RFR (S) 8247367: Shenandoah: pacer should wait on lock instead of exponential backoff

[Zhengyu Gu]

Looks good.

-Zhengyu

On 6/11/20 4:49 AM, Aleksey Shipilev wrote:
> RFE:
>    https://bugs.openjdk.java.net/browse/JDK-8247367
> 
> After JDK-8247310, we can just use the wait/notify on the newly introduced lock to coordinate
> wakeups of paced threads. This avoids doing exponential backoff that introduces additional latency.
> 
> Fix:
>    https://cr.openjdk.java.net/~shade/8247367/webrev.01/
> 
> Testing: hotspot_gc_shenandoah; benchmarks
>