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

[Per Liden]

Hi,

On 2/26/20 4:17 PM, Erik Gahlin wrote:
> 
>> On 26 Feb 2020, at 15:02, Per Liden <per.liden at oracle.com> wrote:
>>
>> Hi,
>>
>> On 2/26/20 2:50 PM, Erik Gahlin wrote:
>>> Hi Per,
>>> My thinking was that users expect the timestamp of the event to happen before the GarbageCollection event, so I made the event untimed.
>>
>> I would have expected the event start time to be before the collection starts, and the end time when it's done.
> 
> We have sometimes sorted events by their end time, or when they are committed (written to the buffer).This works better than start time in some cases.
> 
>>
>>> I could make the event timed, but what happens if a concurrent gc is used and it is already in progress. Would a new gc cycle start, or will it complete the existing cycle before returning?
>>
>> Unless -XX:+ExplicitGCInvokesConcurrent is used (off by default), they do the same, which is wait until the "System.gc" collection has completed. For ZGC, should a GC cycle be in progress when a call to System.gc() is made, it will first wait for the in progress cycle to finish, and then execute the "System.gc" cycle before returning.
> 
> If users expect the System GC event to measure the time the Java thread is blocked, making the event timed makes sense.
> 
> If users expect the duration to be the length of the triggered GC, or even the pause time, it would mislead users to make it timed.

I agree. I'm thinking the event time should just reflect how long time 
the Java thread was blocked, waiting for System.gc() to complete.

cheers,
Per

> 
> Erik
> 
>>
>> cheers,
>> Per
>>
>>> Thanks
>>> Erik
>>>> On 26 Feb 2020, at 13:56, Per Liden <per.liden at oracle.com> wrote:
>>>>
>>>> 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?
>>>>
>>>> cheers,
>>>> Per
>>>>
>>>>> Testing:
>>>>> tier1+tier2+jdk/jdk/jfr
>>>>> Thanks
>>>>> Erik
>