>>>  Of course, it adds memory bandwidth costs as
>>> well
>>
>> Compared to copying collectors and write barriers?
>
> Well a write barrier just needs a write not a read modify write cycle 
> so
> it should be better

But it has to monitor every write.

>  but that depends on the hardware write buffers working
> well. I'm assuming a card marking write barrier. New space doesn't need
> any write barrier protection.

Sure, but I think it's really not a very clear-cut matter, with a lot 
of dependencies on how the hardware works and how programs behave.

> The copy only happens when collecting so it only effects live objects
> and with a generational collector we are hoping that most objects die
> young.

Sure, but you are copying the entire object from one place to another, 
instead of modifying a single word.  Also, hope is just that:  hope.  
Not a guarantee.  If environmental parameters change (larger objects, 
not so many dying quite so young) then it doesn't work out the same 
way.  For (one) example, Objective-C objects tend to be larger and also 
tend to stick around longer.

So I think the matter is not quite so cut and dry.

Marcek

-- 
Marcel Weiher				Metaobject Software Technologies
marcel at metaobject.com		www.metaobject.com
Metaprogramming for the Graphic Arts.   HOM, IDEAs, MetaAd etc.