RFR: 8222145: ZGC: Introduce SoftMaxHeapSize flag

[Per Liden]

For now, I withdraw this RFR. I'll break out the ZGC specific part into 
a separate patch and re-post this RFR when the CSR has been 
reviewed/approved.

cheers,
Per

On 4/9/19 11:41 AM, Per Liden wrote:
> I realized that I also need to file an CSR for this, which I'll do.
> 
> /Per
> 
> On 4/9/19 11:13 AM, Per Liden wrote:
>> This patch introduces a manageable SoftMaxHeapSize flag. When set, the 
>> GC should strive to not grow heap size beyond the specified size. But 
>> as the flag name suggests, this is a soft limit. In other words, the 
>> GC is allowed to grow the heap size beyond SoftMaxHeapSize if needed.
>>
>> The SoftMaxHeapSize is not be allowed to be set to a value smaller 
>> than min heap size (-Xms) or greater than max heap size (-Xmx). When 
>> not set on the command-line, this flag defaults to the max heap size.
>>
>> There are a few different use cases where this flag could be useful. 
>> For example, when you want to keep the heap footprint down, while 
>> maintaining the capability to deal with a temporary increase in heap 
>> space requirement. Or when you want to play it safe, with lots of 
>> margin, to increase confidence that you will not run into an 
>> allocation stall because of an unforeseen increase in allocation rate.
>>
>> This flag also fit well with the coming JEP 351: ZGC: Uncommit Unused 
>> Memory.
>>
>> Since the flag is manageable, the soft max heap size can be adjusted 
>> (for example via jcmd VM.set_flag) in a live JVM.
>>
>> This option is not prefixed with Z, since other GCs will likely use 
>> this in the future too (at least G1).
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8222145
>> Webrev: http://cr.openjdk.java.net/~pliden/8222145/webrev.0
>>
>> /Per