Java heap Xms 和 linux free memory 不同
我有一个在 centos Box 中运行的 java 程序.我的 -Xmx 和 -Xms 设置为 4000 Mb.
I have a java program running in centos Box. My -Xmx and -Xms set to 4000 Mb.
程序运行良好.
但是当我执行 free -m 时，已用内存显示为 506 MB.根据我的理解，XMS内存应该是为JVM保留的.为什么free命令不显示java使用的内存?
But when i do free -m , the used memory is showing as 506 MB. As per my understanding , XMS memory should be reserved for JVM.Why does free command not showing the java used memory ?
我也完成了 jstat -gccapacity $(pidof java) 并且 NGCMN 和 NGCMX 更新并且具有相同的值?任何支持都会有所帮助.我正在运行我的程序 java -Xms41000m -Xmx42000m -jar
I have also done jstat -gccapacity $(pidof java) and there NGCMN and NGCMX updated and have the same value ? Any support would be helpful. I'm running my program as java -Xms41000m -Xmx42000m -jar
推荐答案
即使 -Xmx 和 -Xms 设置为相同的值，也会为 Java Heap 保留空间不会立即分配到 RAM 中.
Even when -Xmx and -Xms set to the same value, the space reserved for Java Heap is not immediately allocated in RAM.
操作系统通常会延迟分配物理内存，仅在第一次访问虚拟页面时.因此，虽然没有触及 Java Heap 的未使用部分，但它不会真正消耗内存.
Operating System typically allocates physical memory lazily, only on the first access to a virtual page. So, while unused part of Java Heap is not touched, it won't really consume memory.
你可以使用 -XX:+AlwaysPreTouch 选项在 JVM 启动时强制触摸所有堆页面.
You may use -XX:+AlwaysPreTouch option to forcibly touch all heap pages on JVM start.
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