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.
gc-edges-NNNN.log. It also creates a file named
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.
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
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:
MOZ_DISABLE_CONTENT_SANDBOX=t MOZ_CC_LOG_DIRECTORY=/full/path/to/log/directory/ MOZ_CC_LOG_SHUTDOWN=1 MOZ_CC_ALL_TRACES=shutdown ./mach ...
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
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
NS_OS_TEMP_DIR). Note that just
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
variable. To log only shutdown collections, set
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
the MOZ_GCTIMER environment variable. See the Statistics
API page for
details on values.
Set the environment variable
main to only log
main thread CCs, or to
worker to only log worker CCs. The default
all, which will log all CCs.
To get cycle collector logs on Try server, set
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
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
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
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.