问题
I am running a server app on Java 8 with the below flags:
-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
The metaspace start value is 512m and max value is 1gb.
In the GC logs I see different metaspace values than the above flag values. My log sample is below:
{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
}
So my question is: why don't I see the 512m and 1gb values for metaspace in the GC logs? Are the log values not directly related to flags?
回答1:
As explained in “Advanced Garbage Collection Options”:
- -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.
In other words, it doesn’t specify an initial size, but a first time gc threshold, so it matches your observed behavior of not having a full gc while the actual metaspace size is smaller than the specified size.
Once the threshold has been exceeded the first time, the JVM may choose a new threshold, based on the actual usage.
来源：https://stackoverflow.com/questions/41264844/why-is-garbage-collection-log-metaspace-value-different-from-provided-flag