# Weird JVM Heap Usage **URL:** \ **Category:** Elasticsearch\ **Created:** [November 24, 2016, 11:18am UTC](https://discuss.elastic.co/t/weird-jvm-heap-usage/67097 "2016-11-24T11:18:03Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Brad\_Jungsu\_Heo](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/brad_jungsu_heo/32/13135_2.png) [@Brad\_Jungsu\_Heo](https://discuss.elastic.co/u/Brad_Jungsu_Heo)\ **Post date:** [November 24, 2016, 11:18am UTC](https://discuss.elastic.co/t/weird-jvm-heap-usage/67097/1 "2016-11-24T11:18:03Z")
(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](https://cdn.pbrd.co/images/1LccApSIZ.png) ES only uses less than 4GB (20% of heap of 20GB) But Java process uses 16GB of resident memory! ```auto $ 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: ```auto $ 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$Arc 5: 34735 5001840 org.apache.lucene.codecs.blocktree.SegmentTermsEnumFrame 6: 145887 4668384 org.apache.lucene.store.ByteArrayDataInput 7: 34736 3334656 org.apache.lucene.codecs.lucene50.Lucene50PostingsFormat$IntBlockTermState 8: 92266 2952512 java.util.HashMap$Node ... ``` Thank you for advenced answer. ---
**Author:** ![jprante](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/jprante/32/44941_2.png) [@jprante](https://discuss.elastic.co/u/jprante)\ **Post date:** [November 24, 2016, 7:36pm UTC](https://discuss.elastic.co/t/weird-jvm-heap-usage/67097/2 "2016-11-24T19:36:00Z")
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. ---
**Author:** ![Brad\_Jungsu\_Heo](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/brad_jungsu_heo/32/13135_2.png) [@Brad\_Jungsu\_Heo](https://discuss.elastic.co/u/Brad_Jungsu_Heo)\ **Post date:** [November 25, 2016, 1:00am UTC](https://discuss.elastic.co/t/weird-jvm-heap-usage/67097/3 "2016-11-25T01:00:45Z")
@jprante 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) Thanks ---
**Author:** ![jprante](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/jprante/32/44941_2.png) [@jprante](https://discuss.elastic.co/u/jprante)\ **Post date:** [November 25, 2016, 8:02am UTC](https://discuss.elastic.co/t/weird-jvm-heap-usage/67097/4 "2016-11-25T08:02:25Z")
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. ---
**Author:** ![Brad\_Jungsu\_Heo](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/brad_jungsu_heo/32/13135_2.png) [@Brad\_Jungsu\_Heo](https://discuss.elastic.co/u/Brad_Jungsu_Heo)\ **Post date:** [November 25, 2016, 9:18am UTC](https://discuss.elastic.co/t/weird-jvm-heap-usage/67097/5 "2016-11-25T09:18:29Z")
OK. I see. I'm not a JVM guy. So I missed `-Xms 20g` option. Thanks alot. ---
**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\ **Post date:** [December 23, 2016, 9:19am UTC](https://discuss.elastic.co/t/weird-jvm-heap-usage/67097/6 "2016-12-23T09:19:06Z")
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