RFR: Fix wb-assist for Traversal

[Aleksey Shipilev]

On 10/24/2018 05:11 PM, Roman Kennke wrote:
> WB-assist is currently not working for Traversal. It is watching out for
> marked objects close to the one just evacuated, but that wouldn't work
> for traversal GC because it's only ever marking evacuated object,
> nothing in cset gets marked.
> 
> In order to fix this, we need to actually do a little sub-traversal
> starting from the object just evacuated. That's what the patch does:
> 
> http://cr.openjdk.java.net/~rkennke/traversal-wb-assist/webrev.00/

I am against adding more complicated code that handles traversal assists. This is one of the reasons
why there is no concurrent-mark assist as well. Not only this code is complicated, it also has
interaction with gc-worker traversal.

Unless this has clear performance benefits, I'd back off doing this.

-Aleksey