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jmap语法格式如下：
- jmap [option] pid
- jmap [option] executable core
- jmap [option] [server-id@]remote-hostname-or-ip
如果运行在64位JVM上，可能需要指定-J-d64命令选项参数。
- jmap -permstat pid
打印进程的类加载器和类加载器加载的持久代对象信息，输出：类加载器名称、对象是否存活(不可靠)、对象地址、父类加载器、已加载的类大小等信息，如下图：
使用jmap -heap pid查看进程堆内存使用情况，包括使用的GC算法、堆配置参数和各代中堆内存使用情况。比如下面的例子：
- root@ubuntu:/# jmap -heap 21711
- Attaching to process ID 21711, please wait...
- Debugger attached successfully.
- Server compiler detected.
- JVM version is 20.10-b01
-
- using thread-local object allocation.
- Parallel GC with 4 thread(s)
-
- Heap Configuration:
- MinHeapFreeRatio = 40
- MaxHeapFreeRatio = 70
- MaxHeapSize = 2067791872 (1972.0MB)
- NewSize = 1310720 (1.25MB)
- MaxNewSize = 17592186044415 MB
- OldSize = 5439488 (5.1875MB)
- NewRatio = 2
- SurvivorRatio = 8
- PermSize = 21757952 (20.75MB)
- MaxPermSize = 85983232 (82.0MB)
-
- Heap Usage:
- PS Young Generation
- Eden Space:
- capacity = 6422528 (6.125MB)
- used = 5445552 (5.1932830810546875MB)
- free = 976976 (0.9317169189453125MB)
- 84.78829520089286% used
- From Space:
- capacity = 131072 (0.125MB)
- used = 98304 (0.09375MB)
- free = 32768 (0.03125MB)
- 75.0% used
- To Space:
- capacity = 131072 (0.125MB)
- used = 0 (0.0MB)
- free = 131072 (0.125MB)
- 0.0% used
- PS Old Generation
- capacity = 35258368 (33.625MB)
- used = 4119544 (3.9287033081054688MB)
- free = 31138824 (29.69629669189453MB)
- 11.683876009235595% used
- PS Perm Generation
- capacity = 52428800 (50.0MB)
- used = 26075168 (24.867218017578125MB)
- free = 26353632 (25.132781982421875MB)
- 49.73443603515625% used
- ....
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