jdk1.8 默认垃圾收集器为：Parallel Scavenge（新生代）+ Parallel Old（老年代）

$  java -XX:+PrintCommandLineFlags -version
-XX:InitialHeapSize=268435456 -XX:MaxHeapSize=4294967296 -XX:+PrintCommandLineFlags -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC
java version "1.8.0_191"
Java(TM) SE Runtime Environment (build 1.8.0_191-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.191-b12, mixed mode)

$ java -XX:+PrintGCDetails -version
java version "1.8.0_191"
Java(TM) SE Runtime Environment (build 1.8.0_191-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.191-b12, mixed mode)
Heap
 PSYoungGen      total 76288K, used 2621K [0x000000076ab00000, 0x0000000770000000, 0x00000007c0000000)
  eden space 65536K, 4% used [0x000000076ab00000,0x000000076ad8f748,0x000000076eb00000)
  from space 10752K, 0% used [0x000000076f580000,0x000000076f580000,0x0000000770000000)
  to   space 10752K, 0% used [0x000000076eb00000,0x000000076eb00000,0x000000076f580000)
 ParOldGen       total 175104K, used 0K [0x00000006c0000000, 0x00000006cab00000, 0x000000076ab00000)
  object space 175104K, 0% used [0x00000006c0000000,0x00000006c0000000,0x00000006cab00000)
 Metaspace       used 2234K, capacity 4480K, committed 4480K, reserved 1056768K
  class space    used 243K, capacity 384K, committed 384K, reserved 1048576K