Weird JVM Heap Usage

(Sorry if this is not directly related to ES. But I don't know where to ask)

my environments

  • ES Version: 2.3.2
  • GC: CMS gc
  • Heap Size: 20GB
  • Total Ram: 64GB
  • Java Version: 1.8.0_60 Java HotSpot(TM) 64-Bit
  • OS: CentOS 7.2

Here is Heap Usage of one of my test servers

heap usage

ES only uses less than 4GB (20% of heap of 20GB)

But Java process uses 16GB of resident memory!

$ jps
53089 Elasticsearch

$ ps aux | grep 53089
username   53089  112 35.7 168712820 17672036 ?  Sl   Nov16 13599:09 /path/to/java ... -Xms20g -Xmx20g -Des.path.home=/path/to/elasticsearch -cp /path/to/elasticsearch/lib/elasticsearch-2.3.2.jar:/path/to/elasticsearch/lib/* org.elasticsearch.bootstrap.Elasticsearch start -d

Q: Does this means, almost the most of resident memory are fragmented?

Here is jmap output:

$ jmap -histo:live 53089 | head -n 20

 num     #instances         #bytes  class name
   1:        264406     1274923328  [B
   2:        105756      102571056  [J
   3:         98842       12137408  [C
   4:        125753        9054216  org.apache.lucene.util.fst.FST$Arc
   5:         34735        5001840  org.apache.lucene.codecs.blocktree.SegmentTermsEnumFrame
   6:        145887        4668384
   7:         34736        3334656  org.apache.lucene.codecs.lucene50.Lucene50PostingsFormat$IntBlockTermState
   8:         92266        2952512  java.util.HashMap$Node

Thank you for advenced answer.

You configured 20 GB heap for Elasticsearch, so I do not understand why you are concerned about 16 GB process size in memory. This is to be expected.


Thank you for your reply.

Yes. It is not a problem. What I'd like to know is that...

ES uses only 2~4GB of heap but 16GB of redisdent memory. So I'd like to know what is occupying 12~14GB resident memory (heap usage graph and jmap output show this)


The JVM allocates the whole heap at startup, no matter if it is used or not, and keeps it allocated from the OS. This for performance reasons. See -Xms and -Xmx JVM flags.

1 Like

OK. I see.

I'm not a JVM guy. So I missed -Xms 20g option.

Thanks alot.

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