RFR 8134995(M): [REDO] GC: implement ranges (optionally constraints) for those flags that have them missing

[Kim Barrett]

On Oct 5, 2015, at 4:35 PM, sangheon.kim <sangheon.kim at oracle.com> wrote:
> 
> On 10/05/2015 11:31 AM, Kim Barrett wrote:
>>> I can revert to webrev.02 and change groups to AfterMemoryInit.
>> I would prefer that.
> Okay, I will revert to webrev.02 and change the group to AfterMemoryInit.

Thanks.

>>>> But this discussion is making me question all of the FLAG_IS_xxx uses
>>>> in commandLineFlagConstraintsGC.cpp.
>>> For some options, they are automatically added to validate.
>>> (e.g. SurvivorRatio for CMS GC )
>>> So FLAG_IS_xxx is needed for some cases.
>> The (pre-existing) ergonomics around SurvivorRatio seem quite strange
>> to me.  Why is the behavior be different when the value is defaulted
>> vs when it is explicitly specified with the same value as the default?
> SurvivorRatio is a little bit special as it will be automatically validated and its default value is 8.
> And this behavior would make hard to constraint function to decide.
> For example, when we set 4m of heap for G1, the SurvivorRatio is 8 but we have only 4 regions from default setting.
> In this case, without FLAG_IS_xxx it is hard to decide whether the result is FAIL or SUCCESS.
> And I think it is okay in this case.
> This is why explicitly specified option is only validated.

So the “default" value isn’t *really* the default, but more of a suggested value to use 
if it would work and is not overridden from the command line.  Specifying the purported
default value explicitly and getting different behavior would, for me, violate the principal
of least surprise.  The present implementation also violates the “rule” that when there is
a default that can be overridden, there should always be a way to explicitly request the
default.  But none of this really has anything to do with 8134995.

>> I don't expect anything to be done about this right now, but I think
>> this does help make the case for using FLAG_IS_CMDLINE in the pause
>> time options under discussion.

Drat.  I meant “… this does *not* help make the case …”

> I agreed to revert to webrev.02 + changing group.
> What is next? :)

Nothing that I know of.  I think you’re good to go.