RFR: 8020277: Young GC could be extremely slow due to assertion in ObjectStartArray::object_starts_in_range

[Stefan Johansson]

Hi,

Can I have a couple of reviews for this fix for:
https://bugs.openjdk.java.net/browse/JDK-8020277

Webrev:
http://cr.openjdk.java.net/~sjohanss/8020277/webrev.00/

Summary:
There have been a couple of bugs filed about varying/slow gc times for 
ParallelScavenge in debug builds. The reason for the slow down is an 
assertion in object_starts_in_range() that tries to verify that the 
range (mapping to one card) really doesn't have any objects starting in 
it. The assertion uses object_start() to find the first object starting 
before the end of the range. The reason for this being really slow is 
that object_start() won't stop looking for an object until it finds it, 
even if it's out of the range we're currently verifying. So on a heap 
with many large objects that span more than one card, a lot of extra 
work is done.

My proposed fix is to just remove this assertion. An alternative would 
be to limit the verification to just check the range we are interested 
in. I started looking at doing this but realized that such a limitation 
would end up being very similar to what is currently done in 
object_starts_in_range() and with this in mind I think we can just 
remove the assertion. I checked JBS to see if there were many bugs 
reported with this assertion as cause, but I only found this bug and 
8009462 which also lists this assertion as the reason for slow GC times.

Testing:
* Manually verified that jit/graph/cgt10 doesn't hang anymore.

Thanks,
Stefan