java - 为什么垃圾收集日志元空间值与提供的标志不同？

Question

我正在使用以下标志在 Java 8 上运行服务器应用程序:

-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。