apache/spark — pull request #3378 by zsxwing (2014-11-20 05:57:54+00:00)
Title: [SPARK-4505][Core] Add a ClassTag parameter to CompactBuffer[T]

Added a ClassTag parameter to CompactBuffer. So CompactBuffer[T] can create primitive arrays for primitive types. It will reduce the memory usage for primitive types significantly and only pay minor performance lost.

Here is my test code:
```Scala
// Call org.apache.spark.util.SizeEstimator.estimate
def estimateSize(obj: AnyRef): Long = {
val c = Class.forName("org.apache.spark.util.SizeEstimator$")
val f = c.getField("MODULE$")
val o = f.get(c)
val m = c.getMethod("estimate", classOf[Object])
m.setAccessible(true)
m.invoke(o, obj).asInstanceOf[Long]
}

sc.parallelize(1 to 10000).groupBy(_ => 1).foreach {
case (k, v) =>
println(v.getClass() + " size: " + estimateSize(v))
}
```

Using the previous CompactBuffer outputed
```
class org.apache.spark.util.collection.CompactBuffer size: 313358
```

Using the new CompactBuffer outputed
```
class org.apache.spark.util.collection.CompactBuffer size: 65712
```

In this case, the new `CompactBuffer` only used 20% memory of the previous one. It's really helpful for `groupByKey` when using a primitive value.