[SPARK-3649] Remove GraphX custom serializers

As [reported][1] on the mailing list, GraphX throws

```
java.lang.ClassCastException: java.lang.Long cannot be cast to scala.Tuple2
        at org.apache.spark.graphx.impl.RoutingTableMessageSerializer$$anon$1$$anon$2.writeObject(Serializers.scala:39)
        at org.apache.spark.storage.DiskBlockObjectWriter.write(BlockObjectWriter.scala:195)
        at org.apache.spark.util.collection.ExternalSorter.spillToMergeableFile(ExternalSorter.scala:329)
```

when sort-based shuffle attempts to spill to disk. This is because GraphX defines custom serializers for shuffling pair RDDs that assume Spark will always serialize the entire pair object rather than breaking it up into its components. However, the spill code path in sort-based shuffle [violates this assumption][2].

GraphX uses the custom serializers to compress vertex ID keys using variable-length integer encoding. However, since the serializer can no longer rely on the key and value being serialized and deserialized together, performing such encoding would either require writing a tag byte (costly) or maintaining state in the serializer and assuming that serialization calls will alternate between key and value (fragile).

Instead, this PR simply removes the custom serializers. This causes a **10% slowdown** (494 s to 543 s) and **16% increase in per-iteration communication** (2176 MB to 2518 MB) for PageRank (averages across 3 trials, 10 iterations per trial, uk-2007-05 graph, 16 r3.2xlarge nodes).

[1]: http://apache-spark-user-list.1001560.n3.nabble.com/java-lang-ClassCastException-java-lang-Long-cannot-be-cast-to-scala-Tuple2-td13926.html#a14501
[2]: https://github.com/apache/spark/blob/f9d6220c792b779be385f3022d146911a22c2130/core/src/main/scala/org/apache/spark/util/collection/ExternalSorter.scala#L329

Author: Ankur Dave <ankurdave@gmail.com>

Closes #2503 from ankurdave/SPARK-3649 and squashes the following commits:

a49c2ad [Ankur Dave] [SPARK-3649] Remove GraphX custom serializers

1 files changed, 0 insertions, 122 deletions

diff --git a/graphx/src/test/scala/org/apache/spark/graphx/SerializerSuite.scala b/graphx/src/test/scala/org/apache/spark/graphx/SerializerSuite.scala
deleted file mode 100644
index 864cb1fdf0..0000000000
--- a/graphx/src/test/scala/org/apache/spark/graphx/SerializerSuite.scala
+++ /dev/null
@@ -1,122 +0,0 @@
-/*
- * Licensed to the Apache Software Foundation (ASF) under one or more
- * contributor license agreements.  See the NOTICE file distributed with
- * this work for additional information regarding copyright ownership.
- * The ASF licenses this file to You under the Apache License, Version 2.0
- * (the "License"); you may not use this file except in compliance with
- * the License.  You may obtain a copy of the License at
- *
- *    http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.apache.spark.graphx
-
-import java.io.{EOFException, ByteArrayInputStream, ByteArrayOutputStream}
-
-import scala.util.Random
-import scala.reflect.ClassTag
-
-import org.scalatest.FunSuite
-
-import org.apache.spark._
-import org.apache.spark.graphx.impl._
-import org.apache.spark.serializer.SerializationStream
-
-
-class SerializerSuite extends FunSuite with LocalSparkContext {
-
-  test("IntAggMsgSerializer") {
-    val outMsg = (4: VertexId, 5)
-    val bout = new ByteArrayOutputStream
-    val outStrm = new IntAggMsgSerializer().newInstance().serializeStream(bout)
-    outStrm.writeObject(outMsg)
-    outStrm.writeObject(outMsg)
-    bout.flush()
-    val bin = new ByteArrayInputStream(bout.toByteArray)
-    val inStrm = new IntAggMsgSerializer().newInstance().deserializeStream(bin)
-    val inMsg1: (VertexId, Int) = inStrm.readObject()
-    val inMsg2: (VertexId, Int) = inStrm.readObject()
-    assert(outMsg === inMsg1)
-    assert(outMsg === inMsg2)
-
-    intercept[EOFException] {
-      inStrm.readObject()
-    }
-  }
-
-  test("LongAggMsgSerializer") {
-    val outMsg = (4: VertexId, 1L << 32)
-    val bout = new ByteArrayOutputStream
-    val outStrm = new LongAggMsgSerializer().newInstance().serializeStream(bout)
-    outStrm.writeObject(outMsg)
-    outStrm.writeObject(outMsg)
-    bout.flush()
-    val bin = new ByteArrayInputStream(bout.toByteArray)
-    val inStrm = new LongAggMsgSerializer().newInstance().deserializeStream(bin)
-    val inMsg1: (VertexId, Long) = inStrm.readObject()
-    val inMsg2: (VertexId, Long) = inStrm.readObject()
-    assert(outMsg === inMsg1)
-    assert(outMsg === inMsg2)
-
-    intercept[EOFException] {
-      inStrm.readObject()
-    }
-  }
-
-  test("DoubleAggMsgSerializer") {
-    val outMsg = (4: VertexId, 5.0)
-    val bout = new ByteArrayOutputStream
-    val outStrm = new DoubleAggMsgSerializer().newInstance().serializeStream(bout)
-    outStrm.writeObject(outMsg)
-    outStrm.writeObject(outMsg)
-    bout.flush()
-    val bin = new ByteArrayInputStream(bout.toByteArray)
-    val inStrm = new DoubleAggMsgSerializer().newInstance().deserializeStream(bin)
-    val inMsg1: (VertexId, Double) = inStrm.readObject()
-    val inMsg2: (VertexId, Double) = inStrm.readObject()
-    assert(outMsg === inMsg1)
-    assert(outMsg === inMsg2)
-
-    intercept[EOFException] {
-      inStrm.readObject()
-    }
-  }
-
-  test("variable long encoding") {
-    def testVarLongEncoding(v: Long, optimizePositive: Boolean) {
-      val bout = new ByteArrayOutputStream
-      val stream = new ShuffleSerializationStream(bout) {
-        def writeObject[T: ClassTag](t: T): SerializationStream = {
-          writeVarLong(t.asInstanceOf[Long], optimizePositive = optimizePositive)
-          this
-        }
-      }
-      stream.writeObject(v)
-
-      val bin = new ByteArrayInputStream(bout.toByteArray)
-      val dstream = new ShuffleDeserializationStream(bin) {
-        def readObject[T: ClassTag](): T = {
-          readVarLong(optimizePositive).asInstanceOf[T]
-        }
-      }
-      val read = dstream.readObject[Long]()
-      assert(read === v)
-    }
-
-    // Test all variable encoding code path (each branch uses 7 bits, i.e. 1L << 7 difference)
-    val d = Random.nextLong() % 128
-    Seq[Long](0, 1L << 0 + d, 1L << 7 + d, 1L << 14 + d, 1L << 21 + d, 1L << 28 + d, 1L << 35 + d,
-      1L << 42 + d, 1L << 49 + d, 1L << 56 + d, 1L << 63 + d).foreach { number =>
-      testVarLongEncoding(number, optimizePositive = false)
-      testVarLongEncoding(number, optimizePositive = true)
-      testVarLongEncoding(-number, optimizePositive = false)
-      testVarLongEncoding(-number, optimizePositive = true)
-    }
-  }
-}