RFR(M): 8000244: G1: Ergonomically set MarkStackSize and use virtual space for global marking stack

[John Cuthbertson]

Hi Bengt,

Thanks for looking over the code again.

I've submitted 8004669 to track the reuse enhancement and I've attached 
the patch.

>> To reuse the previous space we need to cache the ReservedSpace in the 
>> CMMarkStack instance to re-initialize the _virtual_space. As a result 
>> we need to provide a default constructor for ReservedSpace.
>
>
> If we move the code in CMMarkStack::allocate() into the constructor of 
> CMMarkStack we would not need to add a default consturctor to 
> ReservedSpace. I kind of think it would be more natural to have the 
> setup in the constructor anyway.
>
>> Another concern is that I'm not sure that re-committing the original 
>> unexpanded space won't cause a problem on some platforms. I've tested 
>> the code with forced re-commits and it seems to work but would rather 
>> it was pushed as a separate change.
>
> Pushing it as a separate change is definitely a good idea. If 
> re-committting does not work on a particular platform I guess we are 
> just back to the old behaviour, right? That a failed expand of the 
> mark stack will exit the VM.
>
That's the idea and the code new code in CMMarkStack::expand() should do 
that. I've also added the above comments to the CR.

Thanks,

JohnC