RFR: Shortcut reference processing when no work is available

[Zhengyu Gu]

Looks great!

-Zhengyu

On 06/06/2017 01:54 PM, Aleksey Shipilev wrote:
> Shenandoah has parallel reference processing enabled by default.
>
> Current RP implementation submits the processing task to the supplied executor,
> in hope that parallelism would be beneficial. However, in many cases, either the
> lists of references is empty, or the number of actual references is zero (e.g.
> after precleaning), or the number of references falls to zero in the following
> phases.
>
> This makes RP start the executor task, even if we know there is no work. We
> would then waste time waiting for threads to wake up, discover there is no work,
> rendezvous and terminate.
>
> While there is the potential upstream work [1] that can solve the issue
> completely for smaller number of tasks, we can handle the corner case of zero
> tasks in current Shenandoah:
>
>   1) Shortcut with has_discovered_references() in precleaning and weakrefs_work;
>   2) Shortcut in the Shenandoah-specific RP task;
>
> http://cr.openjdk.java.net/~shade/shenandoah/refproc-shortcut-empty/webrev.01/
>
> These two improvements trim down the weak ref processing time significantly,
> even on small desktop:
>  http://cr.openjdk.java.net/~shade/shenandoah/refproc-shortcut-empty/before.txt
>  http://cr.openjdk.java.net/~shade/shenandoah/refproc-shortcut-empty/after.txt
>
> Thanks,
> -Aleksey
>
> [1] https://bugs.openjdk.java.net/browse/JDK-8043575
>