[SPARK-4531] [MLlib] cache serialized java object

The Pyrolite is pretty slow (comparing to the adhoc serializer in 1.1), it cause much performance regression in 1.2, because we cache the serialized Python object in JVM, deserialize them into Java object in each step.

This PR change to cache the deserialized JavaRDD instead of PythonRDD to avoid the deserialization of Pyrolite. It should have similar memory usage as before, but much faster.

Author: Davies Liu <davies@databricks.com>

Closes #3397 from davies/cache and squashes the following commits:

7f6e6ce [Davies Liu] Update -> Updater
4b52edd [Davies Liu] using named argument
63b984e [Davies Liu] fix
7da0332 [Davies Liu] add unpersist()
dff33e1 [Davies Liu] address comments
c2bdfc2 [Davies Liu] refactor
d572f00 [Davies Liu] Merge branch 'master' into cache
f1063e1 [Davies Liu] cache serialized java object

1 files changed, 2 insertions, 2 deletions

diff --git a/python/pyspark/mllib/recommendation.py b/python/pyspark/mllib/recommendation.py
index 2bcbf2aaf8..97ec74eda0 100644
--- a/python/pyspark/mllib/recommendation.py
+++ b/python/pyspark/mllib/recommendation.py
@@ -19,7 +19,7 @@ from collections import namedtuple
 
 from pyspark import SparkContext
 from pyspark.rdd import RDD
-from pyspark.mllib.common import JavaModelWrapper, callMLlibFunc, _to_java_object_rdd
+from pyspark.mllib.common import JavaModelWrapper, callMLlibFunc
 
 __all__ = ['MatrixFactorizationModel', 'ALS', 'Rating']
 
@@ -110,7 +110,7 @@ class ALS(object):
                 ratings = ratings.map(lambda x: Rating(*x))
             else:
                 raise ValueError("rating should be RDD of Rating or tuple/list")
-        return _to_java_object_rdd(ratings, True)
+        return ratings
 
     @classmethod
     def train(cls, ratings, rank, iterations=5, lambda_=0.01, blocks=-1, nonnegative=False,