[webkit-dev] Leak reports during shutdown

Zoltan Herczeg zherczeg at inf.u-szeged.hu
Tue Sep 29 12:39:15 PDT 2009

Hi Kevin,

WebKit approach is NOT to free memory before quit to make the quit process
as fast as possible. The memory manager should free the unfreed objects.
However, this approach makes really hard to find the "real" leaks (which
are unreferenced objects).

In my experince the unfreed (non-leak) objects come from several sources:
  - JavaScriptCore garbage collector is not called before quit
  - Caches are not cleared before quit
The previous two are performed by QtLauncher in debug mode. See
WebKit/qt/QtLauncher/main.cpp launcherMain()
  - In case of workers, the gc() does not called for each worker threads
when the application quits. The JavaScript threads are simply aborted.
  - Global objects are not freed at all. There was an attempt to free them
(see https://bugs.webkit.org/show_bug.cgi?id=27980 ) but seems not worth
to continue that work, although it helped to find leaks such as this:

The remaining unfreed objects are possible leaks. Many of them are
probably port specific (not all port maintainers have resources to keep
leak hunters).


> Hi all,
> For a while, ports like wx and Win/Cairo have been seeing various
> leaks reported on shutdown, but at least on Mac I've been unable to
> see these leaks using memory leak checkers like the one in
> Instruments, so I decided to poke into the code a bit more and try to
> understand what's going on a bit better.
> What I found was that the reported object leaks were pretty much all
> related to objects that JSC has references to. Changing
> ~ScriptController to do a garbageCollectNow() instead of
> garbageCollectSoon(), for example, drastically reduced the number of
> reported leaks, cleaning up all the CachedResource leaks and almost
> all the WebCoreNode leaks. The remaining leaks were almost all
> JSC::Structure objects. I've been digging through the code to try and
> find the place where these JSC objects are finally deleted, but I
> haven't found anyplace obvious in the code, neither in common code nor
> in the ports' code.
> My question is, is there somewhere these objects are being deleted on
> final shutdown that apparently happens after the leaks are reported,
> or does WebKit have assumptions such as that all still running timers
> must fire before final shutdown that ports such as ours are not
> honoring? (e.g. in my tests garbageCollectSoon() does not end up
> firing the callback because the app shuts down too fast.)
> Thanks,
> Kevin
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