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https://issues.apache.org/jira/browse/SPARK-24938?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16558798#comment-16558798
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Imran Rashid commented on SPARK-24938:
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This should be an easy change to make, its just a question of running a test.  
I keep meaning to do it, but have too many other things in flight, so anybody 
is welcome to do it.  You could use SPARK-24918 and 
https://github.com/squito/spark-memory to check the memory usage before and 
after.

> Understand usage of netty's onheap memory use, even with offheap pools
> ----------------------------------------------------------------------
>
>                 Key: SPARK-24938
>                 URL: https://issues.apache.org/jira/browse/SPARK-24938
>             Project: Spark
>          Issue Type: Improvement
>          Components: Spark Core
>    Affects Versions: 2.4.0
>            Reporter: Imran Rashid
>            Priority: Major
>              Labels: memory-analysis
>
> We've observed that netty uses large amount of onheap memory in its pools, in 
> addition to the expected offheap memory when I added some instrumentation 
> (using SPARK-24918 and https://github.com/squito/spark-memory). We should 
> figure out why its using that memory, and whether its really necessary.
> It might be just this one line:
> https://github.com/apache/spark/blob/master/common/network-common/src/main/java/org/apache/spark/network/protocol/MessageEncoder.java#L82
> which means that even with a small burst of messages, each arena will grow by 
> 16MB which could lead to a 128 MB spike of an almost entirely unused pool.  
> Switching to requesting a buffer from the default pool would probably fix 
> this.



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