英文:
JVM MaxHeapSize := 0 - what does this mean?
问题
我看到这里有其他类似的问题：https://stackoverflow.com/questions/4667483/how-is-the-default-max-java-heap-size-determined 和 https://stackoverflow.com/questions/26631498/understanding-java-heap
我运行了命令来查看我的堆大小 java -XX:+PrintFlagsFinal -version，我得到了MaxHeapSize的输出：
uintx MaxHeapSize := 0 {product}
这是什么意思？
英文:
I have seen other questions on here like: https://stackoverflow.com/questions/4667483/how-is-the-default-max-java-heap-size-determined and https://stackoverflow.com/questions/26631498/understanding-java-heap
I run the command to see my heap size java -XX:+PrintFlagsFinal -version and I get the output for MaxHeapSize:
uintx MaxHeapSize := 0 {product}
What does this mean?
答案1
得分: 6
这是JDK 8中的一个bug。
MaxHeapSize在HotSpot源代码中被定义为uintx，代表64位无符号整数。
在JDK 8中，打印uintx标志值的格式是"%-16lu"，它将输入视为unsigned long。
然而，C++中unsigned long的大小在Windows和Unix上有所不同：
- 大多数类Unix系统是LP64，其中
unsigned long的大小为64位。 - Windows上的Visual C++是LLP64，其中
unsigned long的大小为32位。
因此，JDK 8在Windows上仅打印uintx标志的低32位。这就是为什么如果MaxHeapSize恰好是4 GiB的整数倍，你会看到uintx MaxHeapSize := 0。这只是打印错误；实际的最大堆大小是正确的。
这个bug在JDK 9中作为JDK-8042893更改的一部分被修复了：
} else if (is_uintx()) {
- st->print("%-16lu", get_uintx());
+ st->print(UINTX_FORMAT_W(-16), get_uintx());
英文:
This is a bug in JDK 8.
MaxHeapSize is defined in HotSpot sources as uintx, which stands for 64-bit unsigned integer.
In JDK 8, the format for printing uintx flag values is "%-16lu", which treats input as unsigned long.
However, the size of C++ unsigned long differs on Windows and Unix:
- Most Unix-like systems are LP64, where the size of
unsigned longis 64 bit. - Visual C++ on Windows is LLP64, where the size of
unsigned longis 32 bit.
So, JDK 8 on Windows prints only low 32 bits of uintx flags. That's why if MaxHeapSize is an exact multiple of 4 GiB, you'll see uintx MaxHeapSize := 0. This is just the printing error; the actual max heap size is correct.
The bug was fixed in JDK 9 as a part of JDK-8042893 change:
} else if (is_uintx()) {
- st->print("%-16lu", get_uintx());
+ st->print(UINTX_FORMAT_W(-16), get_uintx());
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