做了一个实验。Vm 参数是，-Xmx100m -Xms100m -Xmn20m 用的默认SurvivorRatio是没设置的，也就是8。然后我jmap -heap 得到的结果是：

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

Heap Configuration:
   MinHeapFreeRatio = 40
   MaxHeapFreeRatio = 70
   MaxHeapSize      = 104857600 (100.0MB)
   NewSize          = 20971520 (20.0MB)
   MaxNewSize       = 20971520 (20.0MB)
   OldSize          = 5439488 (5.1875MB)
   NewRatio         = 2
   SurvivorRatio    = 8
   PermSize         = 21757952 (20.75MB)
   MaxPermSize      = 85983232 (82.0MB)
   G1HeapRegionSize = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 15728640 (15.0MB)
   used     = 4357336 (4.155479431152344MB)
   free     = 11371304 (10.844520568847656MB)
   27.703196207682293% used
From Space:
   capacity = 2621440 (2.5MB)
   used     = 2588848 (2.4689178466796875MB)
   free     = 32592 (0.0310821533203125MB)
   98.7567138671875% used
To Space:
   capacity = 2621440 (2.5MB)
   used     = 0 (0.0MB)
   free     = 2621440 (2.5MB)
   0.0% used
PS Old Generation
   capacity = 83886080 (80.0MB)
   used     = 13364112 (12.745010375976562MB)
   free     = 70521968 (67.25498962402344MB)
   15.931262969970703% used
PS Perm Generation
   capacity = 23068672 (22.0MB)
   used     = 23043336 (21.97583770751953MB)
   free     = 25336 (0.02416229248046875MB)
   99.89017139781605% used

10662 interned Strings occupying 920664 bytes.

注意看两个survivior都是2.5M

但是我手动设置一下SurvivorRatio=8。也就是vm设置成 -Xmx100m -Xms100m -Xmn20m -XX:SurvivorRatio=8

得到的结果是

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

Heap Configuration:
   MinHeapFreeRatio = 40
   MaxHeapFreeRatio = 70
   MaxHeapSize      = 104857600 (100.0MB)
   NewSize          = 20971520 (20.0MB)
   MaxNewSize       = 20971520 (20.0MB)
   OldSize          = 5439488 (5.1875MB)
   NewRatio         = 2
   SurvivorRatio    = 8
   PermSize         = 21757952 (20.75MB)
   MaxPermSize      = 85983232 (82.0MB)
   G1HeapRegionSize = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 16777216 (16.0MB)
   used     = 689544 (0.6576004028320312MB)
   free     = 16087672 (15.342399597167969MB)
   4.110002517700195% used
From Space:
   capacity = 2097152 (2.0MB)
   used     = 2089152 (1.99237060546875MB)
   free     = 8000 (0.00762939453125MB)
   99.6185302734375% used
To Space:
   capacity = 2097152 (2.0MB)
   used     = 0 (0.0MB)
   free     = 2097152 (2.0MB)
   0.0% used
PS Old Generation
   capacity = 83886080 (80.0MB)
   used     = 14887688 (14.198005676269531MB)
   free     = 68998392 (65.80199432373047MB)
   17.747507095336914% used
PS Perm Generation
   capacity = 23068672 (22.0MB)
   used     = 23043440 (21.975936889648438MB)
   free     = 25232 (0.0240631103515625MB)
   99.89062222567472% used

10647 interned Strings occupying 919560 bytes.

注意看两个survivior确是2.0M
很明显第二个才应该是对的。。那么问题来了。为什么默认是8的情况下，得到的结果确实2.5呢？

操作系统是win7
JDK是:
java version "1.7.0_51"
Java(TM) SE Runtime Environment (build 1.7.0_51-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.51-b03, mixed mode)

R大已经解释了。 位置在：http://hllvm.group.iteye.com/group/topic/35468