为什么垃圾收集日志元空间值与提供的标志不同？

Question

我是运行 Java 8 上的服务器应用程序，具有以下标志：

-XX:GCLogFileSize=2097152 -XX:InitialBootClassLoaderMetaspaceSize=33554432 -XX:InitialHeapSize=1610612736 -XX:MaxHeapSize=1610612736 -XX:MaxMetaspaceSize=1073741824 -XX:MetaspaceSize=536870912 -XX:NumberOfGCLogFiles=5 -XX:+PrintGC -XX:+PrintGCApplicationConcurrentTime -XX:+PrintGCApplicationStoppedTime -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintHeapAtGC -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseGCLogFileRotation -XX:-UseLargePagesIndividualAllocation -XX:+UseParallelGC

元空间起始值为 512m，最大值为 1gb。

在 GC 日志中，我看到与上述标志值不同的元空间值。我的日志示例如下：

{Heap before GC invocations=1 (full 0):
 PSYoungGen      total 458752K, used 393216K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000)
  eden space 393216K, 100% used [0x00000000e0000000,0x00000000f8000000,0x00000000f8000000)
  from space 65536K, 0% used [0x00000000fc000000,0x00000000fc000000,0x0000000100000000)
  to   space 65536K, 0% used [0x00000000f8000000,0x00000000f8000000,0x00000000fc000000)
 ParOldGen       total 1048576K, used 0K [0x00000000a0000000, 0x00000000e0000000, 0x00000000e0000000)
  object space 1048576K, 0% used [0x00000000a0000000,0x00000000a00003a0,0x00000000e0000000)
 Metaspace       used 26975K, capacity 54120K, committed 54400K, reserved 1114112K
  class space    used 3346K, capacity 4426K, committed 4480K, reserved 1048576K
2016-12-21T16:41:10.569+0300: 1.423: [GC (GCLocker Initiated GC) [PSYoungGen: 393216K->29645K(458752K)] 393216K->29734K(1507328K), 0.0241803 secs] [Times: user=0.11 sys=0.00, real=0.02 secs] 
Heap after GC invocations=1 (full 0):
 PSYoungGen      total 458752K, used 29645K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000)
  eden space 393216K, 0% used [0x00000000e0000000,0x00000000e0000000,0x00000000f8000000)
  from space 65536K, 45% used [0x00000000f8000000,0x00000000f9cf3460,0x00000000fc000000)
  to   space 65536K, 0% used [0x00000000fc000000,0x00000000fc000000,0x0000000100000000)
 ParOldGen       total 1048576K, used 88K [0x00000000a0000000, 0x00000000e0000000, 0x00000000e0000000)
  object space 1048576K, 0% used [0x00000000a0000000,0x00000000a00163b0,0x00000000e0000000)
 Metaspace       used 26975K, capacity 54120K, committed 54400K, reserved 1114112K
  class space    used 3346K, capacity 4426K, committed 4480K, reserved 1048576K
}

所以我的问题是：为什么我在 GC 日志中看不到元空间的 512m 和 1gb 值？日志值是否与标志没有直接关系？

Answer 1

如 “Advanced Garbage Collection Options” 中所述：

-XX:MetaspaceSize=size

Sets the size of the allocated class metadata space that will trigger a garbage collection the first time it is exceeded. This threshold for a garbage collection is increased or decreased depending on the amount of metadata used. The default size depends on the platform.

换句话说，它没有指定初始大小，而是指定了第一次 gc 阈值，因此它符合您观察到的没有完整 gc 而实际元空间大小小于指定大小的行为。

一旦第一次超过阈值，JVM 可能会根据实际使用情况选择一个新的阈值。

为什么垃圾收集日志元空间值与提供的标志不同？

Why is garbage collection log Metaspace value different from provided flag?

java

garbage-collection

java-8

metaspace