RFR: 8254758: Change G1ServiceThread to be task based

[Stefan Johansson]

On Mon, 19 Oct 2020 14:02:40 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>     1. There's a sentinel node in the task list. I wonder if it's possible to get rid of it. This way, `G1ServiceTask` can
>     be made abstract, and `G1ServiceTask::execute` and `G1ServiceTask::timeout` can be pure and unreachable statically.
> 
I tried this out first, but I like not having to check for `NULL` when adding. Another idea I had to was create a
specific `G1SentinelTask`, to allow `execute()` and `timeout()` to be pure virtual. If you like this approach I'm happy
to go in that direction.

>     2. Currently, the task timeout is a `double`, which says very little on what precision is actually supported. I think
>     an integral type would be better.
> 
I agree, I would prefer to avoid double all together but it is a quite nice to use `os::elapsedTime()` for the timings
since the logging-framework prints time this way. And since you then have a double as the base value having the timeout
using the same is quite nice, but I'll take a look an see if I can make it look nice with integral type. But the
scheduled time will still be using a double.

>     3. "If a task don't want to be rescheduled it can set a negative timeout and that will cause the task to be dropped
>     from the list." I wonder if `timeout=0` is allowed, and what that means.

Timeout 0 basically means run me again instantly unless someone else is up. For example looking forward the uncommit
task will reschedule itself with a 0 timeout avoiding putting the service thread to sleep but still making it possible
for another task to run between two invocations.

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PR: https://git.openjdk.java.net/jdk/pull/734