RFR (S) 8237586: Shenandoah: provide option to disable periodic GC

[Zhengyu Gu]

Fix looks good to me. Please update copyright years.

Thanks,

-Zhengyu

On 1/21/20 1:29 PM, Aleksey Shipilev wrote:
> RFE:
>    https://bugs.openjdk.java.net/browse/JDK-8237586
> 
> Webrev:
>    https://cr.openjdk.java.net/~shade/8237586/webrev.01/
> 
> The VM option is unsigned, which leaves as with "0" as special value. It also matches the behavior
> of G1PeriodicGCInterval and GuaranteedSafepointInterval.
> 
> Testing: hotspot_gc_shenandoah (includes new testcases)
>