部分JVM参数解释

从大神手里拿过来的JVM调优参数，仅仅是把每个参数的含义查询了一下，越是查询越是感觉得到，小用户量的产品优化无从谈起，用户量起来了一切就要从细节入手慢慢优化。知识还需要继续深化，自己知道的往往是浮在表面的东西。

堆=年轻代(new)+年老代(old)

年轻代 = Eden 和 Survivor*2

-Xmx2048m 最大堆大小
-Xms2048m 初始堆大小
-Xmn1024m 年轻代大小
-Xss1m 每个线程的堆栈大小
-XX:PermSize=256m 初始持久代大小
-XX:MaxPermSize=256m 最大持久代大小
-XX:+UseParNewGC 设置年轻代为并行收集
-XX:+UseConcMarkSweepGC 设置年老代为并发收集
-XX:+CMSParallelRemarkEnabled 降低标记停顿
-XX:SurvivorRatio=8 eden/survivor的比值
-XX:MaxTenuringThreshold=15 这个参数用于控制对象能经历多少次Minor GC才晋升到旧生代
-XX:CMSInitiatingOccupancyFraction=75 CMS堆上， 使用75％后开始CMS收集。
-XX:+UseCMSInitiatingOccupancyOnly 仅仅使用手动定义初始化定义开始CMS收集
-XX:+UseCompressedOops
Most HotSpot JVM in the last year have had it on by default. This option allows references to be 32-bit in a 64-bit JVM and access close to 32 GB of heap. (more than 32-bit pointers can) (You can have near unlimited off heap memory as well). This can save a significant amount of memory and potentially improve performance.
If you want to use this option I suggest you update to a version which has it on by default as there may have been a good reason, such as bugs, why it wasn't enabled previously. Try Java 6 update 23 or Java 7 update 5.
In short, don't turn it on, use a version which has it on by default.

-XX:+PrintGCDetails 打印较为详细的GC信息
-XX:+PrintGCDateStamps 打印GC时间戳，相对于应用程序启动的时间
-XX:+PrintClassHistogram 打印类的直方图 ，用kill -3 pid 会打印
-Xloggc:log/gc.log 打印GC信息到指定文件 可以设置绝对路径的。