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In Java 8, I run GC log on my service and via GCeasy I saw that 1GB is allocated to the metaspace, while the peak was around 40m, so why 1 GB was allocated?

I play with the flags and added "-XX:MaxMetaspaceSize=10M".

Then I got java.lang.OutOfMemoryError: Metaspace, as expected, but still on the GC log I saw that 1 GB was allocated. Is there a bug in the logs? or I'm missing something.

I run grep Metaspace on the GC log file and got the below output:

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

2020-05-19T16:38:18.359+0000: 0.837: [Full GC (Metadata GC Threshold) [PSYoungGen: 7298K->0K(153088K)] [ParOldGen: 8K->6944K(349696K)] 7306K->6944K(502784K), [Metaspace: 9975K->9975K(1058816K)], 0.0249384 secs] [Times: user=0.08 sys=0.01, real=0.02 secs]

2020-05-19T16:38:18.386+0000: 0.864: [Full GC (Metadata GC Threshold) [PSYoungGen: 0K->0K(153088K)] [ParOldGen: 6944K->6944K(349696K)] 6944K->6944K(502784K), [Metaspace: 9975K->9975K(1058816K)], 0.0129850 secs] [Times: user=0.04 sys=0.00, real=0.01 secs]

2020-05-19T16:38:18.400+0000: 0.879: [Full GC (Last ditch collection) [PSYoungGen: 0K->0K(153088K)] [ParOldGen: 6944K->6816K(349696K)] 6944K->6816K(502784K), [Metaspace: 9975K->9965K(1058816K)], 0.0252318 secs] [Times: user=0.07 sys=0.00, real=0.03 secs]

moriya
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  • What is your _exact_ question here? Why there is 1GB, what does 40 MB have to do with it and where do you even see those 40? I am confused – Eugene May 19 '20 at 23:56
  • Maybe I supply too many details, my question is on the 1 GB allocated. Even after I set "-XX:MaxMetaspaceSize=10M", I saw in the GC log: [Metaspace: 9975K->9975K(**1058816K**)], 0.0249384 secs]. (1058816k) - this means 1GB is the capacity of the Metaspace and I don't understand this – moriya May 20 '20 at 05:46

1 Answers1

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The answer is basically here, though not trivial to understand.

The MaxMetaspaceSize limits the committed memory, while your GC logs actually print the reserved memory. So reserved: 1058816K, committed: 9975K, according to your log. Reserved memory does not have to be backed by swap or actual RAM, at all, so usually it is very big.

That is why you see those big numbers in the GC log.

Eugene
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