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https://issues.apache.org/jira/browse/SPARK-4550?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14304864#comment-14304864
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Sandy Ryza commented on SPARK-4550:
-----------------------------------
I had heard rumors to that effect, so I ran some experiments and didn't find
that to be the case:
{code}
import org.apache.spark.serializer.KryoSerializer
import org.apache.spark.SparkConf
import java.io.ByteArrayOutputStream
import java.nio.ByteBuffer
val ser1 = new KryoSerializer(new SparkConf)
def serialize(objs: Array[AnyRef], ser: KryoSerializer): Array[Byte] = {
val instance = ser.newInstance
val baos = new ByteArrayOutputStream()
val stream = instance.serializeStream(baos)
objs.foreach(obj => stream.writeObject(obj))
stream.close()
baos.toByteArray
}
val inner = (0 until 100000).toArray
val bytes1 = serialize(Array((1, inner), (2, inner)), ser1)
val inner1 = (0 until 100000).toArray
val inner2 = (0 until 100000).toArray
val bytes2 = serialize(Array((1, inner1), (2, inner2)), ser1)
val secondHalf = new Array[Byte](bytes1.size / 2)
System.arraycopy(bytes1, bytes1.size / 2, secondHalf, 0, bytes1.size / 2)
ser1.newInstance.deserialize[AnyRef](ByteBuffer.wrap(secondHalf))
{code}
bytes1 ends up the same size as bytes2, and the last line is able to
successfully reproduce the second object.
Are there cases or Kryo versions I'm not thinking about?
> In sort-based shuffle, store map outputs in serialized form
> -----------------------------------------------------------
>
> Key: SPARK-4550
> URL: https://issues.apache.org/jira/browse/SPARK-4550
> Project: Spark
> Issue Type: Improvement
> Components: Shuffle, Spark Core
> Affects Versions: 1.2.0
> Reporter: Sandy Ryza
> Assignee: Sandy Ryza
> Priority: Critical
> Attachments: SPARK-4550-design-v1.pdf
>
>
> One drawback with sort-based shuffle compared to hash-based shuffle is that
> it ends up storing many more java objects in memory. If Spark could store
> map outputs in serialized form, it could
> * spill less often because the serialized form is more compact
> * reduce GC pressure
> This will only work when the serialized representations of objects are
> independent from each other and occupy contiguous segments of memory. E.g.
> when Kryo reference tracking is left on, objects may contain pointers to
> objects farther back in the stream, which means that the sort can't relocate
> objects without corrupting them.
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