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MemChaser – An extension to track Firefox’s memory activities

Today I want to announce a new extension for Firefox. It’s been developed by the Mozilla QA Automation Services team, which exists to support the Mozilla QA community by developing tools and frameworks for ease of testing. And this time, in case of the extension, we even target (web) developers, and our huge Firefox user base. So what is it about?

Mid of December 2011 bug 711900 has been filed. It complained about an increased garbage and cycle collector activity in Firefox 10, which was not existent in Firefox 9. So Mozilla QA has been asked to analyze that behavior and find possible use cases for reproduction. While working on it, we noticed that observing the related information about memory usage and garbage/cycle collector activity wasn’t that comfortable. For the memory you have had to constantly watch and refresh ‘about:memory‘ in a separate tab, and the latter even forced us to always have the Error Console open and a specific hidden preference set.

To make it easier for us I had the idea of creating an extension, which would be able to present all the necessary information at a prominent place and would be visible all the time. There should be no need of switching tabs or scraping the Error Console for output. Scraping was even hard because all entries constantly get wiped out when a maximum threshold of messages has been reached. Further copying out this data is a hassle. So lets create an extension with logging capabilities…

Some days later after an internal discussion across teams we started the development and called it the MemChaser project. Based on the fantastic help from members of the SpiderMonkey team – lets name Nicholas Nethercote and Andrew McCreight – we got a very early prototype setup kinda quickly. But as is turned out having an extension, which needs a restart to get installed, is pretty useless if the user is already facing a strange memory related issue. So one of the bigger tasks was to get familiar with the Add-on SDK project aka Jetpack and to transform the extension into a restartless one. As it turned out the work wasn’t that hard but gave us well better structured code and a nice toolchain to test and build the extension. And finally with all the features implemented we were able to release the MemChaser extension on

The current set of features is not that big but covers all the requirements we got from the QA team. We will make sure to enhance the extension drastically in the next couple of months so that it doesn’t only display current values in the Add-on bar. But what is it doing right now?

As you can see in the image a couple of values are displayed. But what do those stand for? Here the answer:

  • Resident: Memory mapped by the process that is present in physical memory. It’s normally the memory usage you will see in the systems process manager.
  • GC: Duration of the last Garbage Collector activity and in brackets the time when the second last activity happened.
  • CC: Duration of the last Cycle Collector activity and in brackets the time when the second last activity happened.
  • Logging: Starts and stops the internal logger, which writes out the current memory usage and GC/CC activities to a log file, which can be found in the memchaser sub directory of your Firefox profile.

All those information already helps a lot to keep track of memory related activities and enables someone to see if Firefox is using too much memory at all or is having long GC/CC cycles which could freeze the UI of Firefox for short moments.

As I have mentioned above we will continuously enhance the features of the extension. We already have some great ideas and I will certainly blog about those more in the next couple of days, once we have updated our documentation and set milestones for the next releases.

For now please install the extension in your daily profile and watch the memory activities. If you recognize strange behavior turn on logging and let us know by contacting us directly.

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