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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 Version

$ 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

$ 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)

And just after start it was

Total: reserved=2402748KB, committed=150796KB     

-                     Class (reserved=1056956KB, committed=7868KB)
                            (classes #1300)
                            (malloc=188KB #564) 
                            (mmap: reserved=1056768KB, committed=7680KB)
Eugene To
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2 Answers2

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The reason why Java reserves 1G for Classes hides in the way how it manages compressed class pointers.

The long answer: read this doc https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/considerations.html

The short answer: setup the correct size in 'CompressedClassSpaceSize' property -XX:CompressedClassSpaceSize=300m

Eugene To
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    I also liked this detailed explanation about why CompressedClassSpaceSize is a hard limit: https://stuefe.de/posts/metaspace/what-is-compressed-class-space/ – Andrei Damian-Fekete Jan 02 '20 at 11:14
  • BEWARE ... this answer might be wrong. If you are using a limited and small metaspace and the value of the metaspace is smaller than `XX:CompressedClassSpaceSize` then that option might be ignored and JVM might use the formula `CompressedClassSpaceSize = MaxMetaspaceSize – 2 * InitialBootClassLoaderMetaspaceSize` – Rafael Dec 01 '21 at 12:41
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Class (reserved=1221904KB

this isn't memory that's being used, just virtual address space

committed=197904KB

That's 197MB, not 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.

the8472
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    Yes, you are right. That is what I meant. Sorry, English is not my native language. I'll edit the topic. – Eugene To Jun 26 '15 at 15:04
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    But It should not reserve so much memory because I set up MetaspaceSize=300m and MaxMetaspaceSize=400m. Unpredictable reservation is bad. I have other JMV processes on that VM and I can't start them because there is no free memory. – Eugene To Jun 26 '15 at 15:07
  • you can just turn on overcommit to allow it to reserve that much space. – the8472 Jun 26 '15 at 17:31
  • This is a possible workaround. But I want to understand why java reserves 1G for Classes in the first place. – Eugene To Jun 26 '15 at 17:41
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    I think that may have something to do with it needing a contiguous block of memory for compressed class space, which you are using. There's the `CompressedClassSpaceSize` option. – the8472 Jun 26 '15 at 17:43