Spark MapReduce 中的意外结果 [英] Unexpected results in Spark MapReduce

问题描述

我是 Spark 的新手，想了解 MapReduce 是如何在后台完成的，以确保我正确使用它.这篇文章提供了一个很好的答案，但我的结果似乎不符合所描述的逻辑.我在 Scala 中运行 Spark 快速入门 指南命令行.当我正确地添加行长时，结果就很好了.总行长为 1213:

I'm new to Spark and want to understand how MapReduce gets done under the hood to ensure I use it properly. This post provided a great answer, but my results don't seem to follow the logic described. I'm running the Spark Quick Start guide in Scala on command line. When I do line length addition properly, things come out just fine. Total line length is 1213:

scala> val textFile = sc.textFile("README.md")

scala> val linesWithSpark = textFile.filter(line => line.contains("Spark"))

scala> val linesWithSparkLengths = linesWithSpark.map(s => s.length)

scala> linesWithSparkLengths.foreach(println)

Result:
14
78
73
42
68
17
62
45
76
64
54
74
84
29
136
77
77
73
70

scala> val totalLWSparkLength = linesWithSparkLengths.reduce((a,b) => a+b)
    totalLWSparkLength: Int = 1213

当我稍微调整它以使用 (a-b) 而不是 (a+b) 时，

When I tweak it slightly to use (a-b) instead of (a+b),

scala> val totalLWSparkTest = linesWithSparkLengths.reduce((a,b) => a-b)

根据这篇文章中的逻辑，我预计为-1185:

I expected -1185, according to the logic in this post:

List(14,78,73,42,68,17,62,45,76,64,54,74,84,29,136,77,77,73,70).reduce( (x,y) => x - y )
  Step 1 : op( 14, 78 ) will be the first evaluation. 
     x is 14 and y is 78. Result of x - y = -64.
  Step 2:  op( op( 14, 78 ), 73 )
     x is op(14,78) = -64 and y = 73. Result of x - y = -137
  Step 3:  op( op( op( 14, 78 ), 73 ), 42) 
     x is op( op( 14, 78 ), 73 ) = -137 and y is 42. Result is -179.
  ...
  Step 18:  op( (... ), 73), 70) will be the final evaluation.
     x is -1115 and y is 70. Result of x - y is -1185.

然而，奇怪的事情发生了:

However, something strange happens:

scala> val totalLWSparkTest = linesWithSparkLengths.reduce((a,b) => a-b)
totalLWSparkTest: Int = 151

当我再次运行它时......

When I run it again...

scala> val totalLWSparkTest = linesWithSparkLengths.reduce((a,b) => a-b)
totalLWSparkTest: Int = -151

谁能告诉我为什么结果是 151(或 -151)而不是 -1185?

Can anyone tell me why the result is 151 (or -151) instead of -1185?

推荐答案

之所以会这样，是因为减法既不结合也不可交换.让我们从关联性开始:

It happens because subtraction is neither associative nor commutative. Lets start with associativity:

(- (- (- 14 78) 73) 42) 
(- (- -64 73) 42)
(- -137 42) 
-179

不一样

(- (- 14 78) (- 73 42))
(- -64 (- 73 42))
(- -64 31)
-95

现在是交换性的时候了:

Now its time for commutativity:

(- (- (- 14 78) 73) 42) ;; From the previous example

不一样

(- (- (- 42 73) 78) 14)
(- (- -31 78) 14)
(- -109 14)
-123

Spark 首先对各个分区应用 reduce，然后以任意顺序合并部分结果.如果您使用的函数不满足一个或两个标准，则最终结果可能是不确定的.

Spark first applies reduce on individual partitions and then merges partial results in arbitrary order. If function you use doesn't meet one or both criteria final results can be non-deterministic.

这篇关于Spark MapReduce 中的意外结果的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！