RFR (S): 8213307: G1 should clean up RMT with ClassUnloadingWithConcurrentMark

[Aleksey Shipilev]

On 11/05/2018 05:02 PM, Erik Österlund wrote:
>> Talking about surprises in current Hotspot GCs: ZGC does not even do class unloading, even if users
>> "forces" it with -XX:+ClassUnloading(WithConcurrentMark) -- which is probably why we don't see ZGC
>> config in that test.
> 
> We have not implemented STW class unloading in ZGC, because we did not think it is appropriate for a
> concurrent collector to do that in a safepoint. We are going straight for concurrent class unloading
> instead, which is currently in the process of integrating. The infrastructure being built to support
> this is rather generic, and ultimately you will probably be able to hook both G1 and Shenandoah into
> it if you like, on platforms supporting nmethod entry barriers.

I understand that part. I was pointing out that "all current Hotspot GCs" are not necessarily
reacting to class unloading flags in unsurprising manner: current ZGC being prime example today, and
current Shenandoah would be another one, once it is integrated. Therefore, we better not set tests
up assuming some particular "non-surprising" behavior.

Thomas had reverted that part, anyway, so I don't think there is anything else to discuss.

Cheers,
-Aleksey