Cassandra中的堆内存默认分配 [英] Heap Memory default allocation in Cassandra

问题描述

根据cassandra-env.sh，440G总RAM的默认堆内存分配应为32765M(JVM切换到64位引用之前的最大CAP).

As per cassandra-env.sh the default heap memory allocation for a 440G Total RAM should be 32765M (Maximum CAP before JVM Swithches to 64 bit reference).

所以，当我查询"java -XX:+ PrintCommandLineFlags -version"或"java -XX:+ PrintFlagsFinal -version | grep -iE'MaxHeapSize'"时，为什么显示32210157568字节(30718M)

So, why is it showing 32210157568 bytes(30718M) when I query "java -XX:+PrintCommandLineFlags -version" or "java -XX:+PrintFlagsFinal -version | grep -iE 'MaxHeapSize'"

为什么会有大约2G的差异.

Why is there difference, of around 2G.

仅供参考:jvm.options文件是默认&使用DSE 5.1.3.

推荐答案

java -XX:+PrintFlagsFinal与Cassandra无关，而且我不知道您为什么提到cassandra-env.sh.无论如何，让我回答问题的主要部分.

java -XX:+PrintFlagsFinal has nothing to do with Cassandra, and I don't know why you mention cassandra-env.sh. Anyway, let me answer the main part of the question.

在JDK 8中，当未指定-Xmx时，最大堆大小估计为

In JDK 8, when -Xmx is not specified, the maximum heap size is estimated as

MaxHeapSize = min(1/4 RAM, max_heap_for_compressed_oops)

在您的情况下，服务器具有大量的RAM，因此默认堆大小受基于零的压缩oop支持的最大可能大小(即32 GB)限制.

In your case the server has plently of RAM, so the default heap size is limited by the maximum possible size supported by zero-based compressed oops, that is, 32 GB.

堆显然不能从零地址开始(空页面由操作系统保留)，并且

The heap obviously cannot start at zero address (null page is reserved by the OS), and the default heap alignment is 2 MB, so we must subtract at least 2 MB.

然后，JDK倾向于在HeapBaseMinAddress处分配堆，该堆位于

Then, JDK prefers to allocate the heap at HeapBaseMinAddress, which is equal to 2 GB on Linux. This provides some space to grow the native heap of the process. For this reason JVM reduces the default maximum heap size by HeapBaseMinAddress.

这就是为什么最终计算出的堆大小等于

That's why the final computed heap size is equal to

32 GB - 2 MB - 2 GB = 32210157568

如果放弃对从零开始的压缩oop的要求，则可以设置-XX:HeapBaseMinAddress=0.在这种情况下，计算出的堆大小将为

If you give up the requirement for the zero-based compressed oops, you may set -XX:HeapBaseMinAddress=0. In this case the computed heap size would be

32 GB - 2MB = 32766 MB

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