GC interface update

Roman Kennke rkennke at redhat.com
Mon Apr 24 13:46:23 UTC 2017

Am 24.04.2017 um 08:37 schrieb Per Liden:
> On 04/20/2017 02:29 PM, Roman Kennke wrote:
>> Am 20.04.2017 um 14:01 schrieb Per Liden:
>>> On 2017-04-20 12:05, Aleksey Shipilev wrote:
>>>> On 04/20/2017 09:37 AM, Kirk Pepperdine wrote:
>>>>>> Good stuff. However, one thing I'm not quite comfortable with is the
>>>>>> introduction of the GC class (and its sub classes). I don't quite
>>>>>> see the
>>>>>> purpose of this interface split-up between GC and CollectedHeap. I
>>>>>> view
>>>>>> CollectedHeap as _the_ interface (but yes, it needs some love), and
>>>>>> as a
>>>>>> result I think the the functions you've exposed in the GC class
>>>>>> actually
>>>>>> belongs in CollectedHeap.
>>>>> I thought the name CollectedHeap implied the state of the heap
>>>>> after the
>>>>> collector has completed. What is the intent of CollectedHeap?
>>>> No, CollectedHeap is the actual current GC interface. This is the
>>>> entry point to
>>>> GC as far as the rest of runtime is concerned, see e.g. CollectedHeap*
>>>> Universe::create_heap(), etc. Implementing CollectedHeap,
>>>> CollectorPolicy, and
>>>> BarrierSet are the bare minimum required for GC implementation
>>>> today. [1]
>>> Yep, and I'd like us to move towards tightening down the GC interface to
>>> basically be cleaned up versions of CollectedHeap and BarrierSet.
>>> CollectorPolicy and some other things that class drags along, like
>>> AdaptiveSizePolicy, are way too collector specific and I don't think
>>> that should be exposed to the rest of the VM.
>> Right, I totally agree with this.
>> BTW, another reason for making a new GC interface class instead of
>> further bloating CollectedHeap as the central interface was that there
>> is way too much implementation stuff in CollectedHeap. Ideally, I'd like
>> to have a true interface with no or only trivial implementations for the
>> declared methods, and most importantly nothing that's only ever needed
>> by the GC itself (and never called by the runtime). But as I said, I'm
>> not against a serious refactoring and tightening-up of CollectedHeap
>> instead.
> Yes, I'd like to keep CollectedHeap as the main interface, but I
> completely agree that CollectedHeap currently contains too much
> implementation stuff that we probably want to move out.

Ok, I will revert that part of the change to use CollectedHeap as main
interface then. It's no big deal, so far I only had one additional
method for servicability support in the GC interface class anyway.

Would you also prefer keep 'management' of the heap in Universe too?
I.e. Universe::create_heap() and Universe::heap() etc? Or do you see a
benefit in moving it out like I did with gc_factory.cpp? The idea being
that there's only one smallish place that knows about all the existing
GC impls?


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