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From "Andrey Chernyshev" <a.y.chernys...@gmail.com>
Subject Re: [DRLVM][VM] -- which header bits are available for GC mark and GC forwarding use?
Date Fri, 25 Aug 2006 13:02:42 GMT
On 8/25/06, Weldon Washburn <weldonwjw@gmail.com> wrote:
> On 8/24/06, Salikh Zakirov <Salikh.Zakirov@intel.com> wrote:
> > In any way, currently there is no single header file in the system,
> > which would describe the object structure. Rather, DRLVM uses some
> > static assumptions about object header, which are not enforced by any
> > common include file. This would be a nice thing to solve...
>
> Good point.  There needs to be a discussion on harmony-dev regarding
> how the object header bits are "sliced and diced".  From talking to
> the MMTk guys (Steve Blackburn) it seems MMTk wants to have one byte
> of object header for private use.  Its unclear to me if this will be a
> performance problem for a product JVM.  I think the hashCode can be
> reduced to one bit plus the object's current address at first
> HashCode() invocation.  I'd put this hash bit in the GC byte.  And

OK, then what this one bit will be used for? Where do we keep the hash
code after the object is moved and it's address is changed? Perhaps
I'm not getting the idea...

> make the GC byte the lowest byte in the header word.  The remaining
> 3bytes could be used for fat/thin locks.  Are there any remaining

I think this is a good idea to use the different bytes for GC and
Object's monitor purposes. It can help monitor's and GC's algorithms
to work independently and avoid extensive atomic operations whenever
it is possible.

Thanks,
Andrey.


> fields unaccounted for?  Thoughts?
>
> --
> Weldon Washburn
> Intel Middleware Products Division
>
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>


-- 
Andrey Chernyshev
Intel Middleware Products Division

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