RFR: 8254758: Change G1ServiceThread to be task based

[Albert Mingkun Yang]

On Mon, 19 Oct 2020 12:00:40 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

> Please review this enhancement to make `G1ServiceThread` task based. This thread has evolved from having a single
> responsibility to do more things. And there are plans to add more to it. To make this easier and to allow different
> tasks to run at different intervals this change adds a simple task mechanism to the service thread.  The idea is that
> the service thread keeps a list of `G1ServiceTask`-objects that are ordered by when they should execute. When no tasks
> are ready to be run the service thread check how far into the future the next task is scheduled and sleeps/waits for
> that duration. If a new task is added while the thread is waiting it will be woken up to check for ready tasks.   There
> are currently two tasks in the list. They have been extracted from the old service thread into the tasks:
> `G1PeriodicGCTask` and `G1RemSetSamplingTask`. These are more or less identical to the old code, but in the form of
> tasks. The only difference is that `G1PeriodicGCTask` no longer need to keep track of the last attempt, since it will
> be rescheduled with the correct interval.   To reschedule a task is to update the time when it should be executed next
> and adding it back to the list. This insertion is O(n) and it should be fine since the expected number of tasks handled
> by the service thread is low. 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. There are currently no such tasks and adding such tasks come with the
> responsibility of making sure resources are taken care of when the task is no longer active.  I've added a gtest to do
> some basic VM-testing and apart from a lot of local testing I've run mach5 tier 1-4 a few times with good results.  I
> have created a few follow up tasks to look at moving the current tasks to more appropriate locations and also updated a
> few old service thread related tasks with the label:
> [`gc-g1-service-thread`](https://bugs.openjdk.java.net/issues/?jql=labels%20%3D%20gc-g1-service-thread)

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. 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. 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.

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