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From Salikh Zakirov <Salikh.Zaki...@Intel.com>
Subject Re: [rant] Memory options in VM -- why is the default not 'unlimited'
Date Sun, 30 Jul 2006 12:51:52 GMT
Alex Blewitt wrote:
> However, if the GC is generational, why does it need to be contiguous?

As far as I know, there are no theoretical obstacles in making GC heap
non-contiguous. The heap is created in contiguous address range mostly
because of practical reasons.

1) it is very convenient to be able to do assert(object >= heap_base && object
< heap_ceiling)

2) a well-known design for heap traversal during garbage collection involves having so called
mark bit table, which has 1 bit for each potential object start location.
For example, in current DRLVM GC (called "GC v4"), objects are 4 bytes aligned, thus each
4-byte aligned address can potentially be a start of an object, and so has a corresponding
mark bit in the table.

To extend the mark bits techniques on non-contiguous heaps, one have to split the heap space
into 2^n byte blocks and allocate mark bit tables for each block.
GC v4 uses this techniques, with block size = 128k, and first 4k of the block keeping
the mark bit table. However, in case of GC v4, it has a design flaw. 
It tries to organize the LOS (Large object space) uniformly with small object blocks, and
has
large objects aligned at the block level. The objects larger than 1/2 of the block are considered
large. In the worst case, if the heap is filled with the objects of size about 1/2 of block,
more than 50% of the heap space is wasted for alignment.

I think that the space inefficiency can be fixed by introducing separate logic for handling
large objects, and aligning them to a smaller boundary, e.g. on a page size (4k). 



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