RFR: Traveral GC heuristics

[Aleksey Shipilev]

On 01/24/2018 12:17 PM, Roman Kennke wrote:
> Am 24.01.2018 um 12:14 schrieb Aleksey Shipilev:
>> On 01/24/2018 12:12 PM, Roman Kennke wrote:
>>> Am 24.01.2018 um 11:11 schrieb Aleksey Shipilev:
>>>> On 01/23/2018 09:41 PM, Roman Kennke wrote:
>>>>> Differential patch:
>>>>> http://cr.openjdk.java.net/~rkennke/traversal/webrev.05.diff/
>>>>> Full patch, including your changes:
>>>>> http://cr.openjdk.java.net/~rkennke/traversal/webrev.05/
>>>>
>>>> Okay! This looks safe enough to push.
>>>>
>>>> I have a minor question about why this is needed:
>>>>
>>>> 1657     case _degenerated_outside_cycle:
>>>> 1658       if (shenandoahPolicy()->can_do_traversal_gc()) {
>>>> 1659         // Not possible to degenerate from here, upgrade to Full GC right away.
>>>> 1660         cancel_concgc(GCCause::_allocation_failure);
>>>> 1661         op_degenerated_fail();
>>>> 1662         return;
>>>> 1663       }
>>>>
>>>> Aren't we good with the usual Degenerated GC cycle here?
>>>>
>>>
>>> The problem is that degen_outside_cycles goes into normal marking, and something's not up for that.
>>> I'm hitting asserts when I go there.
>>
>> That probably indicates a bug? Traversal GC is ought to leave the heap in the state that is ready
>> for the usual concurrent cycle, no?
> 
> In the middle of traversal GC? I don't know... Also, cannot, in any case, expect *concurrent* cycle
> to work: we don't have the barriers for that. Theoretically, we could do STW normal cycle, but what
> would be the point? I'd rather have a STW degen traversal pickup.

"outside cycle" means you are out of Traversal GC already -- that means outside the *complete*
cycle. So, here is where Traversal differs from Partial? Partial may be followed by the normal
concurrent cycle, and Traversal can only run Traversals?

-Aleksey