Add-on hangs: a brief summary

I have blogged about clustering BHR hangs before. This post is dedicated to the add-on side of things.

In a way BHR could be seen as a distributed profiler: each user runs a local profiler that samples the stack of a thread only when that thread is hanging for over N milliseconds. Then the stacks are sent to our backend infrastructure through a Telemetry submission.

Imagine to profile Firefox for an hour. At the end of the hour you would like to determine the impact of one of your installed add-ons on the browsing experience. Not simple at all, the add-on might have been in use only for a fraction of your session, say 20 seconds. In that fraction it might have slowed down the browser significantly though. Since you are collecting hangs for the whole session, that signal might eventually be dominated by noise. This means that in most cases, add-on hangs are not going to look like a big deal once aggregated.

I aggregated the stacks and partitioned them in browser and add-on specific ones. For a given add-on Foo, I computed for all sessions the ratio of hangs of Foo over the number of hangs of both Firefox and Foo. Finally I averaged those ratios. This final number gives an idea of the average proportion of hangs due to Foo an user of that add-on can expect to find in his session.

That’s not the whole story though, one can imagine scenarios where an add-on triggers an asynchrounous workload in the browser which will not be accounted to the add-on itself, like garbage collection. In a grantedly less common, but still plausible scenario, an add-on could improve the performances of the browser and in doing so reducing the number of browser specific hangs while increasing the ratio. In general I tend to see the final number as a lower bound and even though it’s not precise, it can help identify bottlenecks.

From the most popular add-ons the ones that have a high ratio are:

  1. Lastpass (12%)
  2. Hola Better Internet (11%)
  3. Avira Antivirus (10%)
  4. noscript (9%)
  5. Adblock (9%), note this is not Adblock Plus

LastPass, for instance, has a ratio of hangs of about 12%, which means that if you had just LastPass installed and no other add-on, on average about 12% of the hangs you would experience would likely be due to LastPass. That’s a lot and and the main culprit seems to be the logic to scan for input fields when the document changes dynamically. That said I am glad I can’t see any popular add-ons with a shockingly high ratio of hangs, which is good.

The numbers shouldn’t be used to mark an add-on as bad or good; they are based on a fallible heuristic and they are meant to give us the tools to prioritize which add-ons we should keep an eye on.