[webkit-dev] How does the Javascript garbage collection work?

Maciej Stachowiak mjs at apple.com
Thu Sep 11 01:33:40 PDT 2008

On Sep 11, 2008, at 12:13 AM, Josh Chia (谢任中) wrote:

> I did some more research.  It seems that KJS does mark-and-sweep GC,  
> and the marking is to mark objects that are not known to be  
> unreachable, so that those left unmarked can be removed at the end.   
> Please correct me if I'm wrong.

More specifically, it marks objects that are reachable from the root  

> On Wed, Sep 10, 2008 at 9:23 PM, Josh  
> Chia (谢任中) <joshchia at gmail.com> wrote:
> Hi,
> I'm trying to debug some memory leaks and now need to understand  
> what collector.{h,cpp} are doing.  Could someone point me to some  
> documents to explain how the garbage collector works?  I've also run  
> valgrind and it complained that CollectorBitmap::get() uses an  
> unreferenced value.  I'm not sure whether this is really wrong, so  
> I'll have to first understand how the garbage collector works, the  
> alignment magic used with JSCell and whatever other GC magic I could  
> probably figure out on my own but only after staring at the code for  
> a long time.

We don't have detailed docs, but I can give you this overview:

The basic algorithm is mark and sweep. It's partially conservative -  
it does a conservative scan of the stack for references but is exact  
with respect to the heap (both its own and the C++ heap). Some of the  
code may confuse valgrind but I do not believe there is actual  
uninitialized access.

We arrange it so collector cells are always allocated at a multiple of  
a power of two, this helps in part by making the conservative scan  

It's really pretty straightforward in terms of algorithms, a fairly  
amateur (but surprisingly effective) take on a garbage collector. In  
the future we'd like to consider using a copying collector that  
supports variable-sized allocations.


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