Java HotSpot(TM) 64-Bit Server VM warning: NewSize (1536k) is greater than the MaxNewSize (1024k)

忘是亡心i 2022-10-07 01:56 19阅读 0赞

> 看<<实战java虚拟机>>书，运行一个demo，然后报了以下的错误提示。  
> Java HotSpot™ 64-Bit Server VM warning: NewSize (1536k) is greater than the MaxNewSize (1024k). A new max generation size of 1536k will be used.

测试代码为：

package cn.shutdown.demo.jvm;
    
    /**
     * -Xmx20m -Xms20m -Xmn1m -XX:SurvivorRatio=2 -XX:+PrintGCDetails
     * @author dmn
     * @date 2021/6/15
     */
    public class NewSizeDemo {
        public static void main(String[] args) {
            byte[] b = null;
            for (int i = 0; i < 10; i++) {
                b = new byte[1 * 1024 * 1024];
            }
        }
    }

运行结果为：

Java HotSpot(TM) 64-Bit Server VM warning: NewSize (1536k) is greater than the MaxNewSize (1024k). A new max generation size of 1536k will be used.
    [GC (Allocation Failure) [PSYoungGen: 512K->496K(1024K)] 512K->512K(19968K), 0.0015693 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
    Heap
     PSYoungGen      total 1024K, used 738K [0x00000007bfe80000, 0x00000007c0000000, 0x00000007c0000000)
      eden space 512K, 47% used [0x00000007bfe80000,0x00000007bfebc8d0,0x00000007bff00000)
      from space 512K, 96% used [0x00000007bff00000,0x00000007bff7c010,0x00000007bff80000)
      to   space 512K, 0% used [0x00000007bff80000,0x00000007bff80000,0x00000007c0000000)
     ParOldGen       total 18944K, used 10256K [0x00000007bec00000, 0x00000007bfe80000, 0x00000007bfe80000)
      object space 18944K, 54% used [0x00000007bec00000,0x00000007bf6040a0,0x00000007bfe80000)
     Metaspace       used 2700K, capacity 4486K, committed 4864K, reserved 1056768K
      class space    used 289K, capacity 386K, committed 512K, reserved 1048576K

对这个警告提示很好奇，于是搜了搜，在StackOverflow里找到一个非常靠谱的答案。  
[JDK7 JVM different from JDK8][]

## 说明如下 ##

> The observed behaviour is related to some changes between Java 7 and Java 8.

这个情况与jdk7和jdk8中的变更有关

-Xmn1024k

> this option sets the `initial` and the `maximum` size of the heap for the young generation. It’s a shorthand for `-XX:NewSize=1024k -XX:MaxNewSize=1024k`.

这个jvm参数项设置 青年代堆内存的初始和最大的空间。是 `-XX:NewSize=1024k -XX:MaxNewSize=1024k`的简化写法

// same command in Java 7/8
    java -XX:+PrintFlagsFinal -Xmn2048k -version | grep NewSize
    ...
    uintx MaxNewSize                               := 2097152
    uintx NewSize                                  := 2097152
    ...

> This behaviour seems to be not in line with the documentation.

这种行为似乎与文档不符。

[Java 7][]

> \-Xmnsize or -XX:NewSize Sets the size of the young generation (nursery).

[Java 8][]

> The -XX:NewSize option is equivalent to -Xmn.

> Because `-XX:NewSize=2048k` would set in Java7/8 only the initial size and the maximum size is chosen by the JVM.

因为 `-XX:NewSize=2048k` 会在 Java7/8 中设置只有初始大小，最大大小由 JVM 选择。

> Hence if you specify `-Xmn1024k` it actually means you want to set

因此，如果你指定`-Xmn1024k` 意味着你想要设置如下

-XX:NewSize=1024k -XX:MaxNewSize=1024k

> In Java 7 (tested with 1.7.0\_40-b43) the minimal size (for initial and maximum) is `196608`. If you set the value to low. The start of the JVM fails with

在Java7中，（初始化和最大值）的范围的最小值为 196608，如果设置的值比这个值小，JVM虚拟机启动就会报下面的错误而失败。

Error occurred during initialization of VM
    Too small new size specified

> In Java 8 (tested with 1.8.0\_74-b02) minimal size (for initial and maximum) is `1572864` (1536K).

在Java8中，（初始化和最大值）的范围的最小值为`1572864` (1536K).

> Now why you get this `NewSize (1536k) is greater than the MaxNewSize (1024k)` with Java 8.

所以java8中的，1536k是大于你设置的 -Xmn1m (-Xmn1024k) 的。

> If you choose `NewSize` lower than the minimal size (1536K) it’s adjusted silently to that minimal size.

如果你选择的`NewSize` 初始化的值小于取值范围的最低值，那会自动被设置为最低值(1536k)

java -XX:+PrintFlagsFinal -XX:NewSize=1k -version | grep NewSize
    ...
    uintx NewSize                                  := 1572864
    ...

> If you choose `MaxNewSize` lower than `NewSize` ，`MaxNewSize` will be set silently to `NewSize`

如果 `MaxNewSize`的值小于 `NewSize`， `MaxNewSize` 的值会被自动设置为`NewSize`的值。

java -XX:+PrintFlagsFinal -XX:NewSize=2048k -XX:MaxNewSize=1025k -version | \
        grep NewSize
    ...
    uintx MaxNewSize                               := 2097152
    uintx NewSize                                  := 2097152
    ...

> If you choose `NewSize` and `MaxNewSize` lower than minimal size and `MaxNewSize` bigger than `NewSize`, both are set to the minimal size.

`NewSize` 和`MaxNewSize` 的值如果都低于最小值(1536k)，则都会被设置为最小值(1536k)。

java -XX:+PrintFlagsFinal -XX:NewSize=1024k -XX:MaxNewSize=1025k -version | \
        grep NewSize
    ...
    uintx MaxNewSize                               := 1572864
    uintx NewSize                                  := 1572864
    ...

java -XX:+PrintFlagsFinal -XX:NewSize=1 -XX:MaxNewSize=1025k -version | \
        grep NewSize
    ...
    uintx MaxNewSize                               := 1572864
    uintx NewSize                                  := 1572864
    ...

> But as soon you choose for `MaxNewSize` a value lower than `1025k` equal to `1024k` you get the error message you have discovered. Seems you found an edge case where a check is done before the adjustment.

但是，一旦您为“MaxNewSize”选择一个低于“1025k”的值，即等于“1024k”，您就会收到您发现的错误消息。你的这种情况正好是在在需要调整的界限边缘了。

java -XX:NewSize=1 -XX:MaxNewSize=1025k -version
    java version "1.8.0_74"
    
    java -XX:NewSize=1 -XX:MaxNewSize=1024k -version
    Java HotSpot(TM) 64-Bit Server VM warning: NewSize (1536k) is greater than the \
    MaxNewSize (1024k). A new max generation size of 1536k will be used.

> That’s explains why `java -Xmn1025k -version` runs fine where as `java -Xmn1024k -version` gives you the error message.

这就是为什么执行 `java -Xmn1025k -version`正常跑，但是 `java -Xmn1024k -version` 报错了。

> After some more testing. The error seems to be printed only in case `MaxNewSize` is equal to `1024k`. The next examples are working with the default size for `NewSize` and `MaxNewSize`.

经过测试，这个错误只会在 `MaxNewSize`的值为 `1024k`的时候进行输出。

java -XX:NewSize=1 -XX:MaxNewSize=1023k -version
    java -XX:NewSize=1 -XX:MaxNewSize=1 -version

[JDK7 JVM different from JDK8]: https://stackoverflow.com/questions/45747910/jdk7-jvm-different-from-jdk8
[Java 7]: https://docs.oracle.com/javase/7/docs/technotes/tools/solaris/java.html
[Java 8]: https://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html