JVM 备忘录

Java 虚拟机

GC 日志相关配置

-XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintHeapAtGC -Xloggc:./gc.log

• -XX:+PrintGC 输出GC日志
• -XX:+PrintGCDetails 输出GC的详细日志
• -XX:+PrintGCTimeStamps 输出GC的时间戳（以基准时间的形式）
• -XX:+PrintGCDateStamps 输出GC的时间戳（以日期的形式，如 2013-05-04T21:53:59.234+0800）
• -XX:+PrintHeapAtGC 在进行GC的前后打印出堆的信息
• -Xloggc:./gc.log 日志文件的输出路径

Java HotSpot(TM) 64-Bit Server VM (25.121-b13) for windows-amd64 JRE (1.8.0_121-b13), built on Dec 12 2016 18:21:36 by "java_re" with MS VC++ 10.0 (VS2010)
Memory: 4k page, physical 16599008k(3716028k free), swap 27331352k(3936160k free)
CommandLine flags: -XX:InitialHeapSize=20971520 -XX:MaxHeapSize=20971520 -XX:MaxNewSize=10485760 -XX:NewSize=10485760 -XX:+PrintGC -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintHeapAtGC -XX:SurvivorRatio=8 -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:-UseLargePagesIndividualAllocation -XX:+UseParallelGC 
{Heap before GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 6307K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 76% used [0x00000000ff600000,0x00000000ffc28ce8,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 10240K, used 0K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  object space 10240K, 0% used [0x00000000fec00000,0x00000000fec00000,0x00000000ff600000)
 Metaspace       used 3477K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 384K, capacity 388K, committed 512K, reserved 1048576K
2019-11-28T09:53:34.265+0800: 0.123: [GC (Allocation Failure) [PSYoungGen: 6307K->776K(9216K)] 6307K->4880K(19456K), 0.0022355 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap after GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 776K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 75% used [0x00000000ffe00000,0x00000000ffec2020,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 10240K, used 4104K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  object space 10240K, 40% used [0x00000000fec00000,0x00000000ff002020,0x00000000ff600000)
 Metaspace       used 3477K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 384K, capacity 388K, committed 512K, reserved 1048576K
}
Heap
 PSYoungGen      total 9216K, used 7241K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 78% used [0x00000000ff600000,0x00000000ffc50650,0x00000000ffe00000)
  from space 1024K, 75% used [0x00000000ffe00000,0x00000000ffec2020,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 10240K, used 4104K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  object space 10240K, 40% used [0x00000000fec00000,0x00000000ff002020,0x00000000ff600000)
 Metaspace       used 3499K, capacity 4498K, committed 4864K, reserved 1056768K
  class space    used 387K, capacity 390K, committed 512K, reserved 1048576K

JVM常用内存设置

参考： Oracle Javase8 Doc

-Xms1024M -Xmx1024M -Xmn512M -Xss2M -XX:SurvivorRatio=8 -XX:MaxMetaspaceSize=256M

• -Xms1024M 堆空间设置最小空间限制，一般要大于程序启动时占用内存大小；
• -Xmx1024M 设置堆空间的最大可申请上限，和-XX:MaxHeapSize功能相同，一般-Xms 和-Xmx 设置成相同值；
• -Xmn512M 设置堆空间中的“年轻代”的初始值和最大值大小； 如果设置的过小，会导致频繁的Minor GC，如果设置的过大，会累积成一次Full GC 导致单次GC时间过长；官方推荐的年轻代大小是 堆空间的四分之一到二分之一；
• -Xss2M 设置虚拟机栈（线程栈）的最大值，功能和-XX:ThreadStackSize相同，一般默认1MB左右
• -XX:SurvivorRatio=8 设置eden区和survivor区的比率，默认设置为8，则Eden80%，From_Survivor10%，To_Survivor10%
• -XX:MaxMetaspaceSize=256M 设置元空间在直接内存中的最大值；默认是不限制的，宿主机有多大剩余空间，就可以申请到多少空间。

JVM默认参数配置

java -XX:+PrintFlagsFinal -version