RFR/RFC: Passive should opt-in the barriers, not opt-out

[Christine Flood]

I like it.

Patch looks fine.

On Fri, Nov 17, 2017 at 10:07 AM, Aleksey Shipilev <shade at redhat.com> wrote:

> http://cr.openjdk.java.net/~shade/shenandoah/passive-opt-in/webrev.01/
>
> Current "passive" heuristics is diagnostic and only runs Full GCs. In that
> mode, barriers are not
> required, and our Wiki mentions that is the way to dissect the barriers
> cost. As Shenandoah adoption
> goes up, we might want to have this in more convenient form. That is,
> let's make barriers opt-in,
> not opt-out with "passive" heuristics. This allows users to make
> diagnostic runs with all barriers
> disabled at once, without knowing what barriers they need to disable. New
> code also prints the
> barrier names it implicitly disables, so that users can selectively enable
> them back without looking
> into the source or our wiki.
>
> We still allocate fwdptr slot, because Full GC code needs it for storing
> relocation data.
>
> Our existing testing covers all interesting tests with "passive"
> heuristics already.
>
> Testing: hotspot_gc_shenandoah {fastdebug|release}
>
> Thanks,
> -Aleksey
>
>