GC and CC logs

Garbage collector (GC) and cycle collector (CC) logs give information about why various JS and C++ objects are alive in the heap. Garbage collector logs and cycle collector logs can be analyzed in various ways. In particular, CC logs can be used to understand why the cycle collector is keeping an object alive. These logs can either be manually or automatically generated, and they can be generated in both debug and non-debug builds.

This logs the contents of the Javascript heap to a file named gc-edges-NNNN.log. It also creates a file named cc-edges-NNNN.log to which it dumps the parts of the heap visible to the cycle collector, which includes native C++ objects that participate in cycle collection, as well as JS objects being held alive by those C++ objects.

Generating logs

From within Firefox

To manually generate GC and CC logs, navigate to about:memory and use the buttons under “Save GC & CC logs.” “Save concise” will generate a smaller CC log, “Save verbose” will provide a more detailed CC log. (The GC log will be the same size in either case.)

With multiprocess Firefox, you can’t record logs from the content process, due to sandboxing. You’ll need to disable sandboxing by setting MOZ_DISABLE_CONTENT_SANDBOX=t when you run Firefox.

From the commandline

TLDR: if you just want shutdown GC/CC logs to debug leaks that happen in our automated tests, you probably want something along the lines of:


As noted in the previous section, with multiprocess Firefox, you can’t record logs from the content process, due to sandboxing. You’ll need to disable sandboxing by setting MOZ_DISABLE_CONTENT_SANDBOX=t when you run Firefox.

On desktop Firefox you can override the default location of the log files by setting the MOZ_CC_LOG_DIRECTORY environment variable. By default, they go to a temporary directory which differs per OS - it’s /tmp/ on Linux/BSD, $LOCALAPPDATA\Temp\ on Windows, and somewhere in /var/folders/ on Mac (whatever the directory service returns for TmpD/NS_OS_TEMP_DIR). Note that just MOZ_CC_LOG_DIRECTORY=. won’t work - you need to specify a full path. On Firefox for Android you can use the cc-dump.xpi extension to save the files to /sdcard. By default, the file is created in some temp directory, and the path to the file is printed to the Error Console.

To log every cycle collection, set the MOZ_CC_LOG_ALL environment variable. To log only shutdown collections, set MOZ_CC_LOG_SHUTDOWN. To make all CCs verbose, set MOZ_CC_ALL_TRACES to "all”, or to “shutdown” to make only shutdown CCs verbose.

Live GC logging can be enabled with the pref javascript.options.mem.log. Output to a file can be controlled with the MOZ_GCTIMER environment variable. See the Statistics API page for details on values.

Set the environment variable MOZ_CC_LOG_THREAD to main to only log main thread CCs, or to worker to only log worker CCs. The default value is all, which will log all CCs.

To get cycle collector logs on Try server, set MOZ_CC_LOG_DIRECTORY to MOZ_UPLOAD_DIR, then set the other variables appropriately to generate CC logs. The way to set environment variables depends on the test harness, or you can modify the code in nsCycleCollector to set that directly. To find the CC logs once the try run has finished, click on the particular job, then click on “Job Details” in the bottom pane in TreeHerder, and you should see download links.

To set the environment variable, find the buildBrowserEnv method in the Python file for the test suite you are interested in, and add something like this code to the file:

browserEnv["MOZ_CC_LOG_DIRECTORY"] = os.environ["MOZ_UPLOAD_DIR"]
browserEnv["MOZ_CC_LOG_SHUTDOWN"] = "1"

Analyzing GC and CC logs

There are numerous scripts that analyze GC and CC logs on GitHub

To find out why an object is being kept alive, you should use find_roots.py in the root of the github repository. Calling find_roots.py on a CC log with a specific object or kind of object will produce paths from rooting objects to the specified objects. Most big leaks include an nsGlobalWindow, so that’s a good class to try if you don’t have any better idea.

To fix a leak, the next step is to figure out why the rooting object is alive. For a C++ object, you need to figure out where the missing references are from. For a JS object, you need to figure out why the JS object is reachable from a JS root. For the latter, you can use the corresponding find_roots.py for JS on the GC log.


There are two add-ons that can be used to create and analyze CC graphs.

  • about:cc is simple, ugly, but rather powerful.