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→‎Cycle collector heap dump: update various information on CC heap dumps
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(→‎Cycle collector heap dump: update various information on CC heap dumps)
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==== Cycle collector heap dump ====
==== Cycle collector heap dump ====
The cycle collector heap dump is useful for figuring out why the cycle collector is keeping an object alive.  These can either be manually or automatically generated.
The cycle collector heap dump is useful for figuring out why the cycle collector is keeping an object alive.  These can either be manually or automatically generated.  They can be generated in both debug and non-debug builds.


To manually generate a CC dump on Firefox 29 and up, go 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.
To manually generate a CC dump on Firefox 29 and up, go 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.


To manually generate a CC dump prior to Firefox 29 even in a non-debug build, enable the Error Console, by going to about:config and setting devtools.errorconsole.enabled to true.  Then open a new window, go to Tools, Web Developer, Web Console.
To manually generate a CC dump prior to Firefox 29, enable the Error Console by going to about:config and setting devtools.errorconsole.enabled to true.  Then open a new window, go to Tools, Web Developer, Web Console.


Then evaluate this expression:
Then evaluate this expression:
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This creates a file named cc-edges-NNNN.log and writes a dump of the heap known to the cycle collector, which includes JS objects and also native C++ objects that participate in cycle collection, to the file.  It will also log the contents of the Javascript heap to a file named gc-edges-NNNN.log.
This creates a file named cc-edges-NNNN.log and writes a dump of the heap known to the cycle collector, which includes JS objects and also native C++ objects that participate in cycle collection, to the file.  It will also log the contents of the Javascript heap to a file named gc-edges-NNNN.log.


Since Firefox 18 (night builds after 2012-09-13) one can override the default location of the log files by setting the MOZ_CC_LOG_DIRECTORY environment variable. http://people.mozilla.com/~mleibovic/cc-dump.xpi is an addon for Android Firefox which uses that to save files to /sdcard. The code for this addon is available on [https://github.com/leibovic/skeleton-addon-fxandroid/tree/cc Github].
One can override the default location of the log files by setting the MOZ_CC_LOG_DIRECTORY environment variable. http://people.mozilla.com/~mleibovic/cc-dump.xpi is an addon for Android Firefox which uses that to save files to /sdcard. The code for this addon is available on [https://github.com/leibovic/skeleton-addon-fxandroid/tree/cc Github].  By default, the file is created in some temp directory, and the path to the file is printed to the Error Console.


The file location changed in Firefox 12:
To log every cycle collection, set the MOZ_CC_LOG_ALL environment variable (XPCOM_CC_LOG_ALL in 29 and earlier).  To log only shutdown collections, set MOZ_CC_LOG_SHUTDOWN (XPCOM_CC_LOG_SHUTDOWN in 29 and earlier). To make any shutdown CCs AllTraces() at shutdown, set MOZ_CC_ALL_TRACES_AT_SHUTDOWN (XPCOM_CC_ALL_TRACES_AT_SHUTDOWN in 29 and earlier). The latter two are useful for debugging shutdown leaks.
* Firefox 12 (including nightly builds after 2012-01-02) creates the file in the temp directory and prints the path to the file to the Error Console.
* Older builds created the file in Firefox's current working directory (usually the current working directory where Firefox was started). That meant that on Windows 7 and Vista, you typically had to run Firefox as Administrator to create the file.


To log every cycle collection, set the XPCOM_CC_LOG_ALL environment variable.  To log only shutdown collections, set XPCOM_CC_LOG_SHUTDOWN.  To make any shutdown CCs AllTraces() at shutdown, set XPCOM_CC_ALL_TRACES_AT_SHUTDOWN.  The latter two are useful for debugging shutdown leaks.
In 30 and later, 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 analyze a cycle collector dump, you need the scripts from [https://github.com/amccreight/heapgraph/tree/master/cc Github].  The relevant scripts are find_roots.py and parse_cc_graph.py (which is called by find_roots).  Calling find_roots on a CC file 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 analyze a cycle collector dump, you need the scripts from [https://github.com/amccreight/heapgraph/tree/master/cc Github].  The relevant scripts are find_roots.py and parse_cc_graph.py (which is called by find_roots).  Calling find_roots on a CC file 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.
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