RFR: 8048269: Add flag to turn off class unloading after G1 concurrent mark

[Jon Masamitsu]

I think the reason for this flags is that it increases the pause times
and some applications care more about pause times than about
class bloat.  How about a name like

ReduceClassUnloading

or

UseReducedClassUnloading

and add

ReducedClassUnloadingInterval

where the interval is the number of GC's
between class unloading.  Or don't add
an interval.  We could add it later or
something equivalent if some customer
asks for it.

The same flag would make sense for STW collectors
that might want the same type of option.

Jon


On 7/11/2014 12:52 AM, Bengt Rutisson wrote:
>
> On 2014-07-11 07:44, Stefan Karlsson wrote:
>> On 2014-07-11 01:11, Jon Masamitsu wrote:
>>> Could we use a more GC generic name for the flag such as
>>> GCClassUnloadingEnabled instead of a G1 specific name and
>>> at some point move CMS to use the GC generic name?
>>
>> Yes, me and Bengt was talking about that, but IMHO we couldn't come 
>> up with a flag with a reasonably short and to-the-point flag name. I 
>> don't like the GCClassUnloadingEnabled since it could be mistaken to 
>> mean turn of class unloading entirely, which isn't what the flag is 
>> supposed to do.
>>
>> Some of the names we thought about:
>>   -XX:+UseConcurrentClassUnloading
>>     This sounds we're doing the class unloading concurrently, which 
>> isn't the case.
>>
>>   -XX:+UseConcMarkClassUnloading
>>
>>   -XX:+UseClassUnloadingAfterConcMark
>>
>>   -XX:+ClassUnloadingAfterConcMark
>>
>> Any more suggestions?
>
> Talked some more to Stefan about this.
>
> The suggestion he made to call it -XX:+ClassUnloadingAfterConcMark is 
> based on that the high level flag to control all class unloading is 
> called -XX:+ClassUnloading. So, this name is kind of a specialization 
> of the overall flag. I kind of like that (even if I think the overall 
> flag should have been called .XX:+UseClassUnloading). However, 
> ConcMark sounds very CMS-like for me. I think my preferred flag name 
> for the moment is:
>
> -XX:+ClassUnloadingAfterConcurrentMark
>
> If we introduce a common flag for CMS and G1 I would suggest to 
> deprecate CMSClassUnloadingEnable in JDK9 and have it only be an alias 
> for the new flag. And then remove CMSClassUnloadingEnable in JDK10.
>
> Thanks,
> Bengt
>
>>
>> thanks,
>> StefanK
>>
>>>
>>> Jon
>>>
>>> On 07/10/2014 08:19 AM, Stefan Karlsson wrote:
>>>> Hi all,
>>>>
>>>> Please, review this patch to add code and a flag to turn off class 
>>>> unloading after G1 concurrent mark.
>>>>
>>>> http://cr.openjdk.java.net/~stefank/8048269/webrev.00/
>>>> https://bugs.openjdk.java.net/browse/JDK-8048269
>>>>
>>>> thanks,
>>>> StefanK
>>>
>>
>