RFR: 8003216: Add JFR event indicating explicit System.gc() cal

[Stefan Johansson]

Hi Erik,

On 2020-02-26 18:28, Erik Gahlin wrote:
> Hi Stefan,
> 
> GC-id would be nice, but perhaps not possible in all scenarios, i.e. 
> -XX:+ExplicitGCInvokesConcurrent and Epsilon GC?
For ExplicitGCInvokesConcurrent it would not be a big problem, that 
would start a concurrent cycle and we could use the id for that GC. I 
also realized that we can get the GC-id without any problem. For other 
events sent before the GC-id is properly setup, we use GCId::peek() 
which returns the id that will be used for the next collection.

For Epsilon, I'm not sure an event should be sent at all since they are 
blocked, see: EpsilonHeap::collect(...)

Thanks,
Stefan

> 
> Thanks
> Erik
> 
> On 2020-02-26 14:21, Stefan Johansson wrote:
>> Hi Erik,
>>
>>> 26 feb. 2020 kl. 13:56 skrev Per Liden <per.liden at oracle.com>:
>>>
>>> Hi Erik,
>>>
>>> On 2020-02-26 13:50, Erik Gahlin wrote:
>>>> Hi,
>>>> Could I have a review of a JFR event that is emitted when 
>>>> System.gc() is called.
>>>> Purpose is to collect the stack trace. It is not sufficient with the 
>>>> cause field that the GarbageCollection event has today.
>>>> Bug:
>>>> https://bugs.openjdk.java.net/browse/JDK-8003216
>>>> Webrev:
>>>> http://cr.openjdk.java.net/~egahlin/8003216/
>>> 489     EventSystemGC event;
>>> 490     event.commit();
>>> 491     Universe::heap()->collect(GCCause::_java_lang_system_gc);
>>>
>>> Don't you want the commit() call after the call to collect(), to get 
>>> the timing right?
>> I was thinking the same thing, could also be nice to have the GC-id 
>> associated with the event to make it easy to match it to GC-logs and 
>> other GC-events. Not sure how to easily get the GC-id though, since 
>> it’s not set at the time we commit the event.
>>
>> I guess if the event has the correct span with timestamps it will be 
>> easy to figure out which other events are associated with it, even 
>> without the GC-id.
>>
>> Cheers,
>> Stefan
>>
>>> cheers,
>>> Per
>>>
>>>> Testing:
>>>> tier1+tier2+jdk/jdk/jfr
>>>> Thanks
>>>> Erik