RFR: 8228991: Obsolete -XX:UseAdaptiveGCBoundary

[Kim Barrett]

> On Apr 15, 2020, at 5:40 AM, Stefan Karlsson <stefan.karlsson at oracle.com> wrote:
> 
> Hi Kim,
> 
> On 2020-04-15 00:23, Kim Barrett wrote:
>> Please review this change which obsoletes the boolean product flag
>> -XX:UseAdaptiveGCBoundary and removed the supporting code.
>> As discussed in the associated CSR, this feature has had a long bug
>> tail and does not appear to have ever really worked, so we're going
>> straight to obsoleting the option and removing the code, rather than
>> first deprecating it.
>> This mostly involves removal of a bunch of code.  In some places there
>> are functions which previously needed to be virtual but no longer need
>> that because the derived classes no longer exist.
>> There is an assertion in ParallelScavengeHeap::initialize() that was
>> changed by removing !UseAdaptiveGCBoundary from a disjunction, which
>> is not what one would expect if making that option unconditionally
>> false.  It looks like the presence of a test for that option never
>> mattered, as except in the heterogeneous heap case the virtual spaces
>> have always supposed to meet (it's using AdjoiningGenerations after
>> all) and did so.
>>  CR:
>> https://bugs.openjdk.java.net/browse/JDK-8228991
>> https://bugs.openjdk.java.net/browse/JDK-8242164 (CSR)
>> Webrev:
>> https://cr.openjdk.java.net/~kbarrett/8228991/open.00/
> 
> This looks good to me. If you want to push this, that's fine by me.
> 
> I found a lot of dead/unneeded code when following the trails of what you removed. It was too much to describe, so I created a patch for it instead:
> https://cr.openjdk.java.net/~stefank/8228991/webrev.01/

You are right, there’s more dead code there.  I hadn’t noticed that after my changes
AdjoiningGeneration was only used to provide construction of the PS{Young,Old}Gen
objects, which then get pulled out and cached and the  AdjoiningGeneration object
is unused after that.

> Main change in the patch:
> - Removes AdjoiningGenerations, AdjoiningVirtualSpaces, PSVirtualSpaceHighToLow
> - Cleans up PS{Young,Old}Gen
> -- Removes most virtual declarations from Removes
> -- Moves PS{Young,Old}Gen initialization to constructor and makes initilization functions private.
> -- protected -> private
> 
> Should I propose this as a follow-up cleanup?

We could do that, or I could merge your changes into mine and do it all at once.
Reviewing might be easier as two steps, esp. since you and Thomas have already
looked the initial set of changes.  I’m fine with either, though if done as two steps
I’m going to skip some of Thomas’s comments, since they apply to stuff that’s just
going to get deleted by the followup.