from process start until the first frame drawn
    2. cold_view_nav_start: when opening the browser through an outside link (e.g. a link in gmail), this is the duration from process start until roughly Gecko's Navigation::Start event
  3. After determining the path your changes affect, these are the steps that you should follow:


  • Run located in perf-tools. Note:
    • The usual iteration coumbered list itemnts used is 25. Running less iterations might affect the results due to noise
    • Make sure the application you're testing is a fresh install. If testing the Main intent (which is where the browser ends up on its homepage), make sure to clear the onboarding process before testing
 python3 --count=25 --product=fenix cold_view_nav_start nightly results.txt

where --count refers to the iteration count.

  • Once you have gathered your results, you can analyze them using in perf-tools.
  python3 results.txt

NOTE:For testing before and after to compare changes made to Fenix: repeat these steps, but this time for the code before the changes. Therefore, you could checkout the parent comment (I.e: using git rev-parse ${SHA}^ where ${SHA} is the first commit on the branch where the changes are)

An example of using these steps to review a PR can be found (here).

Testing non start-up changes

Testing for non start-up changes is a bit different than the steps above since the performance team doesn't have tools as of now to test different part of the browser.

  1. The first step here would be to instrument the code to take (manual timings). By getting timings before and after the changes, it could potentially indicate any changes in performance.
  2. Using profiles and markers.
    1. (Profiles) can be a good visual representative for performance changes. A simple way to find your code and its changes could be either through the call tree, the flame graph or stack graph. NOTE: some code may be missing from the stack since pro-guard may inline it, or the sampling rate of the profiler is more than the time taken by the code.
    2. Another useful tool to find changes in performance is markers. Markers can be good to show the time elapsed between point A and point B or to pin point when a certain action happens.