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Thanks for the helpful infomation! Yeah I am trying to understand this. it seems after jdk21 the int[] is turned to FillerArray (or FillerElement?) and I am not sure if it count into the JMX bean for monitoring the Oldgen size of G1GC or not though.</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Aleksey Shipilev <shipilev@amazon.de><br>
<b>Sent:</b> Thursday, July 25, 2024 23:56<br>
<b>To:</b> shanghe chen <chenshanghe@outlook.com>; jdk-dev@openjdk.org <jdk-dev@openjdk.org><br>
<b>Subject:</b> Re: large size of [Ljdk.internal.vm.FillerElement in jmap histo</font>
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On 25.07.24 17:54, shanghe chen wrote:<br>
> Hi in a web service with openjdk 21.0.3 using G1GC with heap size 26GB, we recently sometimes
<br>
> observed a huge jump up of the oldgen like from 9GB to 20GB in about 3 minutes and then slowly
<br>
> returned to 9GB (which is the normal size) in about 30 minutes, checking jmap histo we found:<br>
> <br>
> num #instances #bytes class name (module)<br>
> -------------------------------------------------------<br>
> 1: 6328181 16406573984 [Ljdk.internal.vm.FillerElement; (java.base@21.0.3)<br>
> 2: 18920559 1233399640 [Ljava.lang.Object; (java.base@21.0.3)<br>
> 3: 18813814 848534768 [B (java.base@21.0.3)<br>
> 4: 5939383 771222984 [I (java.base@21.0.3)<br>
> 5: 3093810 654462640 [J (java.base@21.0.3)<br>
> 6: 15519103 496611296 java.util.HashMap$Node (java.base@21.0.3)<br>
> 7: 17985808 431659392 java.util.Date (java.base@21.0.3)<br>
> 8: 17415598 417974352 java.util.ArrayList (java.base@21.0.3)<br>
> 9: 16857329 404575896 java.lang.String (java.base@21.0.3)<br>
> 10: 9743073 389722920 java.math.BigDecimal (java.base@21.0.3)<br>
> 11: 4410896 281417160 [Ljava.util.HashMap$Node; (java.base@21.0.3)<br>
> <br>
> Any idea for why the class jdk.internal.vm.FillerElement has so large size? There seems to be less
<br>
> document for this class. Thanks in advanced!<br>
<br>
Those are filler arrays, they are not "real" reachable objects. See more here:<br>
<a href="https://shipilev.net/jvm/anatomy-quarks/5-tlabs-and-heap-parsability/">
https://shipilev.net/jvm/anatomy-quarks/5-tlabs-and-heap-parsability/</a><br>
<br>
-Aleksey<br>
<br>
<br>
<br>
Amazon Web Services Development Center Germany GmbH<br>
Krausenstr. 38<br>
10117 Berlin<br>
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss<br>
Eingetragen am Amtsgericht Charlottenburg unter HRB 257764 B<br>
Sitz: Berlin<br>
Ust-ID: DE 365 538 597<br>
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