jvm培训教案

垃圾回收统计
jstat -gcutil $pid
    S0：幸存1区当前使用比例
    S1：幸存2区当前使用比例
    E：伊甸园区使用比例
    O：老年代使用比例
    M：元数据区使用比例
    CCS：压缩使用比例
    YGC：年轻代垃圾回收次数
    FGC：老年代垃圾回收次数
    FGCT：老年代垃圾回收消耗时间
    GCT：垃圾回收消耗总时间
    
 S0     S1     E      O      M     CCS    YGC     YGCT    FGC    FGCT     GCT

  0.00   0.00  10.00   0.00  17.20  19.76      0    0.000     0    0.000    0.00
0
    
查看堆空间
jmap -heap $pid
Attaching to process ID 4600, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.144-b01

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

Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 6442450944 (6144.0MB)
   NewSize                  = 4294967296 (4096.0MB)
   MaxNewSize               = 4294967296 (4096.0MB)
   OldSize                  = 2147483648 (2048.0MB)
   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 = 3221225472 (3072.0MB)
   used     = 322122752 (307.2001953125MB)
   free     = 2899102720 (2764.7998046875MB)
   10.000006357828775% used
From Space:
   capacity = 536870912 (512.0MB)
   used     = 0 (0.0MB)
   free     = 536870912 (512.0MB)
   0.0% used
To Space:
   capacity = 536870912 (512.0MB)
   used     = 0 (0.0MB)
   free     = 536870912 (512.0MB)
   0.0% used
PS Old Generation
   capacity = 2147483648 (2048.0MB)
   used     = 0 (0.0MB)
   free     = 2147483648 (2048.0MB)
   0.0% used

6290 interned Strings occupying 486400 bytes.


调优原则
调优不仅仅是要解决oom；而是要减少gc次数和时间，特别是full gc


什么情况下需要调优？
系统响应卡顿次数频繁、系统响应时间变长


gc日志分析

2017-10-19T15:47:24.236+0800: 76438.662:
 [GC (Allocation Failure) 2017-10-19T15:47:24.236+0800: 76438.662:
 [ParNew: 546188K->646K(613440K), 0.0083856 secs]
 648196K->102709K(3057152K),
 0.0084711 secs]
 [Times: user=0.04 sys=0.00, real=0.01 secs]