RFR: Degenerated GC mode for Traversal

[Roman Kennke]

> On 03/27/2018 01:46 PM, Roman Kennke wrote:
>>>>> Ok, that sounds fair. But by this logic, we should not even have the entire block predicated by
>>>>> is_concurrent_traversal_in_progress, right? Instead, we should go to cleanup unconditionally after
>>>>> final-traversal. It would be the same thing we do for final-mark -> cleanup pair in normal cycle.
>>>>
>>>> If final-traversal *fails* (which happens before we trash regions) we
>>>> actually want to dive into degen. Hence the check.
>>>>
>>>> I just had the idea that we might want to have a special degen-point for
>>>> this situation: if we fail after successful traversal GC, but before
>>>> cleanup, we might just want to finish cleanup under STW/degen, and then
>>>> carry on. Then the check would look like:
>>>>
>>>> if (heap->is_concurrent_traversal_in_progress()) {
>>>>   if
>>>> (check_cancellation_or_degen(ShenandoahHeap::_degenerated_traversal))
>>>> return;
>>>> } else {
>>>>   if
>>>> (check_cancellation_or_degen(ShenandoahHeap::_degenerated_traversal_cleanup))
>>>> return;
>>>> }
>>>>
>>>> or such?
>>>>
>>>> WDYT?
>>>>
>>>
>>> Like this:
>>>
>>> Diff:
>>> http://cr.openjdk.java.net/~rkennke/traversal-degen/webrev.03.diff/
>>> Full:
>>> http://cr.openjdk.java.net/~rkennke/traversal-degen/webrev.03/
>>>
>>
>> OTOH, this is a matter of luck. If it runs OOM after the check, it will
>> go to full-gc anyway?
> 
> Yes. So I am thinking that we can just slide to cleanup after final-traversal failure, and then the
> allocation failure handling would restart degen-traversal itself from the known "clean" state? The
> only difference against diving into STW straight away would be non-allocating mutators running while
> alloc-failure is pending. Which seems to be no different from any other alloc-failure pending. No
> need for separate degen point as well.
>

Right, this seems the easiest solution.

Diff:
http://cr.openjdk.java.net/~rkennke/traversal-degen/webrev.04.diff/
Full:
http://cr.openjdk.java.net/~rkennke/traversal-degen/webrev.04/

Ok now?

Pending tests (looking good so far).

Roman