Metaspace分配的比MaxMetaspaceSize更多

Question

在Java8中，我在我的服务上运行GC日志，通过GCeasy，我看到1GB被分配给metaspace，而峰值在40m左右，那么为什么要分配1GB呢？

我玩旗子，加上"-XX:MaxMetaspaceSize=10M“。

然后，我得到了java.lang.OutOfMemoryError: Metaspace，正如预期的那样，但是仍然在GC日志上看到分配了1GB。日志里有错误吗？或者我漏掉了什么。

我在GC日志文件上运行grep Metaspace并获得以下输出：

-XX:MaxMetaspaceSize=10485760 -XX:MetaspaceSize=10485760 CommandLine旗子：-XX:CompressedClassSpaceSize=2097152 -XX:+DisableExplicitGC -XX:GCLogFileSize=104857600 -XX:InitialHeapSize=536870912 -XX:MaxHeapSize=536870912 -XX:NumberOfGCLogFiles=10 -XX:+PrintGC -XX:+PrintGCApplicationConcurrentTime -XX:+PrintGCApplicationStoppedTime -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:ReservedCodeCacheSize=134217728 -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX：+UseCompressedOops-XX:ReservedCodeCacheSize=134217728-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedClassPointers-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedClassPointers-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCDetails-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX：+PrintGC-XX：+PrintGCApplicationConcurrentTime-XX：+PrintGCApplicationStoppedTime-XX：+PrintGCDateStamps-XX：+PrintGCDetails-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedClassPointers-XX：+UseCompressedOops-XX：+UseCompressedOops-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedOops-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX：+PrintGCDetails-XX：+PrintGCTimeStamps-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedOops-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX：+PrintGCDetails-XX：+PrintGCTimeStamps-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedOops-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGC-XX：+PrintGCApplicationConcurrentTime-XX：+PrintGC-XX：+PrintGCApplicationConcurrentTime-XX：+PrintGCApplicationStoppedTime-XX：+PrintGCDateStamps-XX：+PrintGCDetails-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedClassPointers-XX：+UseCompressedOops-XX：+UseCompressedOops-XX：+PrintGCTimeStamps-XX：+PrintGC-XX：+PrintGCApplicationConcurrentTime-XX：+PrintGCApplicationStoppedTime-XX：+PrintGCDateStamps-XX：+PrintGCDetails-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedClassPointers-XX：+UseCompressedOops-XX：+UseCompressedOops-XX：+PrintGCTimeStamps-XX：+PrintGC-XX：+PrintGCApplicationConcurrentTime-XX：+PrintGC-XX：+PrintGCApplicationConcurrentTime-XX：+PrintGCApplicationStoppedTime-XX：+PrintGCDateStamps-XX：+PrintGCDetails-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedClassPointers-XX：+UseCompressedOops-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedOops-XX：+UseCompressedOops-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedClassPointers-XX：+UseCompressedOops-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedOops-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+PrintGCTimeStamps-XX:ReservedCodeCacheSize=134217728-XX：+UseCompressedClassPointers-XX：+UseCompressedOops

2020年-05-19T16:38:18.359+0000: 0.837：[全GC (元数据GC阈值) PSYoungGen: 7298 K->0K(153088K) 7306K->6944K(502784K)，Metaspace: 9975K->9975K(1058816K)，0.0249384秒] Times: user=0.08 sys=0.01，real=0.02 secs

2020年-05-19T16:38:18.386+0000: 0.864：[全GC (元数据GC阈值) PSYoungGen: 0K->0K(153088K) 6944K->6944K(502784K)，Metaspace: 9975K->9975K(1058816K)，0.0129850秒]Time: user=0.04 sys=0.00，real=0.01 secs

2020-05-19T16:38:18.400+0000: 0.879:完全GC (最后一条沟收集) [PSYoungGen: 0K->0K(153088K)] [ParOldGen: 6944K->6816K(3496K)] 6944K->6816K(502784K)，[Metaspace: 9975K->9965K(1058816K)]，0.0252318秒

Answer 1

答案基本上是here，尽管理解起来并不简单。

MaxMetaspaceSize限制提交的内存，而GC日志实际上打印保留的内存。所以保留：1058816K，提交：9975K，根据您的日志。保留内存根本不需要由交换或实际RAM支持，所以通常是非常大的。

这就是为什么你在GC log中看到这么大的数字。