large size of [Ljdk.internal.vm.FillerElement in jmap histo

[shanghe chen]

Hi in a web service with openjdk 21.0.3 using G1GC with heap size 26GB, we recently sometimes observed a huge jump up of the oldgen like from 9GB to 20GB in about 3 minutes and then slowly returned to 9GB (which is the normal size) in about 30 minutes, checking jmap histo we found:

 num     #instances         #bytes  class name (module)
-------------------------------------------------------
   1:       6328181    16406573984  [Ljdk.internal.vm.FillerElement; (java.base at 21.0.3)
   2:      18920559     1233399640  [Ljava.lang.Object; (java.base at 21.0.3)
   3:      18813814      848534768  [B (java.base at 21.0.3)
   4:       5939383      771222984  [I (java.base at 21.0.3)
   5:       3093810      654462640  [J (java.base at 21.0.3)
   6:      15519103      496611296  java.util.HashMap$Node (java.base at 21.0.3)
   7:      17985808      431659392  java.util.Date (java.base at 21.0.3)
   8:      17415598      417974352  java.util.ArrayList (java.base at 21.0.3)
   9:      16857329      404575896  java.lang.String (java.base at 21.0.3)
  10:       9743073      389722920  java.math.BigDecimal (java.base at 21.0.3)
  11:       4410896      281417160  [Ljava.util.HashMap$Node; (java.base at 21.0.3)

Any idea for why the class jdk.internal.vm.FillerElement has so large size? There seems to be less document for this class. Thanks in advanced!

The jdk vendor we using is

openjdk version "21.0.3" 2024-04-16 LTS
OpenJDK Runtime Environment Temurin-21.0.3+9 (build 21.0.3+9-LTS)
OpenJDK 64-Bit Server VM Temurin-21.0.3+9 (build 21.0.3+9-LTS, mixed mode, sharing)

And the gc otpionwe using is

 -Xms26g -Xmx26g -Xss300k -XX:+UseG1GC -XX:MaxGCPauseMillis=200 -XX:ParallelGCThreads=16 -XX:ConcGCThreads=8 -XX:InitiatingHeapOccupancyPercent=30

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