尽管 (Max)MetaspaceSize，Java 8 仍为 Metaspace 保留最小 1G
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Java 8 reserves minimum 1G for Metaspace despite (Max)MetaspaceSize
提问by Eugene To
Java 8 reserves 1G for Metaspace just after it starts. It means that minimum metaspace size is 1G. But I set up MetaspaceSize to 300m and MaxMetaspaceSize to 400m. Why Java reserves more then I allow?
Java 8 刚启动就为 Metaspace 预留了 1G。这意味着最小元空间大小为 1G。但是我将 MetaspaceSize 设置为 300m，将 MaxMetaspaceSize 设置为 400m。为什么 Java 保留的比我允许的多？
Java Version
爪哇版
$ 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 Flags
虚拟机标志
$ 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
神经网络模型
[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)
And just after start it was
就在开始之后
Total: reserved=2402748KB, committed=150796KB
- Class (reserved=1056956KB, committed=7868KB)
(classes #1300)
(malloc=188KB #564)
(mmap: reserved=1056768KB, committed=7680KB)
采纳答案by Eugene To
The reason why Java reserves 1G for Classes hides in the way how it manages compressed class pointers.
Java 为类保留 1G 的原因隐藏在它如何管理压缩类指针的方式中。
The long answer: read this doc https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/considerations.html
长答案：阅读此文档https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/thinkations.html
The short answer: setup the correct size in 'CompressedClassSpaceSize' property -XX:CompressedClassSpaceSize=300m
简短的回答：在 'CompressedClassSpaceSize' 属性中设置正确的大小 -XX:CompressedClassSpaceSize=300m
回答by the8472
Class (reserved=1221904KB
类（保留=1221904KB
this isn't memory that's being used, just virtual address space
这不是正在使用的内存，只是虚拟地址空间
committed=197904KB
已提交=197904KB
That's 197MB, not 1GB
那是 197MB，而不是 1GB
Therefore you're not showing that java actually consumes 1GB of memory for class data, only that it reserves 1GB worth of address space.
因此，您并没有表明 java 实际上为类数据消耗了 1GB 的内存，只是表明它保留了 1GB 的地址空间。
