jmap返回结果解释

先做几个名词解释：永久代(Perm Generation)、年轻代(Young Generation)、老年代(Old Generation)、Eden、Survivor
它们之间的关系可以用如下几个等式来表达：
永久代=方法区
年轻代+老年代=heap
Eden+2 Survivor=年轻代(一般有1个Eden和2个Survivor区域，分别叫From Space和To Space)
对象在这些区域之间的流转过程，参见 Eden&Survivor

贴上jmap返回的数据，如下

[root@l-zcgmid3 ~]# jps
15589 Bootstrap
32498 Jps
[root@l-zcgmid3 ~]# jmap -heap 15589
Attaching to process ID 15589, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 24.60-b09

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

Heap Configuration:
   MinHeapFreeRatio = 0
   MaxHeapFreeRatio = 100
   MaxHeapSize      = 3221225472 (3072.0MB)//最大可分配的Heap大小
   NewSize          = 1310720 (1.25MB)//JVM启动时分配的新生代内存
   MaxNewSize       = 17592186044415 MB//可分配的最大新生代内存。(为啥会这么大=-=?)
   OldSize          = 5439488 (5.1875MB)//JVM启动时分配的老年代内存大小
   NewRatio         = 2//老年代/新生代的堆内存比例，此值表示Heap中，新生代占比1/3
   SurvivorRatio    = 8//Eden和其中一个Survivor的比值,设置为8,则两个Survivor区与一个Eden区的比值为2:8,一个Survivor区占整个年轻代的1/10
   PermSize         = 536870912 (512.0MB)//类比以上，JVM启动时，永久代的大小
   MaxPermSize      = 536870912 (512.0MB)//最大可分配永久代大小。(为啥一启动就占了最大size？跟Heap中不大一样。。)
   G1HeapRegionSize = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 1063256064 (1014.0MB)
   used     = 670623592 (639.5564956665039MB)
   free     = 392632472 (374.4435043334961MB)
   63.07263270872819% used
From Space:
   capacity = 4718592 (4.5MB)
   used     = 3905896 (3.7249526977539062MB)
   free     = 812696 (0.7750473022460938MB)
   82.77672661675348% used
To Space:
   capacity = 5242880 (5.0MB)
   used     = 0 (0.0MB)
   free     = 5242880 (5.0MB)
   0.0% used
PS Old Generation
   capacity = 2147483648 (2048.0MB)
   used     = 1589377200 (1515.7482147216797MB)
   free     = 558106448 (532.2517852783203MB)
   74.01114329695702% used
PS Perm Generation
   capacity = 536870912 (512.0MB)
   used     = 61945112 (59.075462341308594MB)
   free     = 474925800 (452.9245376586914MB)
   11.538176238536835% used 

根据以上数据，我们来check一些ratio：
NewRatio = 2
Eden Space capacity = 1063256064 (1014.0MB)
From Space capacity = 4718592 (4.5MB)
To Space capacity = 5242880 (5.0MB)
简单加法，New size=1014.0+4.5+5.0=1023.5MB
MaxHeapSize = 3221225472 (3072.0MB)
年轻代占比：1023.5/3072.0=1/3。换算NewRatio的计算，老年代/新生代，结果为2

SurvivorRatio = 8
(这个比例的check存疑)
1014.0/5.0=200+?(跟8差距很远呐= =！)

今天的文章jmap参数解释分享到此就结束了，感谢您的阅读。