JVM 参数 -XX:MetaspaceSize 解释错误 #1947

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1. Snailclimb commented on Mar 12, 2023

   

   Snailclimb

   on Mar 12, 2023

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   参见 Oracle 官方文档 Other Considerations 中提到：

   When the space committed for class metadata reaches a certain level (a high-water mark), a garbage collection is induced.

   The high-water mark is initially set to the value of the command-line option MetaspaceSize.

   可见 -XX:MetaspaceSize 是一个提前设定的阈值，会触发 GC，那一定不是初始大小。

   感谢指出，已经修正。

   1、Metaspace 的初始容量并不是 -XX:MetaspaceSize 设置，无论 -XX:MetaspaceSize 配置什么值，对于 64 位 JVM 来说，Metaspace 的初始容量都是 21807104（约 20.8m）。

   可以参考 Oracle 官方文档 Other Considerations中提到的：

   Specify a higher value for the option MetaspaceSize to avoid early garbage collections induced for class metadata. The amount of class metadata allocated for an application is application-dependent and general guidelines do not exist for the selection of MetaspaceSize. The default size of MetaspaceSize is platform-dependent and ranges from 12 MB to about 20 MB.

   MetaspaceSize 的默认大小取决于平台，范围从 12 MB 到大约 20 MB。

   另外，还可以看一下这个试验：JVM 参数 MetaspaceSize 的误解。

   2、Metaspace 由于使用不断扩容到-XX:MetaspaceSize参数指定的量，就会发生 FGC，且之后每次 Metaspace 扩容都会发生 Full GC。

   也就是说，MetaspaceSize 表示 Metaspace 使用过程中触发 Full GC 的阈值，只对触发起作用。

   垃圾搜集器内部是根据变量 _capacity_until_GC来判断 Metaspace 区域是否达到阈值的，初始化代码如下所示：

   void MetaspaceGC::initialize() {
     // Set the high-water mark to MaxMetapaceSize during VM initializaton since
     // we can't do a GC during initialization.
     _capacity_until_GC = MaxMetaspaceSize;
   }

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2. Snailclimb

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   on Mar 12, 2023
3. Snailclimb commented on Aug 24, 2026

   

   Snailclimb

   on Aug 24, 2026

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   感谢反馈！这个问题已经在当前文档中修复：docs/java/jvm/jvm-parameters-intro.md 已补充修正说明：-XX:MetaspaceSize 是触发元数据 GC 的高水位阈值，并非初始容量。 如果觉得仍有问题，欢迎重新打开或新开 issue 说明。
4. Snailclimb

   closed this as completedon Aug 24, 2026