RFR: 8254999: Move G1RemSetSamplingTask to more appropriate location [v3]

[Albert Mingkun Yang]

On Tue, 24 Nov 2020 09:36:11 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

>> Please review this change that moves the `Remembered Set Sampling Task` from `g1ServiceThread.cpp` into `g1RemSet.cpp`. This is basically just a move but with a small edition that the task will reschedule itself instead of running if at start of execution the last GC happend more recently than the value of `G1ConcRefinementServiceIntervalMillis`.
>> 
>> This change does not cover moving the timing measurements from the service thread into the task, that will be covered by: 
>> [JDK-8252645](https://bugs.openjdk.java.net/browse/JDK-8252645)
>> 
>> Testing
>> Tier1-3 + local verification that the tast is scheduled as expected.
>
> Stefan Johansson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove new scheduling logic

Marked as reviewed by ayang (Author).

src/hotspot/share/gc/g1/g1RemSet.cpp line 499:

> 497: G1RemSet::~G1RemSet() {
> 498:   delete _scan_state;
> 499:   if (_sampling_task != NULL) {

Could you add a comment to explain when `_sampling_task` would be null?

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PR: https://git.openjdk.java.net/jdk/pull/1395