JVM基础：GC

Minor GC: 新生代

Major GC: 老年代

Full GC:  新生+老年

垃圾回收器：JDK1.8开始支持G1

垃圾回收器发展路线，随着内存越来越大而演进：从分代演化到不分代；  Serial：几十M； Parallel：几个G； CMS: 几十个G

CMS: Concurrent Mark Sweep    第3列：简称PS&PO

Serial：a stop-the-world, copying collector which uses a signle GC thread

Parallel Scavenge：a stop-the-world, copying collector which uses multiple GC thread

Parallel Old：a compacting collector that uses multiple GC threads

 ParNew: a stop-the-world, copying collector which uses multiple GC threads;it differs from "Paralledl Scavenge" in that it has enhancements that makes it useable with CMS（和CMS配对使用）; 

For example, ParNew does the synchronization needed so that it can run during the concurrent phases of CMS

CMS: concurrent mark sweep 非常重要的一个算法，面试重灾区；  a mostly concurrent, low-pause collector（暂停时间短）

4 phases：　　1.initial mark　　2.concurrent mark　　3.remark　　4.concurent sweep

 https://www.bilibili.com/video/BV1AZ4y147fD?p=3