[SPARK-17817][PYSPARK] PySpark RDD Repartitioning Results in Highly Skewed Partition Sizes
## What changes were proposed in this pull request? Quoted from JIRA description: Calling repartition on a PySpark RDD to increase the number of partitions results in highly skewed partition sizes, with most having 0 rows. The repartition method should evenly spread out the rows across the partitions, and this behavior is correctly seen on the Scala side. Please reference the following code for a reproducible example of this issue: num_partitions = 20000 a = sc.parallelize(range(int(1e6)), 2) # start with 2 even partitions l = a.repartition(num_partitions).glom().map(len).collect() # get length of each partition min(l), max(l), sum(l)/len(l), len(l) # skewed! In Scala's `repartition` code, we will distribute elements evenly across output partitions. However, the RDD from Python is serialized as a single binary data, so the distribution fails. We need to convert the RDD in Python to java object before repartitioning. ## How was this patch tested? Jenkins tests. Author: Liang-Chi Hsieh <viirya@gmail.com> Closes #15389 from viirya/pyspark-rdd-repartition.
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@ -2017,8 +2017,7 @@ class RDD(object):
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>>> len(rdd.repartition(10).glom().collect())
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10
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"""
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jrdd = self._jrdd.repartition(numPartitions)
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return RDD(jrdd, self.ctx, self._jrdd_deserializer)
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return self.coalesce(numPartitions, shuffle=True)
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def coalesce(self, numPartitions, shuffle=False):
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"""
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@ -2029,6 +2028,14 @@ class RDD(object):
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>>> sc.parallelize([1, 2, 3, 4, 5], 3).coalesce(1).glom().collect()
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[[1, 2, 3, 4, 5]]
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"""
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if shuffle:
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# In Scala's repartition code, we will distribute elements evenly across output
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# partitions. However, the RDD from Python is serialized as a single binary data,
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# so the distribution fails and produces highly skewed partitions. We need to
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# convert it to a RDD of java object before repartitioning.
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data_java_rdd = self._to_java_object_rdd().coalesce(numPartitions, shuffle)
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jrdd = self.ctx._jvm.SerDeUtil.javaToPython(data_java_rdd)
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else:
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jrdd = self._jrdd.coalesce(numPartitions, shuffle)
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return RDD(jrdd, self.ctx, self._jrdd_deserializer)
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@ -914,6 +914,16 @@ class RDDTests(ReusedPySparkTestCase):
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self.assertEqual(partitions[0], [(0, 5), (0, 8), (2, 6)])
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self.assertEqual(partitions[1], [(1, 3), (3, 8), (3, 8)])
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def test_repartition_no_skewed(self):
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num_partitions = 20
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a = self.sc.parallelize(range(int(1000)), 2)
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l = a.repartition(num_partitions).glom().map(len).collect()
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zeros = len([x for x in l if x == 0])
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self.assertTrue(zeros == 0)
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l = a.coalesce(num_partitions, True).glom().map(len).collect()
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zeros = len([x for x in l if x == 0])
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self.assertTrue(zeros == 0)
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def test_distinct(self):
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rdd = self.sc.parallelize((1, 2, 3)*10, 10)
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self.assertEqual(rdd.getNumPartitions(), 10)
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