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From "Steve Scaffidi (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-12217) hashCode in DoubleWritable returns same value for many numbers
Date Sat, 11 Jul 2015 17:45:04 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-12217?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14623500#comment-14623500
] 

Steve Scaffidi commented on HADOOP-12217:
-----------------------------------------

Oh! Also, using Murmur Hash here would not help, since the root of the problem is related
to getting the hashCode of an object representing a number, which should (usually) involve
simply using the number itself - typically a highly performant operation. For numeric types
whose representation is larger than an int (32 bits), a typical solution is to simply "or"
the number by itself, shifted by 32 bits, repeating until you have only 32 bits left, then
return that value.

Luckily this operation is not only extremely cheap to perform, it's already built-in to Java's
Double class, and documented here for those who might need to know how it's done:
  http://docs.oracle.com/javase/7/docs/api/java/lang/Double.html#hashCode%28%29

As the hashCode method in DoubleWritable currently does, for whole numbers between +/-MAX_INT
(more or less), casting a double's bitwise representation to an int removes almost all of
the significant (in the sense of significant to representing the desired numerical value)
bits! The implementation at the link above does the right thing, and provides far better distribution
for assigning buckets in a HashMap.

I would personally want to find out why fixing hashCode() in Hadoop's DoubleWritable breaks
bucketing in Hive - I have some suspicions as to how that might happen, but I would need more
info about exactly what you found.


> hashCode in DoubleWritable returns same value for many numbers
> --------------------------------------------------------------
>
>                 Key: HADOOP-12217
>                 URL: https://issues.apache.org/jira/browse/HADOOP-12217
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: io
>    Affects Versions: 0.18.0, 0.18.1, 0.18.2, 0.18.3, 0.19.0, 0.19.1, 0.20.0, 0.20.1,
0.20.2, 0.20.203.0, 0.20.204.0, 0.20.205.0, 1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.1.0, 1.1.1,
1.2.0, 0.21.0, 0.22.0, 0.23.0, 0.23.1, 0.23.3, 2.0.0-alpha, 2.0.1-alpha, 2.0.2-alpha, 0.23.4,
2.0.3-alpha, 0.23.5, 0.23.6, 1.1.2, 0.23.7, 2.1.0-beta, 2.0.4-alpha, 0.23.8, 1.2.1, 2.0.5-alpha,
0.23.9, 0.23.10, 0.23.11, 2.1.1-beta, 2.0.6-alpha, 2.2.0, 2.3.0, 2.4.0, 2.5.0, 2.4.1, 2.5.1,
2.5.2, 2.6.0, 2.7.0, 2.7.1
>            Reporter: Steve Scaffidi
>              Labels: easyfix
>
> Because DoubleWritable.hashCode() is incorrect, using DoubleWritables as the keys in
a HashMap results in abysmal performance, due to hash code collisions.
> I discovered this when testing the latest version of Hive and certain mapjoin queries
were exceedingly slow.
> Evidently, Hive has its own wrapper/subclass around Hadoop's DoubleWritable that overrode
used to override hashCode() with a correct implementation, but for some reason they recently
removed that code, so it now uses the incorrect hashCode() method inherited from Hadoop's
DoubleWritable.
> It appears that this bug has been there since DoubleWritable was created(wow!) so I can
understand if fixing it is impractical due to the possibility of breaking things down-stream,
but I can't think of anything that *should* break, off the top of my head.
> Searching JIRA, I found several related tickets, which may be useful for some historical
perspective: HADOOP-3061, HADOOP-3243, HIVE-511, HIVE-1629, HIVE-7041



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