The default for G1ConcRefinementGreenZone is on the small side. Because of this it can take quite a few GC cycles before it approaches the long-term range. It also isn't accounting for other options in a sensible way, so changes to those options might take it even further from reality. We should use a better default. (A better green zone controller (JDK-8155996) would make the initial value less important, but improving the current default is better than doing nothing.)