CRR (S): 7121623: G1: always be able to reliably calculate the length of a forwarded chunked array
tony.printezis at oracle.com
Wed Dec 28 02:38:10 PST 2011
Quick follow-up on this. See below.
On 12/27/2011 09:20 AM, Tony Printezis wrote:
>> It is probably true that the post-image's length is not used
>> during GC once it's been copied, but it'd be good to check (I'm
>> especially wary of CMS... but of course
>> this is limited to G1 -- does G1 ever need to scan or iterate over
>> regions that are subject to being copied
>> into during an incremental pause?)
> This is of course something I was also worried about. In G1 we should
> not be scanning to-space objects that are being copied during GC, not
> only because the length might be incorrect due to this change but also
> because there are no guarantees that the objects are well formed
> (another thread might be in the process of copying them). For all
> regions we copy objects into we call save_marks() so that we never go
> over saved_mark() during scanning.
The above is correct. However your observation made me think of
something related: we do of course scan the to-image of an object after
we copy it to identify what it points to. When the object is chunked we
use oop_iterate_range() to scan each chunk. I checked the definition of
that method and it does not use the object's size / length when doing
the scanning, it relies only on the start / end parameters passed to it.
So, we're safe. :-) I updated the latest webrev I posted:
to include the following comment:
4674 // Process indexes [start,end). It will also process the header
4675 // along with the first chunk (i.e., the chunk with start == 0).
4676 // Note that at this point the length field of to_obj_array is not
4677 // correct given that we are using it to keep track of the next
4678 // start index. oop_iterate_range() (thankfully!) ignores the length
4679 // field and only relies on the start / end parameters. It does
4680 // however return the size of the object which will be incorrect. So
4681 // we have to ignore it even if we wanted to use it.
4682 to_obj_array->oop_iterate_range(&_scanner, start, end);
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