RFR: 8258431: Provide a JFR event with live set size estimate [v12]

[Stefan Johansson]

On Tue, 16 Mar 2021 13:24:38 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

>>> > Sure, but for the event to be useful we want the reported value to be as close to the reality as possible. I don't understand why you want the lower bound, can you explain why? I would go for the upper bound, which in that case would be used() at the end of the GC. I know used() is not perfect, but for G1 this is the best "cheap" value we have for liveness at the end of any GC.
>>> 
>>> Mostly because `used()` will report all live instances and potential garbage and will make it inconsistent with what the other GCs would report.
>>> 
>> The other STW GCs do report the same, right? 
>> 
>>> > So as a middle road I would suggest to update G1CollectedHeap::gc_epilogue(bool full) to include:
>>> > set_live(used());
>>> > With this you don't need the changes for the G1FullCollector. The liveness calculated at Remark would be used until the next young collection and I think here is where some improvements could be made. During the mixed phase a better solution would make use of the liveness information we have for the old regions together with what is newly allocated, but this needs further investigation.
>>> 
>>> This sounds interesting. Let me try this out.
>> 
>> Glad you like the idea :) I did a quick test locally and it shows the trend ok, even if it is an over estimate of live: 
>>   live = 1.1 GB
>>   live = 1.2 GB
>>   live = 1.5 GB
>>   live = 1.7 GB
>>   live = 2.1 GB
>>   name = "G1Old"
>>   live = 1.4 GB
>>   live = 1.6 GB
>>   live = 1.8 GB
>>   live = 2.0 GB
>>   live = 2.3 GB
>>   live = 2.5 GB
>>   live = 2.8 GB
>>   live = 3.1 GB
>>   live = 3.3 GB
>>   live = 3.7 GB
>>   live = 4.0 GB
>>   live = 4.3 GB
>>   name = "G1Old"
>>   live = 1.2 GB
>> G1Old is from concurrent mark events.
>
>> @kstefanj
>> Just to make sure - `set_live(used())` should be the last call in `G1CollectedHeap::gc_prologue(bool full)` ?
>> 
> Anywhere in there should be fine, we should look if there is anything related in there it could be grouped with.
> 
>> I am getting some funny numbers with this change - basically, last known live size is getting bigger than the current used size 🤷
>> 
>> ![Screen Shot 2021-03-16 at 1 28 27 PM](https://user-images.githubusercontent.com/738413/111308939-97b06d80-865b-11eb-9e2f-d595b49d6401.png)
> 
> This is strange, what kind of GCs are happening around those samples?

Oh, sorry! I messed up, you should put the code in `gc_epilogue()` 😓

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PR: https://git.openjdk.java.net/jdk/pull/2579