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https://issues.apache.org/jira/browse/SPARK-44379?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Shardul Mahadik updated SPARK-44379:
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Attachment: screenshot-2.png
> Broadcast Joins taking up too much memory
> -----------------------------------------
>
> Key: SPARK-44379
> URL: https://issues.apache.org/jira/browse/SPARK-44379
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 3.4.1
> Reporter: Shardul Mahadik
> Priority: Major
> Attachments: screenshot-1.png, screenshot-2.png
>
>
> Context: After migrating to Spark 3 with AQE, we saw a significant increase
> in driver and executor memory usage in our jobs which contains star joins. By
> analyzing heapdump, we saw that majority of the memory was being taken up by
> {{UnsafeHashedRelation}} used for broadcast joins; in this case there were 92
> broadcast joins in the query.
> !image-2023-07-11-10-41-02-251.png|width=851,height=70!
> This took up over 6GB of total memory, even though every table being
> broadcasted was around ~1MB and hence should only have been ~100MB total. I
> found that this is because {{BytesToBytesMap}} used within
> {{UnsafeHashedRelation}} allocates memory in ["pageSize"
> increments|https://github.com/apache/spark/blob/37aa62f629e652ed70505620473530cd9611018e/core/src/main/java/org/apache/spark/memory/MemoryConsumer.java#L117]
> which in our case was 64MB. Based on the [default page size
> calculation|https://github.com/apache/spark/blob/37aa62f629e652ed70505620473530cd9611018e/core/src/main/scala/org/apache/spark/memory/MemoryManager.scala#L251],
> this should be the case for any container with > 1 GB of memory (assuming
> executor.cores = 1) which is far too common. Thus in our case, most of the
> memory requested by {{BytesToBytesMap}} was un-utilized with just trailing 0s.
> !image-2023-07-11-10-52-59-553.png|width=389,height=101!
> I think this is a major inefficiency for broadcast joins (especially star
> joins). I think there are a few ways to tackle the problem.
> 1) Reduce {{spark.buffer.pageSize}} globally to a lower value. This does
> reduce the memory consumption of broadcast joins, but I am not sure what it
> implies for the rest of Spark machinery
> 2) Add a "finalize" operation to {{BytesToBytesMap}} which is called after
> all values are added to the map and allocates a new page only for the
> required bytes.
> 3) Enhance the serialization of {{BytesToBytesMap}} to record the number of
> keys and values, and use those during deserialization to only request the
> required memory.
> 4) Use a lower page size for certain {{BytesToBytesMap}}s based on the
> estimated data size of broadcast joins.
> I believe Option 3 would be simple enough to implement and I have a POC PR
> which I will post soon, but I am interested in knowing other people's
> thoughts here.
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