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JVM堆内存设置和测试

分类: 文章 • 2024-01-05 22:05:34

2019独角兽企业重金招聘Python工程师标准>>> JVM堆内存设置和测试

首先Java虚拟机中gc的原理，可以参见
http://www.360doc.com/content/12/1023/16/9615799_243296263.shtml
http://ifeve.com/useful-jvm-flags-part-5-young-generation-garbage-collection/
以下是一些个人总结和测试。

1. Java虚拟机内存结构

划分新生代和老年代，这样只在新生代分配内存，从而简化了新对象的分配。另外新生代和老年代使用不同的GC算法，可以更有效的清除不再需要的对象。
从上图可以看出，JVM内存由young+old+permanent组成，JVM又进一步将Young分成了eden，from survivor和to survivor三个区域。新对象会首先分配在 Eden 中(如果新对象过大，会直接分配在老年代中)。在GC中，Eden 中的对象会被移动到survivor中，直至对象熬过一定的GC的次数,会被移动到老年代。老年代一般是一些系统级（线程库，classloader等） 的对象，官方推荐新生代占堆大小的3/8，而survivor区各占新生代的1/10。

2. GC原理

JVM堆内存设置和测试
很多对象的生存时间都很短，而新生对象很少引用生存时间长的对象。所以，GC会频繁访问新生代对象，执行Minor GC。在新生代中，GC可以快速标记回收”死对象”，而不需要扫描整个Heap中的存活一段时间的”老对象”(即执行major/FULL GC)。
新生代的GC使用复制算法。在GC前To survivor区保持清空,对象保存在Eden和From survivor区中，GC运行时,Eden中的幸存对象被复制到 To survivor区。针对 From survivor取中的幸存对象，会考虑对象年龄,如果年龄没达到阀值(tenuring threshold)，对象会被复制到To survivor区。如果达到阀值对象被复制到老年代。复制阶段完成后，Eden 和From survivor区中只保存死对象，可以被视为全部清空。如果在复制过程中To survivor区被填满了，剩余的对象会被复制到老年代中。最后 From和To会对换。
JVM堆内存设置和测试
上图演示GC过程，黄色表示死对象，绿色表示剩余空间，红色表示幸存对象
如果新生代过小，会导致新生对象很快就晋升到老年代中，在老年代中对象很难被回收。如果新生代过大，会发生过多的复制过程。所以需要通过不断的测试调优，找到一个合适的JVM参数。

3. JVM内存参数

堆内存大小设置

-Xms:初始堆大小。只要启动，就占用的堆大小
-Xmx:最大堆大小。java.lang.OutOfMemoryError: Java heap这个错误可以通过配置-Xms和-Xmx参数来设置
-Xss：栈大小分配。栈是每个线程私有的区域，通常只有几百K大小，决定了函数调用的深度，而局部变量、参数都分配到栈上。当出现大量局部变量，递归时，会发生栈空间OOM（java.lang.*Error）之类的错误。
-XX:NewSize=n:设置新生代大小的绝对值
-XX:NewRatio=n: 设置年轻代和年老代的比值。比如设置为3，则新生代：老年代=1：3，新生代占1/4的总heap大小。
-XX:SurvivorRatio=n:年轻代中Eden区与两个Survivor区的比值。注意Survivor区有from和to两个。比如设置为8时，那么eden:from:to=8:1:1
-XX:MaxPermSize=n:设置持久代大小 ；java.lang.OutOfMemoryError: PermGen space这个OOM错误需要合理调大PermSize和MaxPermSize大小。
-XX:HeapDumpOnOutOfMemoryError：发生OOM时转储堆到文件，这是一个非常好的诊断方法。
-XX:HeapDumpPath：导出堆的转储文件路径
-XX:OnOutOfMemoryError：OOM时，执行一个脚本，比如发送邮件报警，重启程序。后面跟着一个脚本的路径。

测试1

第一次分配5M，没有超过xms。第二次再次分配5M，total mem会增加。第三次再申请40M，超过xmx限制所以报了OOM错误。

public class JVMTest { public static void main(String args[]) { //=====================Begin========================= 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"); //=====================First Allocated========================= System.out.println("5MB array allocated"); byte[] b1 = new byte[5 * 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"); //=====================Second Allocated========================= System.out.println("10MB array allocated"); byte[] b2 = new byte[10 * 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"); //=====================OOM========================= System.out.println("OOM!!!"); System.gc(); byte[] b3 = new byte[40 * 1024 * 1024]; } }
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public class JVMTest {
     public static void main ( String args [ ] ) {
 
         //=====================Begin=========================
         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" ) ;
 
         //=====================First Allocated=========================
         System . out . println ( "5MB array allocated" ) ;
         byte [ ] b1 = new byte [ 5 * 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" ) ;
 
         //=====================Second Allocated=========================
         System . out . println ( "10MB array allocated" ) ;
         byte [ ] b2 = new byte [ 10 * 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" ) ;
 
         //=====================OOM=========================
         System . out . println ( "OOM!!!" ) ;
         System . gc ( ) ;
         byte [ ] b3 = new byte [ 40 * 1024 * 1024 ] ;
     }
}

以50m XMX和10m XMS的运行测试：

D:\>java -Xmx50m -Xms10m JVMTest Xmx=44.5M free mem=9.804550170898438M total mem=10.5M 5MB array allocated Xmx=44.5M free mem=4.804534912109375M total mem=10.5M 10MB array allocated Xmx=44.5M free mem=5.3045196533203125M total mem=21.0M OOM!!! Exception in thread "main" java.lang.OutOfMemoryError: Java heap space at JVMTest.main(JVMTest.java:43)
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D : \ > java - Xmx50m - Xms10m JVMTest
Xmx = 44.5M
free mem = 9.804550170898438M
total mem = 10.5M
5MB array allocated
Xmx = 44.5M
free mem = 4.804534912109375M
total mem = 10.5M
10MB array allocated
Xmx = 44.5M
free mem = 5.3045196533203125M
total mem = 21.0M
OOM ! ! !
Exception in thread "main" java . lang . OutOfMemoryError : Java heap space
         at JVMTest . main ( JVMTest . java : 43 )

测试2

public class JVMXmn1 { public static void main(String args[]) { byte[] b=null; for(int i=0;i<10;i++) b=new byte[1*1024*1024]; } }
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public class JVMXmn1 {
     public static void main ( String args [ ] ) {
         byte [ ] b = null ;
         for ( int i = 0 ; i < 10 ; i ++ )
             b = new byte [ 1 * 1024 * 1024 ] ;
     }
}

下面按1m的新生代设置，这时对象大于新生代大小，会直接创建在老年代。新生代没有使用。没有触发gc

D:\>java -Xmx20m -Xms20m -Xmn1m -XX:+PrintGCDetails JVMXmn1 Heap PSYoungGen total 512K, used 0K [0x00000000fff00000, 0x0000000100000000, 0x0000000100000000) eden space 0K, -2147483648% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff00000) from space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000) to space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000) ParOldGen total 19456K, used 10836K [0x00000000fec00000, 0x00000000fff00000, 0x00000000fff00000) object space 19456K, 55% used [0x00000000fec00000,0x00000000ff695348,0x00000000fff00000) PSPermGen total 21504K, used 2442K [0x00000000f9a00000, 0x00000000faf00000, 0x00000000fec00000) object space 21504K, 11% used [0x00000000f9a00000,0x00000000f9c628d8,0x00000000faf00000)
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D : \ > java - Xmx20m - Xms20m - Xmn1m - XX : + PrintGCDetails JVMXmn1
Heap
PSYoungGen       total 512K , used 0K [ 0x00000000fff00000 , 0x0000000100000000 , 0x0000000100000000 )
   eden space 0K , - 2147483648 % used [ 0x00000000fff00000 , 0x00000000fff00000 , 0x00000000fff00000 )
   from space 512K , 0 % used [ 0x00000000fff80000 , 0x00000000fff80000 , 0x0000000100000000 )
   to    space 512K , 0 % used [ 0x00000000fff00000 , 0x00000000fff00000 , 0x00000000fff80000 )
ParOldGen       total 19456K , used 10836K [ 0x00000000fec00000 , 0x00000000fff00000 , 0x00000000fff00000 )
   object space 19456K , 55 % used [ 0x00000000fec00000 , 0x00000000ff695348 , 0x00000000fff00000 )
PSPermGen       total 21504K , used 2442K [ 0x00000000f9a00000 , 0x00000000faf00000 , 0x00000000fec00000 )
   object space 21504K , 11 % used [ 0x00000000f9a00000 , 0x00000000f9c628d8 , 0x00000000faf00000 )

下面按15m的新生代设置，全部分配在eden区，老年代没有使用，没有触发gc。

D:\>java -Xmx20m -Xms20m -Xmn15m -XX:+PrintGCDetails JVMXmn1 Heap PSYoungGen total 13824K, used 11525K [0x00000000ff100000, 0x0000000100000000, 0x0000000100000000) eden space 12288K, 93% used [0x00000000ff100000,0x00000000ffc417c8,0x00000000ffd00000) from space 1536K, 0% used [0x00000000ffe80000,0x00000000ffe80000,0x0000000100000000) to space 1536K, 0% used [0x00000000ffd00000,0x00000000ffd00000,0x00000000ffe80000) ParOldGen total 5120K, used 0K [0x00000000fea00000, 0x00000000fef00000, 0x00000000ff100000) object space 5120K, 0% used [0x00000000fea00000,0x00000000fea00000,0x00000000fef00000) PSPermGen total 21504K, used 2442K [0x00000000f9800000, 0x00000000fad00000, 0x00000000fea00000) object space 21504K, 11% used [0x00000000f9800000,0x00000000f9a628d8,0x00000000fad00000)
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D : \ > java - Xmx20m - Xms20m - Xmn15m - XX : + PrintGCDetails JVMXmn1
Heap
PSYoungGen       total 13824K , used 11525K [ 0x00000000ff100000 , 0x0000000100000000 , 0x0000000100000000 )
   eden space 12288K , 93 % used [ 0x00000000ff100000 , 0x00000000ffc417c8 , 0x00000000ffd00000 )
   from space 1536K , 0 % used [ 0x00000000ffe80000 , 0x00000000ffe80000 , 0x0000000100000000 )
   to    space 1536K , 0 % used [ 0x00000000ffd00000 , 0x00000000ffd00000 , 0x00000000ffe80000 )
ParOldGen       total 5120K , used 0K [ 0x00000000fea00000 , 0x00000000fef00000 , 0x00000000ff100000 )
   object space 5120K , 0 % used [ 0x00000000fea00000 , 0x00000000fea00000 , 0x00000000fef00000 )
PSPermGen       total 21504K , used 2442K [ 0x00000000f9800000 , 0x00000000fad00000 , 0x00000000fea00000 )
   object space 21504K , 11 % used [ 0x00000000f9800000 , 0x00000000f9a628d8 , 0x00000000fad00000 )

下面按8m的新生代设置，触发了一次gc，由于from和to的大小小于1M的对象大小，eden区会直接进入老年代。

D:\>java -Xmx20m -Xms20m -Xmn8m -XX:+PrintGCDetails JVMXmn1 [GC [PSYoungGen: 5954K->536K(7168K)] 5954K->1560K(19456K), 0.0021655 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] Heap PSYoungGen total 7168K, used 5971K [0x00000000ff800000, 0x0000000100000000, 0x0000000100000000) eden space 6144K, 88% used [0x00000000ff800000,0x00000000ffd4ecb8,0x00000000ffe00000) from space 1024K, 52% used [0x00000000ffe00000,0x00000000ffe86010,0x00000000fff00000) to space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000) ParOldGen total 12288K, used 1024K [0x00000000fec00000, 0x00000000ff800000, 0x00000000ff800000) object space 12288K, 8% used [0x00000000fec00000,0x00000000fed00010,0x00000000ff800000) PSPermGen total 21504K, used 2445K [0x00000000f9a00000, 0x00000000faf00000, 0x00000000fec00000) object space 21504K, 11% used [0x00000000f9a00000,0x00000000f9c63400,0x00000000faf00000)
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D : \ > java - Xmx20m - Xms20m - Xmn8m - XX : + PrintGCDetails JVMXmn1
[ GC [ PSYoungGen : 5954K -> 536K ( 7168K ) ] 5954K -> 1560K ( 19456K ) , 0.0021655 secs ] [ Times : user = 0.00 sys = 0.00 , real = 0.00 secs ]
Heap
PSYoungGen       total 7168K , used 5971K [ 0x00000000ff800000 , 0x0000000100000000 , 0x0000000100000000 )
   eden space 6144K , 88 % used [ 0x00000000ff800000 , 0x00000000ffd4ecb8 , 0x00000000ffe00000 )
   from space 1024K , 52 % used [ 0x00000000ffe00000 , 0x00000000ffe86010 , 0x00000000fff00000 )
   to    space 1024K , 0 % used [ 0x00000000fff00000 , 0x00000000fff00000 , 0x0000000100000000 )
ParOldGen       total 12288K , used 1024K [ 0x00000000fec00000 , 0x00000000ff800000 , 0x00000000ff800000 )
   object space 12288K , 8 % used [ 0x00000000fec00000 , 0x00000000fed00010 , 0x00000000ff800000 )
PSPermGen       total 21504K , used 2445K [ 0x00000000f9a00000 , 0x00000000faf00000 , 0x00000000fec00000 )
   object space 21504K , 11 % used [ 0x00000000f9a00000 , 0x00000000f9c63400 , 0x00000000faf00000 )

下面按7m的新生代设置，from和to的大小可以为1/xmn,大于触发了3次新生代gc，一共回收了7M左右的空间，最后剩余3M在系统当中，没有使用老年代。

D:\>java -Xmx20m -Xms20m -Xmn7m -XX:SurvivorRatio=2 -XX:+PrintGCDetails JVMXmn1 [GC [PSYoungGen: 3785K->1512K(5632K)] 3785K->1568K(18944K), 0.0024118 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] [GC [PSYoungGen: 4755K->1528K(5632K)] 4811K->1584K(18944K), 0.0013799 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] [GC [PSYoungGen: 4631K->1496K(5632K)] 4687K->1552K(18944K), 0.0010990 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] Heap PSYoungGen total 5632K, used 2561K [0x00000000ff900000, 0x0000000100000000, 0x0000000100000000) eden space 4096K, 26% used [0x00000000ff900000,0x00000000ffa0a448,0x00000000ffd00000) from space 1536K, 97% used [0x00000000ffd00000,0x00000000ffe76020,0x00000000ffe80000) to space 1536K, 0% used [0x00000000ffe80000,0x00000000ffe80000,0x0000000100000000) ParOldGen total 13312K, used 56K [0x00000000fec00000, 0x00000000ff900000, 0x00000000ff900000) object space 13312K, 0% used [0x00000000fec00000,0x00000000fec0e000,0x00000000ff900000) PSPermGen total 21504K, used 2445K [0x00000000f9a00000, 0x00000000faf00000, 0x00000000fec00000) object space 21504K, 11% used [0x00000000f9a00000,0x00000000f9c63400,0x00000000faf00000)
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D : \ > java - Xmx20m - Xms20m - Xmn7m - XX : SurvivorRatio = 2 - XX : + PrintGCDetails JVMXmn1
[ GC [ PSYoungGen : 3785K -> 1512K ( 5632K ) ] 3785K -> 1568K ( 18944K ) , 0.0024118 secs ] [ Times : user = 0.00 sys = 0.00 , real = 0.00 secs ]
[ GC [ PSYoungGen : 4755K -> 1528K ( 5632K ) ] 4811K -> 1584K ( 18944K ) , 0.0013799 secs ] [ Times : user = 0.00 sys = 0.00 , real = 0.00 secs ]
[ GC [ PSYoungGen : 4631K -> 1496K ( 5632K ) ] 4687K -> 1552K ( 18944K ) , 0.0010990 secs ] [ Times : user = 0.00 sys = 0.00 , real = 0.00 secs ]
Heap
PSYoungGen       total 5632K , used 2561K [ 0x00000000ff900000 , 0x0000000100000000 , 0x0000000100000000 )
   eden space 4096K , 26 % used [ 0x00000000ff900000 , 0x00000000ffa0a448 , 0x00000000ffd00000 )
   from space 1536K , 97 % used [ 0x00000000ffd00000 , 0x00000000ffe76020 , 0x00000000ffe80000 )
   to    space 1536K , 0 % used [ 0x00000000ffe80000 , 0x00000000ffe80000 , 0x0000000100000000 )
ParOldGen       total 13312K , used 56K [ 0x00000000fec00000 , 0x00000000ff900000 , 0x00000000ff900000 )
   object space 13312K , 0 % used [ 0x00000000fec00000 , 0x00000000fec0e000 , 0x00000000ff900000 )
PSPermGen       total 21504K , used 2445K [ 0x00000000f9a00000 , 0x00000000faf00000 , 0x00000000fec00000 )
   object space 21504K , 11 % used [ 0x00000000f9a00000 , 0x00000000f9c63400 , 0x00000000faf00000 )

新生代占一半大小（10m），幸存区为3：1：1（6m:2m,2m)，触发了1次gc，回收了7m左右空间，没有使用老年代。对于这种临时对象，减少老年代的使用是gc优化的关键。

D:\>java -Xmx20m -Xms20m -XX:NewRatio=1 -XX:SurvivorRatio=3 -XX:+PrintGCDetails JVMXmn1 [GC [PSYoungGen: 5954K->1624K(8192K)] 5954K->1624K(18432K), 0.0023152 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] Heap PSYoungGen total 8192K, used 7059K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000) eden space 6144K, 88% used [0x00000000ff600000,0x00000000ffb4ecb8,0x00000000ffc00000) from space 2048K, 79% used [0x00000000ffc00000,0x00000000ffd96020,0x00000000ffe00000) to space 2048K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x0000000100000000) ParOldGen total 10240K, used 0K [0x00000000fec00000, 0x00000000ff600000,0x00000000ff600000) object space 10240K, 0% used [0x00000000fec00000,0x00000000fec00000,0x00000000ff600000) PSPermGen total 21504K, used 2445K [0x00000000f9a00000, 0x00000000faf00000, 0x00000000fec00000) object space 21504K, 11% used [0x00000000f9a00000,0x00000000f9c63400,0x00000000faf00000)
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D : \ > java - Xmx20m - Xms20m - XX : NewRatio = 1 - XX : SurvivorRatio = 3 - XX : + PrintGCDetails JVMXmn1
[ GC [ PSYoungGen : 5954K -> 1624K ( 8192K ) ] 5954K -> 1624K ( 18432K ) , 0.0023152 secs ] [ Times : user = 0.00 sys = 0.00 , real = 0.00 secs ]
Heap
PSYoungGen       total 8192K , used 7059K [ 0x00000000ff600000 , 0x0000000100000000 , 0x0000000100000000 )
   eden space 6144K , 88 % used [ 0x00000000ff600000 , 0x00000000ffb4ecb8 , 0x00000000ffc00000 )
   from space 2048K , 79 % used [ 0x00000000ffc00000 , 0x00000000ffd96020 , 0x00000000ffe00000 )
   to    space 2048K , 0 % used [ 0x00000000ffe00000 , 0x00000000ffe00000 , 0x0000000100000000 )
ParOldGen       total 10240K , used 0K [ 0x00000000fec00000 , 0x00000000ff600000 , 0x00000000ff600000 )
   object space 10240K , 0 % used [ 0x00000000fec00000 , 0x00000000fec00000 , 0x00000000ff600000 )
PSPermGen       total 21504K , used 2445K [ 0x00000000f9a00000 , 0x00000000faf00000 , 0x00000000fec00000 )
   object space 21504K , 11 % used [ 0x00000000f9a00000 , 0x00000000f9c63400 , 0x00000000faf00000 )

^^

转载于:https://my.oschina.net/u/658658/blog/550243

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