JVM调优问题记录2

在华为项目中，之前我用的是JDK默认的GC收集器，初始上线数据量不大时没有问题，后来修改为：-XX:+UseConcMarkSweepGC，当时把-Xmn这个新生代的内存设置去掉了，也没有在启动后查看其新生代内存大小，结果一天后就报错了，我的JVM 内存设置为：

-server -Xms4096m -Xmx16000m -XX:PermSize=256m -XX:MaxPermSize=512m -XX:+UseConcMarkSweepGC，

但是用jstat -heap pid 输出结果为：

Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize = 16777216000 (16000.0MB)
NewSize = 697892864 (665.5625MB)
MaxNewSize = 697892864 (665.5625MB)
OldSize = 1395851264 (1331.1875MB)
NewRatio = 2
SurvivorRatio = 8
PermSize = 268435456 (256.0MB)
MaxPermSize = 536870912 (512.0MB)
G1HeapRegionSize = 0 (0.0MB)


Heap Usage:
New Generation (Eden + 1 Survivor Space):
capacity = 628162560 (599.0625MB)
used = 347078104 (330.99947357177734MB)
free = 281084456 (268.06302642822266MB)
55.25291160300926% used
Eden Space:
capacity = 558432256 (532.5625MB)
used = 338771848 (323.07801055908203MB)
free = 219660408 (209.48448944091797MB)
60.664806583092506% used
From Space:
capacity = 69730304 (66.5MB)
used = 8306256 (7.9214630126953125MB)
free = 61424048 (58.57853698730469MB)
11.911974455180921% used
To Space:
capacity = 69730304 (66.5MB)
used = 0 (0.0MB)
free = 69730304 (66.5MB)
0.0% used
concurrent mark-sweep generation:
capacity = 7884152832 (7518.9140625MB)
used = 5756140000 (5489.482879638672MB)
free = 2128012832 (2029.4311828613281MB)
73.00898552647438% used
Perm Generation:
capacity = 268435456 (256.0MB)
used = 99056544 (94.46768188476562MB)
free = 169378912 (161.53231811523438MB)
36.90143823623657% used

其中new size这项明显跟NewRatio = 2的设置不相符，结过大量的测试发现，由于设置了-XX:+UseConcMarkSweepGC，不知什么原因使NewRatio = 2的默认设置不生效，所以在设置并发GC后，必须要设置：-Xmn这个新生代的设置。

之后设置完毕后，重启后new size的大小就正常了，经验啊。

系统环境：

Redhat 6.5 64位，jdk1.7_51 64位

硬件配置：8C 32G, 200G