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)?
hotspot JVM标记-XX:+UseCompressedOops做什么?我应该在什么时候使用它?当在64位Java实例上使用它时，我将看到什么样的性能和内存使用差异(而不是使用它)?
1 个解决方案
#1
72
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.
去年大多数热点JVM都默认打开了它。此选项允许在64位JVM中引用为32位，并访问接近32gb的堆。(32位以上的指针可以)(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。
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.
如果您想要使用这个选项，我建议您更新一个默认启用它的版本，因为可能有一个很好的原因，比如bug，为什么以前没有启用它。试试Java 6 update 23或Java 7 update 5。
In short, don't turn it on, use a version which has it on by default.
简而言之，不要打开它，使用默认打开的版本。
Update:
更新:
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.
在Java 8中，您可以设置-XX: objectalimentinbytes =，事实上，如果您将大小堆为64 GB，它将使用-XX:ObjectAlignmentInBytes=16，并且仍然使用32位引用。
#1
72
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.
去年大多数热点JVM都默认打开了它。此选项允许在64位JVM中引用为32位，并访问接近32gb的堆。(32位以上的指针可以)(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。
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.
如果您想要使用这个选项，我建议您更新一个默认启用它的版本，因为可能有一个很好的原因，比如bug，为什么以前没有启用它。试试Java 6 update 23或Java 7 update 5。
In short, don't turn it on, use a version which has it on by default.
简而言之，不要打开它，使用默认打开的版本。
Update:
更新:
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.
在Java 8中，您可以设置-XX: objectalimentinbytes =，事实上，如果您将大小堆为64 GB，它将使用-XX:ObjectAlignmentInBytes=16，并且仍然使用32位引用。