Use the -XX:CMSInitiatingOccupancyFraction=85 option when using the CMS collector (-XX:+UseConcMarkSweepGC). If the flag is set and the Old Generation is 85% full, the CMS collector starts collecting unreferenced objects. It is not necessary that CMS will start collection only after the Old Generation 85% occupied. If we want CMS to start only at 85%, then we need to set -XX:+UseCMSInitiatingOccupancyOnly. The default value of the -XX:CMSInitiatingOccupancyFraction flag is 65%.