Java8用jmap输出jvm参数

左手的ㄟ右手 2022-05-23 11:52 180阅读 0赞

jmap输出

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[tomcat@n01 ~]$ /opt/java/jdk1.8.0_101/bin/jmap -heap 11368   
    Attaching to process ID 11368, please wait...
    Debugger attached successfully.
    Server compiler detected.
    JVM version is 25.101-b13
    
    using thread-local object allocation.
    Parallel GC with 2 thread(s)
    
    Heap Configuration:
       MinHeapFreeRatio         = 0
       MaxHeapFreeRatio         = 100
       MaxHeapSize              = 2684354560 (2560.0MB)
       NewSize                  = 1073741824 (1024.0MB)
       MaxNewSize               = 1073741824 (1024.0MB)
       OldSize                  = 1610612736 (1536.0MB)
       NewRatio                 = 2
       SurvivorRatio            = 8
       MetaspaceSize            = 21807104 (20.796875MB)
       CompressedClassSpaceSize = 1073741824 (1024.0MB)
       MaxMetaspaceSize         = 17592186044415 MB
       G1HeapRegionSize         = 0 (0.0MB)
    
    Heap Usage:
    PS Young Generation
    Eden Space:
       capacity = 852492288 (813.0MB)
       used     = 420427144 (400.95056915283203MB)
       free     = 432065144 (412.04943084716797MB)
       49.31741317993014% used
    From Space:
       capacity = 113770496 (108.5MB)
       used     = 2299712 (2.19317626953125MB)
       free     = 111470784 (106.30682373046875MB)
       2.021360617079493% used
    To Space:
       capacity = 107479040 (102.5MB)
       used     = 0 (0.0MB)
       free     = 107479040 (102.5MB)
       0.0% used
    PS Old Generation
       capacity = 1610612736 (1536.0MB)
       used     = 50883368 (48.526161193847656MB)
       free     = 1559729368 (1487.4738388061523MB)
       3.1592552860577903% used
    
    27595 interned Strings occupying 3138384 bytes.

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**MaxHeapFreeRatio**: GC后如果发现空闲堆内存占到整个预估堆内存的N%(百分比)，则收缩堆内存的预估最大值, 预估堆内存是堆大小动态调控的重要选项之一. 堆内存预估最大值一定小于或等于固定最大值(-Xmx指定的数值). 前者会根据使用情况动态调大或缩小, 以提高GC回收的效率  
**MinHeapFreeRatio**: GC后如果发现空闲堆内存占到整个预估堆内存的N%(百分比), 则放大堆内存的预估最大值  
  
**MaxHeapSize**: 即-Xmx, 堆内存大小的上限  
**InitialHeapSize**: 即-Xms, 堆内存大小的初始值  
  
**NewSize**: 新生代预估堆内存占用的默认值  
**MaxNewSize**: 新生代占整个堆内存的最大值  
  
**OldSize**: 老年代的默认大小, default size of the tenured generation  
**NewRatio**: 老年代对比新生代的空间大小, 比如2代表老年代空间是新生代的两倍大小. The ratio of old generation to young generation.  
  
**SurvivorRatio**: Eden/Survivor的值. 这个值的说明, 很多网上转载的都是错的. 8表示Survivor:Eden=1:8, 因为survivor区有2个, 所以Eden的占比为8/10. Ratio of eden/survivor space size. -XX:SurvivorRatio=6 sets the ratio between each survivor space and eden to be 1:6, each survivor space will be one eighth of the young generation.   
  
**MetaspaceSize**: 分配给类元数据空间的初始大小(Oracle逻辑存储上的初始高水位，the initial high-water-mark ). 此值为估计值. MetaspaceSize设置得过大会延长垃圾回收时间. 垃圾回收过后, 引起下一次垃圾回收的类元数据空间的大小可能会变大  
**MaxMetaspaceSize**: 是分配给类元数据空间的最大值, 超过此值就会触发Full GC. 此值仅受限于系统内存的大小, JVM会动态地改变此值  
  
**CompressedClassSpaceSize**: 类指针压缩空间大小, 默认为1G  
  
**G1HeapRegionSize**: G1区块的大小, 取值为1M至32M. 其取值是要根据最小Heap大小划分出2048个区块. With G1 the Java heap is subdivided into uniformly sized regions. This sets the size of the individual sub-divisions. The default value of this parameter is determined ergonomically based upon heap size. The minimum value is 1Mb and the maximum value is 32Mb. Sets the size of a G1 region. The value will be a power of two and can range from 1MB to 32MB. The goal is to have around 2048 regions based on the minimum Java heap size.

**指针压缩**1. 64位平台上默认打开  
  1)使用-XX:+UseCompressedOops压缩对象指针  
    "oops"指的是普通对象指针("ordinary" object pointers)。  
    Java堆中对象指针会被压缩成32位。  
    使用堆基地址（如果堆在低26G内存中的话，基地址为0）  
  2)使用-XX:+UseCompressedClassPointers选项来压缩类指针  
    对象中指向类元数据的指针会被压缩成32位  
    类指针压缩空间会有一个基地址  
  
2. 元空间和类指针压缩空间的区别  
  1)类指针压缩空间只包含类的元数据，比如InstanceKlass, ArrayKlass  
    仅当打开了UseCompressedClassPointers选项才生效  
    为了提高性能，Java中的虚方法表也存放到这里  
    这里到底存放哪些元数据的类型，目前仍在减少  
  2)元空间包含类的其它比较大的元数据，比如方法，字节码，常量池等。

[tomcat@n01 ~]$ /opt/java/jdk1.8.0_101/bin/jstat -gcutil 11368   
      S0     S1     E      O      M     CCS       YGC   YGCT     FGC    FGCT     GCT   
      2.02   0.00  49.32   3.16  92.13  88.17     24    1.220     3    0.637    1.857

S0: Survivor 0区的空间使用率 Survivor space 0 utilization as a percentage of the space's current capacity.  
S1: Survivor 1区的空间使用率 Survivor space 1 utilization as a percentage of the space's current capacity.  
E: Eden区的空间使用率 Eden space utilization as a percentage of the space's current capacity.  
O: 老年代的空间使用率 Old space utilization as a percentage of the space's current capacity.  
M: 元数据的空间使用率 Metaspace utilization as a percentage of the space's current capacity.  
CCS: 类指针压缩空间使用率 Compressed class space utilization as a percentage.  
YGC: 新生代GC次数 Number of young generation GC events.  
YGCT: 新生代GC总时长 Young generation garbage collection time.  
FGC: Full GC次数 Number of full GC events.  
FGCT: Full GC总时长 Full garbage collection time.  
GCT: 总共的GC时长 Total garbage collection time.

记录GC日志

加入参数

-verbose:gc -Xloggc:$CATALINA_BASE/logs/gc.log XX:+PrintGCTimeStamps -XX:+PrintGCDetails

例如

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export JAVA_HOME=/opt/java/jdk1.8.0_101
    export CATALINA_HOME=/opt/tomcat/apache-tomcat-8.0.36-redis
    export CATALINA_BASE=/home/tomcat/tomcat8_jdk8
    export JAVA_OPTS="-server -Xms1280m -Xmx1280m -XX:MaxNewSize=896m -Djava.awt.hea
    dless=true -verbose:gc -Xloggc:$CATALINA_BASE/logs/gc.log -XX:+PrintGCTimeStamps
     -XX:+PrintGCDetails"
    cd $CATALINA_HOME/bin
    ./catalina.sh start

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如果jvm进程不明原因退出, 可以查看syslog, 位置是 /var/log/messages

**-XX:MaxTenuringThreshold**     垃圾最大年龄, 如果设置为0的话,则年轻代对象不经过Survivor区,直接进入年老代. 对于年老代比较多的应用,可以提高效率.如果将此值设置为一个较大值,则年轻代对象会在Survivor区进行多次复制,这样可以增加对象再年轻代的存活 时间,增加在年轻代即被回收的概率. 该参数只有在串行GC时才有效.

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