Momentary GUI freezes in Aurora and xul.dll!DispatchToTracer

Ray Satiro raysatiro at
Thu Sep 29 05:12:00 UTC 2016

I'm using Firefox Aurora (20160928004008 atm) in Windows 7 x64 and 
recently I've noticed that occasionally there will be a momentary freeze 
with 15% CPU usage. I haven't been using it regularly so I'm not sure 
how far back it goes, but several days at least. I've tried closing all 
the tabs but as I open new tabs I get the same freeze. When it happens 
it will continue until I close and reopen Aurora. I have disabled some 
extensions after consulting about:performance but not all extensions, 
since I don't have a way to immediately reproduce and I don't want to 
leave my extensions off all the time.

Ordinarily I would use mozregression but there are no steps to 
reproduce, it just randomly appears. I've used Windows' xperf tool to 
monitor the stack and I can see that when the freeze happens the 
following have a very high count at the times consistent with the freezes:

Line #, Process, Stack, Count, Weight, All Count, % Weight
4, ,   |- xul.dll!DispatchToTracer<jsid>, 11611, 11610.545804, 11611, 1.57
5, ,   |- xul.dll!js::Scope::traceChildren, 9930, 9929.610630, 9930, 1.34
6, ,   |- xul.dll!TraversalTracer::onChild, 6556, 6555.706800, 6556, 0.89

Also illustrated in the attached screenshot.

Does that mean anything to anyone here, and do you have any ideas how I 
can trace this further the next time it happens so that I can file a bug 

Also does anyone know why it takes xul.pdb an hour to load in Windows 
Performance Analyzer? I had the same problem a year ago the last time I 
did a trace like this.

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