GitHub user tejasapatil opened a pull request:
https://github.com/apache/spark/pull/15573
[SPARK-18035] [SQL] Unwrapping java maps in HiveInspectors allocates
unnecessary buffer
## What changes were proposed in this pull request?
Jira: https://issues.apache.org/jira/browse/SPARK-18035
In HiveInspectors, I saw that converting Java map to Spark's
`ArrayBasedMapData` spent quite sometime in buffer copying :
https://github.com/apache/spark/blob/master/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveInspectors.scala#L658
The reason being `map.toSeq` allocates a new buffer and copies the map
entries to it:
https://github.com/scala/scala/blob/2.11.x/src/library/scala/collection/MapLike.scala#L323
This copy is not needed as we get rid of it once we extract the key and
value arrays.
Here is the call trace:
```
org.apache.spark.sql.hive.HiveInspectors$$anonfun$unwrapperFor$41.apply(HiveInspectors.scala:664)
scala.collection.AbstractMap.toSeq(Map.scala:59)
scala.collection.MapLike$class.toSeq(MapLike.scala:323)
scala.collection.AbstractMap.toBuffer(Map.scala:59)
scala.collection.MapLike$class.toBuffer(MapLike.scala:326)
scala.collection.AbstractTraversable.copyToBuffer(Traversable.scala:104)
scala.collection.TraversableOnce$class.copyToBuffer(TraversableOnce.scala:275)
scala.collection.mutable.ArrayBuffer.$plus$plus$eq(ArrayBuffer.scala:48)
scala.collection.mutable.ArrayBuffer.$plus$plus$eq(ArrayBuffer.scala:104)
scala.collection.generic.Growable$class.$plus$plus$eq(Growable.scala:59)
scala.collection.AbstractIterable.foreach(Iterable.scala:54)
scala.collection.IterableLike$class.foreach(IterableLike.scala:72)
scala.collection.AbstractIterator.foreach(Iterator.scala:1336)
scala.collection.Iterator$class.foreach(Iterator.scala:893)
scala.collection.generic.Growable$$anonfun$$plus$plus$eq$1.apply(Growable.scala:59)
scala.collection.generic.Growable$$anonfun$$plus$plus$eq$1.apply(Growable.scala:59)
```
Also, earlier code was populating keys and values arrays separately by
iterating twice. The PR avoids double iteration of the map and does it in one
iteration.
## Performance gains
The number is subjective and depends on how many map columns are accessed
in the query and average entries per map. For one the queries that I tried out,
I saw 3% CPU savings (end-to-end) for the query.
## How was this patch tested?
This does not change the end result produced so relying on existing tests.
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/tejasapatil/spark SPARK-18035_avoid_toSeq
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/spark/pull/15573.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #15573
----
commit 563232d095c0877fd06da8da2e789c94a289cdb1
Author: Tejas Patil <[email protected]>
Date: 2016-10-18T23:21:33Z
`map.asScala.toSeq` creates a new copy of the collection and copies the map
entries to it:
https://github.com/scala/legacy-svn-scala/blob/a74698486dc4221af0fb590c776998b7339e58d6/src/library/scala/collection/MapLike.scala#L313
This copy is not needed as we get rid of it once we extract the key and
value arrays.
Here is the call trace:
```
org.apache.spark.sql.hive.HiveInspectors$$anonfun$unwrapperFor$41.apply(HiveInspectors.scala:664)
scala.collection.AbstractMap.toSeq(Map.scala:59)
scala.collection.MapLike$class.toSeq(MapLike.scala:323)
scala.collection.AbstractMap.toBuffer(Map.scala:59)
scala.collection.MapLike$class.toBuffer(MapLike.scala:326)
scala.collection.AbstractTraversable.copyToBuffer(Traversable.scala:104)
scala.collection.TraversableOnce$class.copyToBuffer(TraversableOnce.scala:275)
scala.collection.mutable.ArrayBuffer.$plus$plus$eq(ArrayBuffer.scala:48)
scala.collection.mutable.ArrayBuffer.$plus$plus$eq(ArrayBuffer.scala:104)
scala.collection.generic.Growable$class.$plus$plus$eq(Growable.scala:59)
scala.collection.AbstractIterable.foreach(Iterable.scala:54)
scala.collection.IterableLike$class.foreach(IterableLike.scala:72)
scala.collection.AbstractIterator.foreach(Iterator.scala:1336)
scala.collection.Iterator$class.foreach(Iterator.scala:893)
scala.collection.generic.Growable$$anonfun$$plus$plus$eq$1.apply(Growable.scala:59)
scala.collection.generic.Growable$$anonfun$$plus$plus$eq$1.apply(Growable.scala:59)
```
Also, earlier code was populating keys and values arrays separately by
iterating twice. The PR avoids double iteration of the map and does it in one
iteration.
----
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