UseCompressedOops JVM 标志有什么作用，我应该什么时候使用它? [英] What does the UseCompressedOops JVM flag do and when should I use it?

问题描述

HotSpot JVM 标志 -XX:+UseCompressedOops 有什么作用?我应该什么时候使用它?在 64 位 Java 实例上使用它时(与不使用它相比)，我会看到什么样的性能和内存使用差异?

What does the HotSpot JVM flag -XX:+UseCompressedOops do and when should I use it? What sort of performance and memory-usage differences will I see when using it on a 64-bit Java instance (vs. not using it)?

推荐答案

去年的大多数 HotSpot JVM 都默认启用了它.此选项允许引用在 64 位 JVM 中为 32 位，并允许访问接近 32 GB 的堆.(超过 32 位指针可以)(您也可以拥有近乎无限的堆外内存).这可以节省大量内存并可能提高性能.

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.

如果你想使用这个选项，我建议你更新到一个默认启用它的版本，因为可能有一个很好的理由，比如错误，为什么以前没有启用它.尝试 Java 6 更新 23 或 Java 7 更新 5.

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.

简而言之，不要开启它，使用默认开启它的版本.

In short, don't turn it on, use a version which has it on by default.

更新:

在 Java 8 中，您可以选择设置 -XX:ObjectAlignmentInBytes=，事实上，如果堆大小为 64 GB，它将使用 -XX:ObjectAlignmentInBytes=16 并且仍然使用 32 位引用.

In Java 8 you have the option to set the -XX:ObjectAlignmentInBytes= and in fact if you heap size to 64 GB it will use -XX:ObjectAlignmentInBytes=16 and still use 32-bit references.

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