尽管（Max）MetaspaceSize，Java 8仍为元空间保留最小1G

Question

Java
8在启动后立即为Metaspace保留1G。这意味着最小的元空间大小为1G。但是我将MetaspaceSize设置为300m，将MaxMetaspaceSize设置为400m。为什么Java保留的资源超出我的允许？

Java版本

$ java -version
java version "1.8.0_45"
Java(TM) SE Runtime Environment (build 1.8.0_45-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.45-b02, mixed mode)

VM标志

$ jcmd 21689 VM.flags
21689:
-XX:CICompilerCount=3 -XX:ConcGCThreads=1 -XX:G1HeapRegionSize=1048576 -XX:InitialHeapSize=62914560 -XX:+ManagementServer -XX:MarkStackSize=4194304 -XX:MaxHeapSize=1006632960 -XX:MaxMetaspaceSize=399998976 -XX:MaxNewSize=603979776 -XX:MetaspaceSize=299999232 -XX:MinHeapDeltaBytes=1048576 -XX:NativeMemoryTracking=summary -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseFastUnorderedTimeStamps -XX:+UseG1GC

NMT

[jetty9-proxy@bm01 bin]$ jcmd 21689 VM.native_memory
21689:

Native Memory Tracking:

Total: reserved=2769543KB, committed=1311159KB

-                     Class (reserved=1221904KB, committed=197904KB)
                            (classes #36543)
                            (malloc=3344KB #44041) 
                            (mmap: reserved=1218560KB, committed=194560KB)

刚开始之后

Total: reserved=2402748KB, committed=150796KB

-                     Class (reserved=1056956KB, committed=7868KB)
                            (classes #1300)
                            (malloc=188KB #564) 
                            (mmap: reserved=1056768KB, committed=7680KB)

Answer 1

Java为类保留1G的原因在于它如何管理压缩的类指针。

长答案 ：请阅读此文档
https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/considerations.html

简短的答案 ：在’CompressedClassSpaceSize’属性-XX中设置正确的大小：CompressedClassSpaceSize
= 300m