RFR: Add humongous regions support to partial GC

[Aleksey Shipilev]

On 03/16/2017 03:48 PM, Roman Kennke wrote:
> Am 16.03.2017 um 14:26 schrieb Aleksey Shipilev:
>> On 03/16/2017 02:10 PM, Roman Kennke wrote:
>>> This adds support for humongous regions as root regions in partial GC.
>>>
>>> I extended the ShenandoahHeapRegion::oop_iterate() to iterate over a
>>> subset of a humongous object array (bounded by the region). This way we
>>> avoid scanning the whole humongous object if the matrix only gives us a
>>> single region as root.
>>>
>>> Tested using gc-bench/arrayfragger (which crashed before).
>>>
>>> http://cr.openjdk.java.net/~rkennke/partial-humongous/webrev.00/
>> Um.
>>
>>  *) I am blind! Who sets humongous_obj_array?
> 
> Ugh. I mixed up some stuff... I originally wanted to do it another way,
> but now realize we don't even need to differentiate between obj and
> primitive arrays. We can simply rewind to the head, call oop_iterate()
> with a region, and let the implementation in oops sort it out. I.e. for
> primitive arrays it would be a no-op (plus header), for oop arrays it
> would do the right thing. This simplifies the patch.
>>  *) There is a weird disconnect: assume we call oop_iterate on a humongous
>> region *head*. With primitive array, it would touch the array oop. With object
>> array, it would only touch the array contents?
> 
> The oop_iterate() implementation *always* visits the header, no matter
> which region it iterates over. This might be overkill, but is
> conservative. For partial-GC it currently doesn't matter, as we don't
> traverse through the header.
> 
> http://cr.openjdk.java.net/~rkennke/partial-humongous/webrev.01/

Ok, this one makes sense.

Thanks,
-Aleksey