jmap查询JVM堆内存

时间 2020-12-03

标签 java bootstrap less jvm 工具性能 google atom spa .net 栏目 Java 繁體版

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jmap命令能够获取运行中的jvm的快照,从而离线分析,检查内存泄漏,检查一些严重影响性能的大对象的建立,检查系统中最多的对象,各类对象所占用的内存大小.可使用jmap生成Heap Dump.

什么是堆Dump

堆Dump是反应Java堆使用状况的内存镜像,其中主要包含系统信息,虚拟机属性,完整的线程Dump,全部类和对象的状态等。通常，在内存不足,GC异常等状况下,咱们就会怀疑内存泄漏,这个时候就能够制做堆(Dump)来查询具体状况。

常见的内存错误

> outOfMemoryError 年老代内存不足。
> outOfMemoryError:PermGen Space 永久代内存不足。
> outOfMemoryError:GC overhead limit exceed 垃圾回收时间占用系统运行时间的98%或以上。

`jmap -heap pid`

查看java堆信息

Attaching to process ID 18378, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.261-b12

using thread-local object allocation.
Parallel GC with 4 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 0 # JVM最小空闲比率
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 4164943872 (3972.0MB)
   NewSize                  = 87031808 (83.0MB)
   MaxNewSize               = 1388314624 (1324.0MB)
   OldSize                  = 175112192 (167.0MB)
   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 = 1314914304 (1254.0MB)
   used     = 794405392 (757.6040191650391MB)
   free     = 520508912 (496.39598083496094MB)
   60.41499355383087% used
From Space:
   capacity = 36175872 (34.5MB)
   used     = 23758320 (22.657699584960938MB)
   free     = 12417552 (11.842300415039062MB)
   65.6744915506114% used
To Space:
   capacity = 37224448 (35.5MB)
   used     = 0 (0.0MB)
   free     = 37224448 (35.5MB)
   0.0% used
PS Old Generation
   capacity = 352321536 (336.0MB)
   used     = 206117376 (196.56884765625MB)
   free     = 146204160 (139.43115234375MB)
   58.502633231026785% used

使用了 Parallel GC垃圾收集器

MinHeapFreeRatio 参数用来设置堆空间最小空闲比例，默认值是 0。当堆空间的空闲内存小于这个数值时，JVM 便会扩展堆空间。
MaxHeapFreeRatio 参数用来设置堆空间最大空闲比例，默认值是 100。当堆空间的空闲内存大于这个数值时，便会压缩堆空间，获得一个较小的堆。

当-Xmx 和-Xms 相等时MinHeapFreeRatio和MaxHeapFreeRatio 两个参数无效。

MaxHeapSize 最大堆内存3972MB
NewSize 新生代默认大小 83MB
MaxNewSize 新生代最大大小为 1324MB
OldSize 老年代大小 167MB
NewRatio 新生代和老年代的大小比率 2
SurvivorRatio 年轻代中Eden和Survivor的比率 8
MetaspaceSize 元空间大小 20.796875MB
CompressedClassSpaceSize 若是开启了-XX:+UseCompressedOops及-XX:+UseCompressedClassesPointers(默认是开启)，则UseCompressedOops会使用32-bit的offset来表明java object的引用，而UseCompressedClassPointers则使用32-bit的offset来表明64-bit进程中的class pointer；可使用CompressedClassSpaceSize来设置这块的空间大小
MaxMetaspaceSize 最大元空间大小 1073741824
G1HeapRegionSize G1收集器启用,一个Region的大小能够经过参数-XX:G1HeapRegionSize设定，取值范围从1M到32M，且是2的指数。

`jmap pid`

查看进程的内存映像信息,相似 Solaris pmap 命令
使用不带选项参数的jmap打印共享对象映射，将会打印目标虚拟机中加载的每一个共享对象的起始地址、映射大小以及共享对象文件的路径全称。这与Solaris的pmap工具比较类似。java

[root@rumenz ~]# jmap 18378
Attaching to process ID 18378, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.261-b12
0x000056248b911000      8K      /usr/local/jdk1.8/bin/java
0x00007f35f8b2e000      487K    /usr/local/jdk1.8/jre/lib/amd64/libfontmanager.so
0x00007f35f8d91000      38K     /usr/local/jdk1.8/jre/lib/amd64/libawt_headless.so
0x00007f35f8f99000      741K    /usr/local/jdk1.8/jre/lib/amd64/libawt.so
0x00007f35f9669000      86K     /usr/lib64/libgcc_s-4.8.5-20150702.so.1
0x00007f35f987f000      276K    /usr/local/jdk1.8/jre/lib/amd64/libsunec.so
0x00007f36189ed000      110K    /usr/local/jdk1.8/jre/lib/amd64/libnet.so
0x00007f3618c04000      91K     /usr/local/jdk1.8/jre/lib/amd64/libnio.so
0x00007f361991f000      50K     /usr/local/jdk1.8/jre/lib/amd64/libmanagement.so
0x00007f363caee000      124K    /usr/local/jdk1.8/jre/lib/amd64/libzip.so
0x00007f363cd0a000      60K     /usr/lib64/libnss_files-2.17.so
0x00007f363cf1d000      226K    /usr/local/jdk1.8/jre/lib/amd64/libjava.so
0x00007f363d14c000      64K     /usr/local/jdk1.8/jre/lib/amd64/libverify.so
0x00007f363d35b000      42K     /usr/lib64/librt-2.17.so
0x00007f363d563000      1110K   /usr/lib64/libm-2.17.so
0x00007f363d865000      16698K  /usr/local/jdk1.8/jre/lib/amd64/server/libjvm.so
0x00007f363e85a000      2105K   /usr/lib64/libc-2.17.so
0x00007f363ec28000      18K     /usr/lib64/libdl-2.17.so
0x00007f363ee2c000      106K    /usr/local/jdk1.8/lib/amd64/jli/libjli.so
0x00007f363f044000      138K    /usr/lib64/libpthread-2.17.so
0x00007f363f260000      159K    /usr/lib64/ld-2.17.so

`jmap -histo:live pid`

堆中对象统计
其中包括每一个Java类、对象数量、内存大小(单位：字节)、彻底限定的类名。打印的虚拟机内部的类名称将会带有一个 *前缀。若是指定了live子选项，则只计算活动的对象。

[root@Server-i-b2l6uawirw ~]# jmap -histo:live 18378 | more

 num     #instances         #bytes  class name
----------------------------------------------
   1:        221097       19915168  [C
   2:        220601        5294424  java.lang.String
   3:         44932        3954016  java.lang.reflect.Method
   4:         67973        3769400  [Ljava.lang.Object;
   5:         98145        3140640  java.util.HashMap$Node
   6:        115945        2782680  java.util.concurrent.atomic.AtomicLong
   7:         19301        2126520  java.lang.Class
   8:         61752        1976064  java.util.concurrent.ConcurrentHashMap$Node
   9:         14120        1665080  [Ljava.util.HashMap$Node;
  10:          8997        1548592  [B
  11:         15452        1236160  com.google.common.cache.LocalCache$Segment
  12:         34289        1097248  java.util.Hashtable$Entry
  13:         37325         895800  java.util.ArrayList
  14:          2899         790728  [Z
  15:         49080         785280  java.lang.Object
  16:         19315         772600  com.google.common.cache.AbstractCache$SimpleStatsCounter
  17:         21096         675072  java.lang.ref.WeakReference
  18:         16237         649480  java.lang.ref.SoftReference
  19:         13368         641664  java.util.HashMap
  20:         15945         637800  java.util.LinkedHashMap$Entry
  21:           114         588392  [F
  22:         18238         583616  java.util.concurrent.locks.ReentrantLock$NonfairSync
  23:           486         533312  [Ljava.util.concurrent.ConcurrentHashMap$Node;
  24:         16521         528672  java.lang.ref.ReferenceQueue
  25:          6928         498816  java.lang.reflect.Field
  26:          3863         494464  com.google.common.cache.LocalCache
  27:          7680         491520  java.net.URL
  28:          5948         475840  java.lang.reflect.Constructor
  29:          8359         463928  [I

[B 表明 byte
[C 表明 char
[D 表明 double
[F 表明 float
[I 表明 int
[J 表明 long
[Z 表明 boolean

`jmap -clstats pid`

打印类加载信息
-clstats是-permstat的替代方案，在JDK8以前，-permstat用来打印类加载器的数据
打印Java堆内存的永久保存区域的类加载器的智能统计信息。对于每一个类加载器而言，它的名称、活跃度、地址、父类加载器、它所加载的类的数量和大小都会被打印。此外，包含的字符串数量和大小也会被打印。

[root@rumenz ~]# jmap -clstats 18378
Attaching to process ID 18378, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.261-b12
finding class loader instances ..
done.
computing per loader stat ..done.
please wait.. computing liveness.liveness analysis may be inaccurate ...
class_loader    classes bytes   parent_loader   alive?  type

<bootstrap>     3365    5881890   null          live    <internal>
0x00000006cb8abb88      1       880     0x00000006c7c2a4c8      dead    sun/reflect/DelegatingClassLoader@0x00000007c000a0a0
0x00000006cafd0af0      1       878     0x00000006c8520248      dead    sun/reflect/DelegatingClassLoader@0x00000007c000a0a0
0x00000006c8600870      0       0       0x00000006c82362b8      dead    hudson/ClassicPluginStrategy$DependencyClassLoader@0x00000007c0275308
0x00000006c86a9878      81      74744   0x00000006c86a98e8      dead    hudson/ClassicPluginStrategy$AntClassLoader2@0x00000007c0274dc8
0x00000006ca739a60      1       880     0x00000006c7c2a4c8      dead    sun/reflect/DelegatingClassLoader@0x00000007c000a0a0
0x00000006cafceae8      1       880     0x00000006c7c2a4c8      dead    sun/reflect/DelegatingClassLoader@0x00000007c000a0a0
0x00000006cb541b40      1       880     0x00000006c7c2a4c8      dead    sun/reflect/DelegatingClassLoader@0x00000007c000a0a0
0x00000006cb75fb68      1       880     0x00000006c86b09b8      dead    sun/reflect/DelegatingClassLoader@0x00000007c000a0a0
0x00000006ca5f9a40      1       1474      null          dead    sun/reflect/DelegatingClassLoader@0x00000007c000a0a0
0x00000006cb07fb18      1       880     0x00000006c82bcba8      dead    sun/reflect/DelegatingClassLoader@0x00000007c000a0a0
0x00000006cb083b28      1       1474    0x00000006c7c2a4c8      dead    sun/reflect/DelegatingClassLoader@0x00000007c000a0a0
0x00000006c86ad848      8       8716    0x00000006c86ad8b8      dead    hudson/ClassicPluginStrategy$AntClassLoader2@0x00000007c0274dc8
0x00000006c8bde898      49      155511  0x00000006c8237a80      dead    com/google/inject/internal/BytecodeGen$BridgeClassLoader@0x00000007c05f9028

`jmap -dump:format=b,file=dump.phrof pid`

生成堆转储快照文件
这个命令执行，JVM会将整个heap的信息dump写入到一个文件，heap若是比较大的话，就会致使这个过程比较耗时，而且执行的过程当中为了保证dump的信息是可靠的，因此会暂停应用，线上系统慎用。bootstrap

[root@rumenz ~]# jmap -dump:format=b,file=dump.hprof 18378
Dumping heap to /root/dump.hprof ...
Heap dump file created