# Lucene Merge Thread causing high CPU usage

**URL:** <https://discuss.elastic.co/t/lucene-merge-thread-causing-high-cpu-usage/270599>\
**Category:** Elasticsearch\
**Created:** [April 19, 2021, 2:49pm UTC](https://discuss.elastic.co/t/lucene-merge-thread-causing-high-cpu-usage/270599 "2021-04-19T14:49:03Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![souravsahoo](https://avatars.discourse-cdn.com/v4/letter/s/cc9497/32.png) [@souravsahoo](https://discuss.elastic.co/u/souravsahoo)\
**Post date:** [April 19, 2021, 2:49pm UTC](https://discuss.elastic.co/t/lucene-merge-thread-causing-high-cpu-usage/270599/1 "2021-04-19T14:49:03Z")

</div>

Hi all,

I have been facing issue with elasticsearch consuming a very high CPU i.e., 80-100% when a heavy data load is ingested.

Below are few inputs:-

Elasticsearch is running on. 7.10.2 version  
We have data tier architecture in place with 12 hot nodes and the data is ingested to elasticsearch with data streams. As the data is only logs data.

Elasticsearch indexing rate is close to 231k docs/sec  
CPU usage is \> 90%  
Memory usage is below 50%

Output from hot\_threads API

```
 101.7% (508.5ms out of 500ms) cpu usage by thread 'elasticsearch[elasticstackstage-hot-v004-v4tp][[.ds-carbonelastic-000155][2]: Lucene Merge Thread #130]'
     10/10 snapshots sharing following 12 elements
       app//org.apache.lucene.codecs.blocktree.BlockTreeTermsWriter$TermsWriter.write(BlockTreeTermsWriter.java:907)
       app//org.apache.lucene.codecs.blocktree.BlockTreeTermsWriter.write(BlockTreeTermsWriter.java:318)
       app//org.apache.lucene.codecs.FieldsConsumer.merge(FieldsConsumer.java:105)
       app//org.apache.lucene.codecs.perfield.PerFieldPostingsFormat$FieldsWriter.merge(PerFieldPostingsFormat.java:197)
       app//org.apache.lucene.index.SegmentMerger.mergeTerms(SegmentMerger.java:244)
       app//org.apache.lucene.index.SegmentMerger.merge(SegmentMerger.java:139)
       app//org.apache.lucene.index.IndexWriter.mergeMiddle(IndexWriter.java:4760)
       app//org.apache.lucene.index.IndexWriter.merge(IndexWriter.java:4364)
       app//org.apache.lucene.index.IndexWriter$IndexWriterMergeSource.merge(IndexWriter.java:5923)
       app//org.apache.lucene.index.ConcurrentMergeScheduler.doMerge(ConcurrentMergeScheduler.java:624)
       app//org.elasticsearch.index.engine.ElasticsearchConcurrentMergeScheduler.doMerge(ElasticsearchConcurrentMergeScheduler.java:100)
       app//org.apache.lucene.index.ConcurrentMergeScheduler$MergeThread.run(ConcurrentMergeScheduler.java:682)

```

There are several nodes with similar usage.

Lucene merge threads seem to be causing the CPU spikes.

Looking for some help on this.

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<div class="post-metadata">

**Author:** ![Christian\_Dahlqvist](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/christian_dahlqvist/32/4617_2.png) [@Christian\_Dahlqvist](https://discuss.elastic.co/u/Christian_Dahlqvist)\
**Post date:** [April 19, 2021, 3:31pm UTC](https://discuss.elastic.co/t/lucene-merge-thread-causing-high-cpu-usage/270599/2 "2021-04-19T15:31:18Z")

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What type of storage are you using on these nodes? SSDs?

What does disk I/O and iowait look like?

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<div class="post-metadata">

**Author:** ![souravsahoo](https://avatars.discourse-cdn.com/v4/letter/s/cc9497/32.png) [@souravsahoo](https://discuss.elastic.co/u/souravsahoo)\
**Post date:** [April 19, 2021, 5:37pm UTC](https://discuss.elastic.co/t/lucene-merge-thread-causing-high-cpu-usage/270599/3 "2021-04-19T17:37:06Z")

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@Christian_Dahlqvist yes, i am using SSDs.

The i/o operations were looking good during the Spike. I have attached a screencap for your reference.

 ![image](https://us1.discourse-cdn.com/elastic/original/3X/c/c/ccbd6cbca64fad13c859c7f6f1d34a9f2cc2e846.png)

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<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\
**Post date:** [May 17, 2021, 5:37pm UTC](https://discuss.elastic.co/t/lucene-merge-thread-causing-high-cpu-usage/270599/4 "2021-05-17T17:37:36Z")

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