Java垃圾回收—— concurrent collector

With the serial collector a major collection is started when the tenured generation becomes full and all application threads are stopped while the collection is done. In contrast a concurrent collection should be started at a time such that the collection can finish before the tenured generation becomes full. There are several ways a concurrent collection can be started.

 

The concurrent collector keeps statistics on the time remaining before the tenured generation is full (T-until-full) and on the time needed to do a concurrent collection (T-collect). When the T-until-full approaches T-collect, a concurrent collection is started. This test is appropriately padded so as to start a collection conservatively early.

A concurrent collection will also start if the occupancy of the tenured generation grows above the initiating occupancy (i.e., the percentage of the current heap that is used before a concurrent collection is started). The initiating occupancy by default is set to about 68%. It can be set with the parameter CMSInitiatingOccupancyFraction which can be set on the command line with the flag

-XX:CMSInitiatingOccupancyFraction=<nn>

The value <nn> is a percentage of the current tenured generation size.