RFR: ClassUnloadingWithConcurrentMark should be opt-in with Shenandoah

[Zhengyu Gu]

Okay.

-Zhengyu

On 05/23/2018 09:39 AM, Aleksey Shipilev wrote:
> http://cr.openjdk.java.net/~shade/shenandoah/class-unload-optin/webrev.01/
> 
> Let's make class unloading with concurrent mark opt-in for Shenandoah. This saves occasional hiccups
> on applications that do not need class unloading to begin with. Since ClassUnloading is still
> enabled by default, Full GC would unload classes, if it comes to that. "aggressive" heuristics would
> unload the classes anyway, so the path that we are disabling for default heuristics is still tested.
> 
> The downside is that once the classes start to leak, the class unloading is not there to save the
> day, and Init/Final Mark pause time would grow up scanning string table, SystemDictionary and
> friends. We can do nothing about it, until concurrent class unloading is implemented upstream.
> 
> I have ran our usual benchmarks and saw no regressions in pause times.
> 
> Testing: hotspot_gc_shenandoah, specjvm, specjbb
> 
> Thanks,
> -Aleksey
>