Github user catap commented on the pull request:
https://github.com/apache/spark/pull/6795#issuecomment-118237792
@feynmanliang I think you mean `tf.persist()`, don't you? Because
`tf.collect()` convert RDD to local array and I can't use this on
DecisionTree.trainClassifier.
So, when I re-run test with `tf.persist()` and `transformed_tf.persist()`,
I had result:
```
Elapsed time 2 min, 5 sec, 711 millis (total 125711 millis)
Elapsed transformed time 0 min, 2 sec, 750 millis (total 2750 millis)
```
the code:
```scala
import java.util.Random
import org.apache.spark.mllib.feature.{HashingTF, SignificantSelector}
import org.apache.spark.mllib.regression.LabeledPoint
import org.apache.spark.mllib.tree.DecisionTree
import org.apache.spark.rdd.RDD
import scala.concurrent.duration._
import scala.language.implicitConversions
val rnd = new Random
val length = 20
implicit def bool2Double(b:Boolean): Double = if (b) 1d else 0d
val localDocs = (1 to length).map(_ =>
(rnd.nextBoolean().toDouble, (1 to 1000).map(_ =>
rnd.nextDouble().toString)))
val docs = sc.parallelize(localDocs, 2)
val hashingTF = new HashingTF()
val tf: RDD[LabeledPoint] = docs.map { case (label, words) =>
LabeledPoint(label, hashingTF.transform(words)) }
val transformer = new SignificantSelector().fit(tf.map(_.features))
val transformed_tf = tf.map(p => p.copy(features =
transformer.transform(p.features)))
tf.persist()
val start = System.currentTimeMillis()
val model = DecisionTree.trainClassifier(tf, 2, Map[Int, Int](), "gini",
30, length)
val end = System.currentTimeMillis()
tf.unpersist()
transformed_tf.persist()
val transformed_start = System.currentTimeMillis()
val transformed_model = DecisionTree.trainClassifier(transformed_tf, 2,
Map[Int, Int](), "gini", 30, length)
val transformed_end = System.currentTimeMillis()
transformed_tf.unpersist()
val elapsed = DurationLong(end - start).millis
val transformed_elapsed = DurationLong(transformed_end -
transformed_start).millis
println("Ok, done.")
println(f"Elapsed time ${elapsed.toMinutes} min, ${elapsed.toSeconds % 60}
sec, ${elapsed.toMillis % 1000} millis (total ${elapsed.toMillis} millis)")
println(f"Elapsed transformed time ${transformed_elapsed.toMinutes} min,
${transformed_elapsed.toSeconds % 60} sec, ${transformed_elapsed.toMillis %
1000} millis (total ${transformed_elapsed.toMillis} millis)")
```
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