RFR 8247808: Move JVMTI strong oops to OopStorage

[coleen.phillimore at oracle.com]

Hi, This patch has been reviewed and I was waiting for the ability to 
define different OopStorages, but I'd like to fix that in a further 
change after the GC changes have been agreed upon and reviewed.  Adding 
a new JVMTI OopStorage in the new mechanism is a smaller change.

open webrev at http://cr.openjdk.java.net/~coleenp/2020/8247808.01/webrev

Retested with tier1-3.

Thanks,
Coleen



On 6/18/20 3:48 PM, coleen.phillimore at oracle.com wrote:
>
>
> On 6/18/20 3:58 AM, Thomas Schatzl wrote:
>> Hi,
>>
>> On 18.06.20 03:09, coleen.phillimore at oracle.com wrote:
>>>
>>>
>>> On 6/17/20 7:49 PM, David Holmes wrote:
>>>> Hi Coleen,
>>>>
>>>> On 18/06/2020 7:25 am, coleen.phillimore at oracle.com wrote:
>>>>> Summary: Remove JVMTI oops_do calls from JVMTI and GCs
>>>>>
>>>>> Tested with tier1-3, also built shenandoah to verify shenandoah 
>>>>> changes.
>>>>>
>> [...]
>>>
>>> Kim noticed that G1 and ParallelGC should be processing these roots 
>>> in parallel (with many threads, since OopStorage has that support) 
>>> and he's going to or has filed a bug to fix it. As we add more 
>>> things to OopStorage (see upcoming RFRs), this will become important.
>>>
>>
>> I do not know which exact roots you want to move into OopStorage, but 
>> I would like to mention this concern: with moving everything into a 
>> single OopStorage (i.e. vm_globals in this case), I am worried that 
>> every time important information about the source for these gets lost.
>>
>> Which makes it hard to understand from where these oops came from 
>> when there is a performance problem in the "VM Globals" bucket.
> Hi Thomas,
>
> I understand this concern.  On the GC list there is a discussion about 
> having the ability to create different strong OopStorages, changing 
> the OopStorage code to process these roots and report statistics in 
> parallel (and/or concurrent), and not having to cascade the code 
> through all the GCs.
>
> I'm going to hold this change until this discussion is complete and 
> move the JVMTI and services/management oops_do oops into a different 
> OopStorage that can make use of this.  Then you'll have your 
> statistics and we won't have classes needing traversal with oops_do.
>
> Thanks,
> Coleen
>
>>
>> This may not apply to JVMTI oops, but others may occasionally have a 
>> significant amount of oops where it would be very interesting to know 
>> from where a particular slowdown comes from.
>>
>> So I would prefer keep some accounting here.
>>
>> Thanks,
>>   Thomas
>