RFR: Degenerated GC mode for Traversal

[Roman Kennke]

Am 27.03.2018 um 13:38 schrieb Roman Kennke:
> Am 27.03.2018 um 13:30 schrieb Roman Kennke:
>> Am 27.03.2018 um 13:21 schrieb Aleksey Shipilev:
>>> On 03/27/2018 01:18 PM, Roman Kennke wrote:
>>>>>  *) In service_concurrent_traversal_cycle, why do we need this check? I thought that coming out with
>>>>> cancellation from final_traversal means we have the failure during degen, and it should upgrade to
>>>>> Full GC right away?
>>>>>
>>>>>  275   if (heap->is_concurrent_traversal_in_progress()) {
>>>>>  276     if (check_cancellation_or_degen(ShenandoahHeap::_degenerated_traversal)) return;
>>>>>  277   }
>>>>
>>>> We must not break out of the cycle between successful final-traversal
>>>> and cleanup phases. This might happen because we're back into Java land
>>>> by then, and allocations may fail before we managed to clean up. If this
>>>> happens, we would get into degen with trashed regions. Do you have any
>>>> suggestions how else to deal with that?
>>>
>>> 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/
> 
> ?
> 
> Roman
> 

OTOH, this is a matter of luck. If it runs OOM after the check, it will
go to full-gc anyway?

WDYT?

Roman