问题遇到的现象和发生背景
最近在尝试修改jvm内存大小。
首先我根据博客Runtime.getRunTime.maxMemory为啥比Xmx指定的内存小提供的代码,使用idea调用Runtime库中的函数查看当前jvm内存。
代码如下:
import java.lang.management.ManagementFactory;
import java.lang.management.MemoryMXBean;
import java.lang.management.MemoryPoolMXBean;
public class MemoryDemo {
static String mb(long s) {
return String.format("%d (%.2f M)", s, (double) s / (1024 * 1024));
}
public static void main(String[] args) {
System.out.println("Runtime max: " + mb(Runtime.getRuntime().maxMemory()));
MemoryMXBean m = ManagementFactory.getMemoryMXBean();
System.out.println("Non-heap: " + mb(m.getNonHeapMemoryUsage().getMax()));
System.out.println("Heap: " + mb(m.getHeapMemoryUsage().getMax()));
for (MemoryPoolMXBean mp : ManagementFactory.getMemoryPoolMXBeans()) {
System.out.println("Pool: " + mp.getName() + " (type " + mp.getType() + ")" + " = " + mb(mp.getUsage().getMax()));
}
}
}
结果如下:
Runtime max: 3722444800 (3550.00 M)
Non-heap: -1 (-0.00 M)
Heap: 3722444800 (3550.00 M)
Pool: CodeHeap 'non-nmethods' (type Non-heap memory) = 7602176 (7.25 M)
Pool: Metaspace (type Non-heap memory) = -1 (-0.00 M)
Pool: CodeHeap 'profiled nmethods' (type Non-heap memory) = 122028032 (116.38 M)
Pool: Compressed Class Space (type Non-heap memory) = 1073741824 (1024.00 M)
Pool: G1 Eden Space (type Heap memory) = -1 (-0.00 M)
Pool: G1 Old Gen (type Heap memory) = 3722444800 (3550.00 M)
Pool: G1 Survivor Space (type Heap memory) = -1 (-0.00 M)
Pool: CodeHeap 'non-profiled nmethods' (type Non-heap memory) = 122028032 (116.38 M)
根据上述结果我了解到运行内存大小为3550.00 M。
然后我就想更改idea的jvm默认内存,查看这个大小是否可以改变。于是我又参考了另外一篇博客IDEA 开启内存显示与修改内存。首先打开了内存显示,发现内存显示最大为1774MB,接着打开了idea64.exe.vmoptions文件,其中的设置也是1774MB。如下图所示:
然后我修改了vmoptions中的-Xmx参数,设为4096MB并重启了idea,内存显示器的改变了。如下图所示:
但是上述代码的运行结果仍然是3550MB。
我的问题
为什么运行结果和配置结果不一样?
为什么修改后配置后代码的运行结果仍然是3550MB?
那么运行时可用的内存实际是多大?
操作环境、软件版本等信息
目前idea的版本是:IntelliJ IDEA 2021.2.3 (Community Edition)
java版本是:openjdk version "11.0.11" 2021-04-20