java堆分析工具jmap

jmap是比較地道本土的堆分析工具，應該掌握，下面講講如何運用。

// 打印jvm的堆狀況，主要是年輕代和老年代信息
jmap -heap <pid>
如：
Heap Configuration:
   MinHeapFreeRatio = 40
   MaxHeapFreeRatio = 70
   MaxHeapSize      = 536870912 (512.0MB)
   NewSize          = 1310720 (1.25MB)
   MaxNewSize       = 17592186044415 MB
   OldSize          = 5439488 (5.1875MB)
   NewRatio         = 2
   SurvivorRatio    = 8
   PermSize         = 21757952 (20.75MB)
   MaxPermSize      = 268435456 (256.0MB)
   G1HeapRegionSize = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 46661632 (44.5MB)
   used     = 1139464 (1.0866775512695312MB)
   free     = 45522168 (43.41332244873047MB)
   2.441972025324789% used
From Space:
   capacity = 524288 (0.5MB)
   used     = 0 (0.0MB)
   free     = 524288 (0.5MB)
   0.0% used
To Space:
   capacity = 1048576 (1.0MB)
   used     = 0 (0.0MB)
   free     = 1048576 (1.0MB)
   0.0% used
PS Old Generation
   capacity = 112721920 (107.5MB)
   used     = 54152304 (51.64366149902344MB)
   free     = 58569616 (55.85633850097656MB)
   48.04061534792878% used
PS Perm Generation
   capacity = 94371840 (90.0MB)
   used     = 94050296 (89.69335174560547MB)
   free     = 321544 (0.30664825439453125MB)
   99.65927971733942% used

37542 interned Strings occupying 3976528 bytes.