[Spark] RDD take() method: overestimate too much

In the comment (Line 1083), it says: "Otherwise, interpolate the number of partitions we need to try, but overestimate it by 50%."

`(1.5 * num * partsScanned / buf.size).toInt` is the guess of "num of total partitions needed". In every iteration, we should consider the increment `(1.5 * num * partsScanned / buf.size).toInt - partsScanned`
Existing implementation 'exponentially' grows `partsScanned ` ( roughly: `x_{n+1} >= (1.5 + 1) x_n`)

This could be a performance problem. (unless this is the intended behavior)

Author: yingjieMiao <yingjie@42go.com>

Closes #2648 from yingjieMiao/rdd_take and squashes the following commits:

d758218 [yingjieMiao] scala style fix
a8e74bb [yingjieMiao] python style fix
4b6e777 [yingjieMiao] infix operator style fix
4391d3b [yingjieMiao] typo fix.
692f4e6 [yingjieMiao] cap numPartsToTry
c4483dc [yingjieMiao] style fix
1d2c410 [yingjieMiao] also change in rdd.py and AsyncRDD
d31ff7e [yingjieMiao] handle the edge case after 1 iteration
a2aa36b [yingjieMiao] RDD take method: overestimate too much

1 files changed, 4 insertions, 1 deletions

diff --git a/python/pyspark/rdd.py b/python/pyspark/rdd.py
index e13bab946c..15be4bfec9 100644
--- a/python/pyspark/rdd.py
+++ b/python/pyspark/rdd.py
@@ -1070,10 +1070,13 @@ class RDD(object):
                 # If we didn't find any rows after the previous iteration,
                 # quadruple and retry.  Otherwise, interpolate the number of
                 # partitions we need to try, but overestimate it by 50%.
+                # We also cap the estimation in the end.
                 if len(items) == 0:
                     numPartsToTry = partsScanned * 4
                 else:
-                    numPartsToTry = int(1.5 * num * partsScanned / len(items))
+                    # the first paramter of max is >=1 whenever partsScanned >= 2
+                    numPartsToTry = int(1.5 * num * partsScanned / len(items)) - partsScanned
+                    numPartsToTry = min(max(numPartsToTry, 1), partsScanned * 4)
 
             left = num - len(items)