JVM之常用JVM配置参数

Trace跟踪参数

-verbose:gc

-XX:+printGC

打印GC简要信息

[GC (Allocation Failure)  3344K->1296K(9728K), 0.0004129 secs][GC (Allocation Failure)  3344K->1296K(9728K), 0.0003914 secs][GC (Allocation Failure)  3344K->1296K(9728K), 0.0003756 secs]

-XX:+PrintGCDetails

打印GC详细信息,1.4.0引入，默认不启用

[GC [PSYoungGen: 2560K->0K(3072K)] 3068K->508K(10240K), 0.0005989 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] [GC [PSYoungGen: 2560K->0K(3072K)] 3068K->508K(10240K), 0.0004555 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] [GC [PSYoungGen: 2560K->0K(3072K)] 3068K->508K(10240K), 0.0004692 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]

• 程序运行完后的输出，这是JDK 7

Heap PSYoungGen      total 3072K, used 1229K [0x00000000ffc80000, 0x0000000100000000, 0x0000000100000000)  eden space 2560K, 48% used [0x00000000ffc80000,0x00000000ffdb36d8,0x00000000fff00000)  from space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)  to   space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000) ParOldGen       total 7168K, used 508K [0x00000000ff580000, 0x00000000ffc80000, 0x00000000ffc80000)  object space 7168K, 7% used [0x00000000ff580000,0x00000000ff5ff070,0x00000000ffc80000) PSPermGen       total 21504K, used 2527K [0x00000000fa380000, 0x00000000fb880000, 0x00000000ff580000)  object space 21504K, 11% used [0x00000000fa380000,0x00000000fa5f7d68,0x00000000fb880000)

• 这是JDK 8，可以看到，JDK 8把永生代PSPermGen给去掉了，换成了Metaspace

Heap PSYoungGen      total 2560K, used 205K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)  eden space 2048K, 10% used [0x00000000ffd00000,0x00000000ffd33480,0x00000000fff00000)  from space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)  to   space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000) ParOldGen       total 7168K, used 1308K [0x00000000ff600000, 0x00000000ffd00000, 0x00000000ffd00000)  object space 7168K, 18% used [0x00000000ff600000,0x00000000ff747070,0x00000000ffd00000) Metaspace       used 4405K, capacity 4716K, committed 4992K, reserved 1056768K  class space    used 480K, capacity 531K, committed 640K, reserved 1048576K

-XX:+PrintGCTimeStamps

打印GC发生的时间戳

-Xloggc:log/gc.log

指定GC log的位置，以文件输出，帮助开发人员分析问题

-XX:+PrintHeapAtGC

每一次GC后， 都打印堆的信息

{Heap before GC invocations=1117 (full 0): PSYoungGen      total 2560K, used 2048K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)  eden space 2048K, 100% used [0x00000000ffd00000,0x00000000fff00000,0x00000000fff00000)  from space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)  to   space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000) ParOldGen       total 7168K, used 1328K [0x00000000ff600000, 0x00000000ffd00000, 0x00000000ffd00000)  object space 7168K, 18% used [0x00000000ff600000,0x00000000ff74c080,0x00000000ffd00000) Metaspace       used 4396K, capacity 4716K, committed 4992K, reserved 1056768K  class space    used 478K, capacity 531K, committed 640K, reserved 1048576KHeap after GC invocations=1117 (full 0): PSYoungGen      total 2560K, used 0K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)  eden space 2048K, 0% used [0x00000000ffd00000,0x00000000ffd00000,0x00000000fff00000)  from space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)  to   space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000) ParOldGen       total 7168K, used 1328K [0x00000000ff600000, 0x00000000ffd00000, 0x00000000ffd00000)  object space 7168K, 18% used [0x00000000ff600000,0x00000000ff74c080,0x00000000ffd00000) Metaspace       used 4396K, capacity 4716K, committed 4992K, reserved 1056768K  class space    used 478K, capacity 531K, committed 640K, reserved 1048576K}

-XX:+TraceClassLoading

监控类的加载

[Loaded java.lang.invoke.LambdaFormEditor from C:\Java\jdk1.8.0_101\jre\lib\rt.jar][Loaded java.lang.invoke.LambdaFormEditor$Transform$Kind from C:\Java\jdk1.8.0_101\jre\lib\rt.jar][Loaded java.lang.invoke.LambdaFormEditor$Transform from C:\Java\jdk1.8.0_101\jre\lib\rt.jar]

-XX:+PrintClassHistogram

按下Ctrl+Break后，打印类的信息。分别序号、实例数量、总大小、类型

-Xmx -Xms

指定最大堆，最小堆

-XX:+TraceClassLoading, -XX:+TraceClassUnLoading

查看类的加载和卸载信息

堆的分配参数

-Xmn

设置新生代大小,绝对值

-XX:NewRatio=数字

新生代(eden+ 2 * survivor)和老年代(不含永久区)的比值。例如4,表示新生代:老年代=1:4，即年轻代占堆的1/5.对JDK 7来说

-XX:SurvivorRatio=数字

设置连个Survivor区和eden的比值，例如8，表示2*Survivor:eden=2:8

-XX:+HeapDumpOnOutOfMemoryError

OOM时道出堆到文件

-XX:+HeapDumpPath

导出OOM的路径

-XX:OnOutOfMemoryError

OOM时，执行一个脚本

总结

• 根据实际，调整新生代和Survivor的大小
• 官方推荐新生代占堆的3/8
• Survivor占新生代的1/10
• OOM时，Dump出堆，确保可以排查现场问题

栈的分配参数

• 线程私有，每个线程都有独立的栈空间
• 主要构成是栈帧，栈帧里边有局部变量表，操作数栈，常量池引用，地址等等，分配在栈上

-Xss

通常只有几百k，决定了函数调用的深度

抛出java.lang.StackOverflowError

永久区分配参数(JDK 7有，JDK 8没有永久区)

-XX:PermSize

设置初始空间，表示一个系统可以容纳多少个类型

-XX:MaxPermSize

设置最大空间，表示一个系统可以容纳多少个类型

-Xnoclassgc

是否对方法区的类回收

如果堆空间没有用完，却抛出OOM，那可能是永久区导致的