RFR(S): 8068579: Running with -XX:-UseParallelGC does not turn ParalleGC off

[Per Liden]

Hi Dima,

On 2016-03-15 17:38, Dmitry Fazunenko wrote:
> Hi Per,
>
> A couple of comments:
> 1) globals_extension.hpp.frames.html
> requires copyright update
> 2) such fix should go with a regression test, it could be done separately.

Good point. I was about to add a test for this just now and realized 
that the new semantics is not kept for minimal VMs (i.e. when 
INCLUDE_ALL_GCS==0). I'll rework the patch a bit to have the same 
semantics also on minimal VMs. New webrev coming.

thanks,
Per

>
> Thanks,
> Dima
>
>
> On 15.03.2016 16:48, Per Liden wrote:
>> Summary: Explicitly disabling the default collector, without
>> specifying another collector, will silently re-enable the default
>> collector. This is a bit counter intuitive. A better approach would be
>> for the VM to complain about the situation.
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8068579
>> Webrev: http://cr.openjdk.java.net/~pliden/8068579/webrev.0/
>> Testing: manual, jprt
>>
>> Example (assuming it's a platform/configuration with G1 as default):
>>
>> $ java -XX:-UseG1GC ...
>>
>> Today, the above command will happily start a VM with G1 enabled.
>>
>> With this patch the VM will instead complain, like this:
>>
>> $ java -XX:-UseG1GC ...
>> Error occurred during initialization of VM
>> Garbage collector not selected (default collector explicitly disabled)
>>
>> And of course, we're good with disabling the default collector as long
>> as you specify some other collector, e.g.:
>>
>> $ java -XX:-UseG1GC -XX:+UseParallelGC ...
>>
>> cheers,
>> Per
>