[转帖]JVM jmap命令的用法小结

2020-05-17 17:54:15 阅读：288 来源： 互联网

标签：java JVM dump MB 转帖 live heap jmap

JVM jmap命令的用法小结

https://www.jianshu.com/p/19ba425cc8f8

jmap

-heap

-dump

-histo

jmap

之前已经总结过了jstack命令的用法。今天时间不太够了，没法写长篇大论，来简单看看jmap命令吧。

jmap是JVM自带的堆内存转储（heap dump）生成工具，可以用来分析某JVM进程的堆内存占用，以及所有对象的概况。其用法说明如下所示。

~ jmap -help
Usage:
    jmap [option] <pid>
        (to connect to running process)
    jmap [option] <executable <core>
        (to connect to a core file)
    jmap [option] [server_id@]<remote server IP or hostname>
        (to connect to remote debug server)

where <option> is one of:
    <none>               to print same info as Solaris pmap
    -heap                to print java heap summary
    -histo[:live]        to print histogram of java object heap; if the "live"
                         suboption is specified, only count live objects
    -clstats             to print class loader statistics
    -finalizerinfo       to print information on objects awaiting finalization
    -dump:<dump-options> to dump java heap in hprof binary format
                         dump-options:
                           live         dump only live objects; if not specified,
                                        all objects in the heap are dumped.
                           format=b     binary format
                           file=<file>  dump heap to <file>
                         Example: jmap -dump:live,format=b,file=heap.bin <pid>
    -F                   force. Use with -dump:<dump-options> <pid> or -histo
                         to force a heap dump or histogram when <pid> does not
                         respond. The "live" suboption is not supported
                         in this mode.
    -h | -help           to print this help message
    -J<flag>             to pass <flag> directly to the runtime system

不再逐字解释了，直接上示例。

-heap：打印堆配置信息和使用概况

~ jmap -heap 3065
Attaching to process ID 3065, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.172-b11

using parallel threads in the new generation.
using thread-local object allocation.
Concurrent Mark-Sweep GC

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 1073741824 (1024.0MB)
   NewSize                  = 357892096 (341.3125MB)
   MaxNewSize               = 357892096 (341.3125MB)
   OldSize                  = 715849728 (682.6875MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
New Generation (Eden + 1 Survivor Space):
   capacity = 322109440 (307.1875MB)
   used     = 151490152 (144.4722671508789MB)
   free     = 170619288 (162.7152328491211MB)
   47.03064647841429% used
Eden Space:
   capacity = 286326784 (273.0625MB)
   used     = 147538816 (140.7039794921875MB)
   free     = 138787968 (132.3585205078125MB)
   51.52812249656672% used
From Space:
   capacity = 35782656 (34.125MB)
   used     = 3951336 (3.7682876586914062MB)
   free     = 31831320 (30.356712341308594MB)
   11.042601197630495% used
To Space:
   capacity = 35782656 (34.125MB)
   used     = 0 (0.0MB)
   free     = 35782656 (34.125MB)
   0.0% used
concurrent mark-sweep generation:
   capacity = 715849728 (682.6875MB)
   used     = 324490944 (309.45867919921875MB)
   free     = 391358784 (373.22882080078125MB)
   45.32947786494095% used

16612 interned Strings occupying 1535376 bytes.

在Heap Configuration一节，会输出与堆相关的JVM参数。在Heap Usage一节，会输出新生代（Eden区、Survivor From/To区）和老生代的容量、用量和空闲量。除此之外，还会输出采用的垃圾收集器，以及字符串常量池中驻留的字符串数量、大小等。关于字符串常量池，可以参见很久很久之前写的这篇文章。

-histo：生成类的实例统计直方图

~ jmap -F -histo 8766 > histo_8766.txt
Iterating over heap. This may take a while...
Heap traversal took 102.943 seconds.

~ head -30 histo_8766.txt 
Attaching to process ID 8766, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.172-b11
Object Histogram:

num       #instances    #bytes  Class description
--------------------------------------------------------------------------
1:      68784   312237944   byte[]
2:      306815  29853976    char[]
3:      32350   8154032 int[]
4:      90110   7610008 java.lang.Object[]
5:      250755  6018120 java.lang.String
6:      19090   5345200 org.apache.spark.status.TaskDataWrapper
7:      111298  3561536 java.util.HashMap$Node
8:      79859   2555488 java.util.concurrent.ConcurrentHashMap$Node
9:      102353  2456472 scala.collection.immutable.$colon$colon
10:     18024   1996520 java.lang.Class
11:     77193   1852632 java.lang.Long
12:     6853    1385888 java.util.HashMap$Node[]
13:     53685   1288440 java.util.concurrent.ConcurrentSkipListMap$Node
14:     35352   1131264 com.codahale.metrics.WeightedSnapshot$WeightedSample
15:     24500   980000  org.apache.spark.streaming.kafka010.OffsetRange
16:     39209   941016  scala.Tuple2
17:     28115   899680  scala.collection.mutable.ListBuffer
18:     3269    875352  boolean[]
19:     35097   842328  java.lang.Double
20:     1248    818688  io.netty.util.internal.shaded.org.jctools.queues.MpscArrayQueue
21:     349 757232  java.util.concurrent.ConcurrentHashMap$Node[]
22:     6330    658320  java.io.ObjectStreamClass

打印的结果有实例数、占用内存总大小和类的全限定名，并按占用内存降序排序。如果在-histo后面加上:live开关的话，表示只统计存活的对象，即在统计之前会触发一次Full GC。

特别注意，遍历堆并生成直方图的过程中，目标JVM都是stop-the-world的，所以对于较大的堆或者生产环境上的程序，要谨慎执行。如果目标JVM无响应，就加上-F参数强制执行之（同jstack），此时:live开关无效化。

-dump：生成堆转储快照文件

~ jmap -dump:live,format=b,file=dump_8766.hprof 8766
Dumping heap to /root/dump_8766.hprof ...
Heap dump file created

生成的二进制快照文件可以使用jhat、MAT、VisualVM等带有分析heap dump功能的工具查看详情，比如通过保留大小（retained size）指标来观察有哪些对象在引用大对象。

:live开关和-F参数的功能与-histo选项下相同，并且生成快照文件的过程同样会stop-the-world。

-finalizerinfo：输出等待finalize的对象数

~ jmap -finalizerinfo 3065
Attaching to process ID 3065, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.172-b11
Number of objects pending for finalization: 0

关于finalize和F-Queue的细节，应该另外写篇文章专门说说。今天就酱紫吧，民那晚安。

标签：java,JVM,dump,MB,转帖,live,heap,jmap
来源： https://www.cnblogs.com/jinanxiaolaohu/p/12906066.html

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