Trace跟踪参数
-verbose:gc
-XX:+printGC
可以打印GC的简要信息
[GC 4790K->374K(15872K), 0.0001606 secs] [GC 4790K->374K(15872K), 0.0001474 secs] [GC 4790K->374K(15872K), 0.0001563 secs] [GC 4790K->374K(15872K), 0.0001682 secs]
-XX:+PrintGCDetails
打印GC详细信息-XX:+PrintGCTimeStamps
打印CG发生的时间戳
例
[GC[DefNew: 4416K->0K(4928K), 0.0001897 secs] 4790K->374K(15872K), 0.0002232 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
-XX:+PrintGCDetails的输出
Heap def new generation total 13824K, used 11223K [0x27e80000, 0x28d80000, 0x28d80000) eden space 12288K, 91% used [0x27e80000, 0x28975f20, 0x28a80000)from space 1536K, 0% used [0x28a80000, 0x28a80000, 0x28c00000) to space 1536K, 0% used [0x28c00000, 0x28c00000, 0x28d80000) tenured generation total 5120K, used 0K [0x28d80000, 0x29280000, 0x34680000) the space 5120K, 0% used [0x28d80000, 0x28d80000, 0x28d80200, 0x29280000) compacting perm gen total 12288K, used 142K [0x34680000, 0x35280000, 0x38680000) the space 12288K, 1% used [0x34680000, 0x346a3a90, 0x346a3c00, 0x35280000) ro space 10240K, 44% used [0x38680000, 0x38af73f0, 0x38af7400, 0x39080000) rw space 12288K, 52% used [0x39080000, 0x396cdd28, 0x396cde00, 0x39c80000)
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
每次一次GC后，都打印堆信息
{Heap before GC invocations=0 (full 0):
def new generation total 3072K, used 2752K [0x33c80000, 0x33fd0000, 0x33fd0000)
eden space 2752K, 100% used [0x33c80000, 0x33f30000, 0x33f30000)from space 320K, 0% used [0x33f30000, 0x33f30000, 0x33f80000)
to space 320K, 0% used [0x33f80000, 0x33f80000, 0x33fd0000)
tenured generation total 6848K, used 0K [0x33fd0000, 0x34680000, 0x34680000)
the space 6848K, 0% used [0x33fd0000, 0x33fd0000, 0x33fd0200, 0x34680000)
compacting perm gen total 12288K, used 143K [0x34680000, 0x35280000, 0x38680000)
the space 12288K, 1% used [0x34680000, 0x346a3c58, 0x346a3e00, 0x35280000)
ro space 10240K, 44% used [0x38680000, 0x38af73f0, 0x38af7400, 0x39080000)
rw space 12288K, 52% used [0x39080000, 0x396cdd28, 0x396cde00, 0x39c80000)
[GC[DefNew: 2752K->320K(3072K), 0.0014296 secs] 2752K->377K(9920K), 0.0014604 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap after GC invocations=1 (full 0):
def new generation total 3072K, used 320K [0x33c80000, 0x33fd0000, 0x33fd0000)
eden space 2752K, 0% used [0x33c80000, 0x33c80000, 0x33f30000)from space 320K, 100% used [0x33f80000, 0x33fd0000, 0x33fd0000)
to space 320K, 0% used [0x33f30000, 0x33f30000, 0x33f80000)
tenured generation total 6848K, used 57K [0x33fd0000, 0x34680000, 0x34680000)
the space 6848K, 0% used [0x33fd0000, 0x33fde458, 0x33fde600, 0x34680000)
compacting perm gen total 12288K, used 143K [0x34680000, 0x35280000, 0x38680000)
the space 12288K, 1% used [0x34680000, 0x346a3c58, 0x346a3e00, 0x35280000)
ro space 10240K, 44% used [0x38680000, 0x38af73f0, 0x38af7400, 0x39080000)
rw space 12288K, 52% used [0x39080000, 0x396cdd28, 0x396cde00, 0x39c80000)
}-XX:+TraceClassLoading
监控类的加载
[Loaded java.lang.Object from shared objects file]
[Loaded java.io.Serializable from shared objects file]
[Loaded java.lang.Comparable from shared objects file]
[Loaded java.lang.CharSequence from shared objects file]
[Loaded java.lang.String from shared objects file]
[Loaded java.lang.reflect.GenericDeclaration from shared objects file]
[Loaded java.lang.reflect.Type from shared objects file]
跟踪调试的时候 那些类被加载出来-XX:+PrintClassHistogram
按下Ctrl+Break后，打印类的信息：
num #instances #bytes class name---------------------------------------------- 1: 890617 470266000 [B 2: 890643 21375432 java.util.HashMap$Node 3: 890608 14249728 java.lang.Long 4: 13 8389712 [Ljava.util.HashMap$Node; 5: 2062 371680 [C 6: 463 41904 java.lang.Class
分别显示：序号、实例数量、总大小、类型
堆的分配参数
Xmx –Xms
指定最大堆和最小堆
VM options: -Xmx20m -Xms5m 运行代码：
public static void main(String[] args) {
System.out.print("Xmx=");
System.out.println(Runtime.getRuntime().maxMemory()/1024.0/1024+"M");
System.out.print("free mem=");
System.out.println(Runtime.getRuntime().freeMemory()/1024.0/1024+"M");
System.out.print("total mem=");
System.out.println(Runtime.getRuntime().totalMemory()/1024.0/1024+"M");
}结果：
Xmx=18.0Mfree mem=4.660301208496094M total mem=5.5M
public static void main(String[] args) { byte[] b= new byte[4 * 1024 * 1024];
System.out.print("Xmx=");
System.out.println(Runtime.getRuntime().maxMemory()/1024.0/1024+"M");
System.out.print("free mem=");
System.out.println(Runtime.getRuntime().freeMemory()/1024.0/1024+"M");
System.out.print("total mem=");
System.out.println(Runtime.getRuntime().totalMemory()/1024.0/1024+"M");
}结果：
Xmx=18.0Mfree mem=5.074928283691406M total mem=10.0M
总内存变多
public static void main(String[] args) {
System.gc();
System.out.print("Xmx=");
System.out.println(Runtime.getRuntime().maxMemory()/1024.0/1024+"M");
System.out.print("free mem=");
System.out.println(Runtime.getRuntime().freeMemory()/1024.0/1024+"M");
System.out.print("total mem=");
System.out.println(Runtime.getRuntime().totalMemory()/1024.0/1024+"M");
}结果：
Xmx=18.0Mfree mem=5.9815826416015625M total mem=6.5M
空闲内存变多
-Xmn
设置新生代大小-XX:NewRatio
新生代（eden+2*s）和老年代（不包含永久区）的比值
4 表示 新生代:老年代=1:4，即年轻代占堆的1/5-XX:SurvivorRatio
设置两个Survivor区和eden的比
8表示 两个Survivor :eden=2:8，即一个Survivor占年轻代的1/10
public static void main(String[] args) { byte[] b=null; for(int i=0;i<10;i++)
b=new byte[1*1024*1024];
}VM options: -Xmx20m -Xms20m -Xmn3m -XX:+PrintGCDetails
输出：
Heap PSYoungGen total 2560K, used 1490K [0x00000007bfd00000, 0x00000007c0000000, 0x00000007c0000000) eden space 2048K, 72% used [0x00000007bfd00000,0x00000007bfe748a8,0x00000007bff00000) from space 512K, 0% used [0x00000007bff80000,0x00000007bff80000,0x00000007c0000000) to space 512K, 0% used [0x00000007bff00000,0x00000007bff00000,0x00000007bff80000) ParOldGen total 17408K, used 10240K [0x00000007bec00000, 0x00000007bfd00000, 0x00000007bfd00000) object space 17408K, 58% used [0x00000007bec00000,0x00000007bf6000a0,0x00000007bfd00000) Metaspace used 3054K, capacity 4496K, committed 4864K, reserved 1056768K class space used 334K, capacity 388K, committed 512K, reserved 1048576K
没有触发GC
全部分配在老年代
VM options: -Xmx20m -Xms20m -Xmn15m -XX:+PrintGCDetails
输出：
Heap PSYoungGen total 13824K, used 11966K [0x00000007bf100000, 0x00000007c0000000, 0x00000007c0000000) eden space 12288K, 97% used [0x00000007bf100000,0x00000007bfcaf8d0,0x00000007bfd00000) from space 1536K, 0% used [0x00000007bfe80000,0x00000007bfe80000,0x00000007c0000000) to space 1536K, 0% used [0x00000007bfd00000,0x00000007bfd00000,0x00000007bfe80000) ParOldGen total 5120K, used 0K [0x00000007bec00000, 0x00000007bf100000, 0x00000007bf100000) object space 5120K, 0% used [0x00000007bec00000,0x00000007bec00000,0x00000007bf100000) Metaspace used 3051K, capacity 4496K, committed 4864K, reserved 1056768K class space used 334K, capacity 388K, committed 512K, reserved 1048576K
没有触发GC
全部分配在eden
老年代没有使用
VM options: -Xmx20m -Xms20m -Xmn15m -XX:+PrintGCDetails
输出
[GC (Allocation Failure) [PSYoungGen: 5797K->496K(6656K)] 6861K->2604K(19968K), 0.0011469 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] Heap PSYoungGen total 6656K, used 1622K [0x00000007bf900000, 0x00000007c0000000, 0x00000007c0000000) eden space 6144K, 18% used [0x00000007bf900000,0x00000007bfa19ab8,0x00000007bff00000) from space 512K, 96% used [0x00000007bff80000,0x00000007bfffc010,0x00000007c0000000) to space 512K, 0% used [0x00000007bff00000,0x00000007bff00000,0x00000007bff80000) ParOldGen total 13312K, used 2108K [0x00000007bec00000, 0x00000007bf900000, 0x00000007bf900000) object space 13312K, 15% used [0x00000007bec00000,0x00000007bee0f030,0x00000007bf900000) Metaspace used 2947K, capacity 4496K, committed 4864K, reserved 1056768K class space used 325K, capacity 388K, committed 512K, reserved 1048576K
进行了1次新生代GC
s0 s1 太小需要老年代担保
VM options: -Xmx20m -Xms20m -Xmn6m -XX:SurvivorRatio=2 -XX:+PrintGCDetails
[GC (Allocation Failure) [PSYoungGen: 2357K->1520K(4608K)] 2357K->1560K(18944K), 0.0010973 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] [GC (Allocation Failure) [PSYoungGen: 3658K->1504K(4608K)] 3698K->1552K(18944K), 0.0009730 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] [GC (Allocation Failure) [PSYoungGen: 3621K->1504K(4608K)] 3669K->1576K(18944K), 0.0004732 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] [GC (Allocation Failure) [PSYoungGen: 3604K->1520K(4608K)] 3676K->1608K(18944K), 0.0004172 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] [GC (Allocation Failure) [PSYoungGen: 3623K->1520K(4608K)] 3711K->1616K(18944K), 0.0004710 secs] [Times: user=0.01 sys=0.00, real=0.01 secs] Heap PSYoungGen total 4608K, used 2619K [0x00000007bfa00000, 0x00000007c0000000, 0x00000007c0000000) eden space 3072K, 35% used [0x00000007bfa00000,0x00000007bfb12e48,0x00000007bfd00000) from space 1536K, 98% used [0x00000007bfd00000,0x00000007bfe7c020,0x00000007bfe80000) to space 1536K, 0% used [0x00000007bfe80000,0x00000007bfe80000,0x00000007c0000000) ParOldGen total 14336K, used 96K [0x00000007bec00000, 0x00000007bfa00000, 0x00000007bfa00000) object space 14336K, 0% used [0x00000007bec00000,0x00000007bec18000,0x00000007bfa00000) Metaspace used 2929K, capacity 4496K, committed 4864K, reserved 1056768K class space used 319K, capacity 388K, committed 512K, reserved 1048576K
进行了5次新生代GC
s0 s1 增大
-XX:+HeapDumpOnOutOfMemoryError
OOM时导出堆到文件-XX:+HeapDumpPath
导出OOM的路径-XX:OnOutOfMemoryError
在OOM时，执行一个脚本
"-XX:OnOutOfMemoryError=D:/tools/jdk1.7_40/bin/printstack.bat %p“
当程序OOM时，在D:/a.txt中将会生成线程的dump
可以在OOM时，发送邮件，甚至是重启程序
总结
根据实际事情调整新生代和幸存代的大小
官方推荐新生代占堆的3/8
幸存代占新生代的1/10
在OOM时，记得Dump出堆，确保可以排查线程
永久区分配参数
-XX:PermSize -XX:MaxPermSize
设置永久区的初始空间和最大空间
他们表示，一个系统可以容纳多少个类型
使用CGLIB等库的时候，可能会产生大量的类，这些类，有可能撑爆永久区导致OOM
for(int i=0;i<100000;i++){
CglibBean bean = new CglibBean("geym.jvm.ch3.perm.bean"+i,new HashMap());
}不断产生新的类
打开堆的Dump
堆空间实际占用非常少
但是永久区溢出 一样抛出OOM
如果堆空间没有用完也抛出了OOM，有可能是永久区导致的
栈大小分配
通常只有几百K
决定了函数调用的深度
每个线程都有独立的栈空间
局部变量、参数 分配在栈上
private static int count=0; public static void recursion(long a,long b,long c){ long e=1,f=2,g=3,h=4,i=5,k=6,q=7,x=8,y=9,z=10;
count++;
recursion(a,b,c);
} public static void main(String args[]){ try{
recursion(0L,0L,0L);
}catch(Throwable e){
System.out.println("deep of calling = "+count);
e.printStackTrace();
}
}VM options: -Xss160K
deep of calling = 235java.lang.StackOverflowError ...
VM options: -Xss280K
deep of calling = 662java.lang.StackOverflowError
作者：香沙小熊
链接：https://www.jianshu.com/p/c4c07e964198
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