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

[Stefan Karlsson]

On 2014-01-21 14:17, Stefan Johansson wrote:
> 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/

Looks good.

StefanK

>
> 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