Re: [rvm-research] BlockAllocator Block Meta Data

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The BlockAllocator class reserves 4 words per block
(BlockAllocator.LOG_BYTES_IN_BLOCK_META), of which it uses 3 words.

The Block Mark word (at offset FL_META_OFFSET) is currently set using
a Word write operation - this is described in [1].  You could probably
use the high-order 24 bits if you change the block mark to a byte
write, but making it a bit (and having to do a read-modify-write)
would have a performance impact.

You also have a full word available at the next word offset.

cheers

[1] Garner, R., Blackburn, S. M., Frampton, D.
Effective Prefetch for Mark-Sweep Garbage Collection
ISMM '07
http://cs.anu.edu.au/people/Robin.Garner/pf-ismm-2007.pdf

On 21/11/13 06:46, Nathan Ricci wrote:
> Hello all,
> 
> I had a question about the block meta data in the block allocator. 
> It seems like there is an entire word of meta data, but as far as I
> can tell looking at markBlockMeta, checkBlockMeta, and
> clearBlockMeta functions, only one bit is actually used.
> 
> What I am trying to do is to keep the sweeper from collecting 
> certain objects (building off the Mark Sweep collector), and (in
> addition to tracking the objects themselves) I think I need to
> "mark" their blocks in a way that is not reset after each
> collection. So I'm trying to use a bit of the block meta data to do
> this.
> 
> Are the other 31 bits of block metadata up for grabs? Or are they 
> used in some way I haven't discovered?
> 
> Thanks, Nathan Ricci Tufts University
> 
> 
> 
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