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From Robin Garner <robin.gar...@anu.edu.au>
Subject Re: [drlvm] Class unloading support - tested one approach
Date Thu, 09 Nov 2006 15:50:07 GMT
Alexey Varlamov wrote:
> 2006/11/9, Robin Garner <robin.garner@anu.edu.au>:
>> Etienne Gagnon wrote:
>> > Alexey Varlamov wrote:
>> >> Sorry if it was already discussed, but I believe this approach also
>> >> requires marking vtable bit/byte on each object allocation, unitl the
>> >> "unloading" GC pass is strictly stop-the-world full-heap collection.
>> >> Robin, did you include this particular overhead too in your
>> >> measurements?
>>
>> I didn't include it - having established that it's cheap during GC where
>> memory bandwidth is at a premium, I kind of took this for granted.
>>
>> > My proposal already argued that vtable bit/byte/word marking is
>> > unnecessary for "nursery allocations".  You only need to mark the 
>> vtable
>> > of objects that survive collection and pretenured objects.
> 
> I may have missed it, but I only recall you argued that we just need
> to collect mature space for the *final unloading* as CL and classes
> are unlikely to die young, which I agree. But chances that a live
> object of a candidate class appeared in the nursery are higher.
> Otherwise I just do not grok how this algorithm can be proven for 
> correctness.

There is definitely some kind of barrier required here.  If no 
references to classes belonging to a c/l exist, but references to one of 
the j.l.classloaders exist, classloader may get marked for collection. 
Objects get created (via reflection, in nursery), references to c/l are 
dropped, classloader unloads.

I believe a barrier in one or more of the reflective methods used to 
create objects from j.l.class/j.l.c/loader references is probably necessary.

Weak references can only be collected at the end of a reachability epoch 
in any case, so I think there may be some stronger guarantees that we 
can use, but I'm too sleepy to thing of them right now :)

>> And this is a persuasive argument.  But I can probably find time to
>> measure it tomorrow if you aren't convinced.
> 
> That would be very kindly, thank you.
> 
> -- 
> Alexey

-- 
Robin Garner
Dept. of Computer Science
Australian National University

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