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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DRILL-4237) Skew in hash distribution
Date Fri, 08 Apr 2016 00:57:25 GMT

    [ https://issues.apache.org/jira/browse/DRILL-4237?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15231446#comment-15231446

ASF GitHub Bot commented on DRILL-4237:

Github user amansinha100 commented on the pull request:

    @jacques-n regarding re-implementation, agree with the general concern; however  in this
case the core logic is directly taken from the java based murmur hash implementation  (compared
to the XXHash that was ported from C).  The OpenHFT discussion happened a little late in the
cycle but in any case it seems fairly young project that is run by 1 company.  We could re-visit
it in future.  When discussing the hash functions with @chunhui-shi we really want to get
to a stage where Drill could allow a few different hash functions/library to exist and choose
based on an external setting.  
    Regarding performance, we have done a couple of rounds of testing but likely will do another

> Skew in hash distribution
> -------------------------
>                 Key: DRILL-4237
>                 URL: https://issues.apache.org/jira/browse/DRILL-4237
>             Project: Apache Drill
>          Issue Type: Bug
>          Components: Functions - Drill
>    Affects Versions: 1.4.0
>            Reporter: Aman Sinha
>            Assignee: Chunhui Shi
> Apparently, the fix in DRILL-4119 did not fully resolve the data skew issue.  It worked
fine on the smaller sample of the data set but on another sample of the same data set, it
still produces skewed values - see below the hash values which are all odd numbers. 
> {noformat}
> 0: jdbc:drill:zk=local> select columns[0], hash32(columns[0]) from `test.csv` limit
> +-----------------------------------+--------------+
> |              EXPR$0               |    EXPR$1    |
> +-----------------------------------+--------------+
> | f71aaddec3316ae18d43cb1467e88a41  | 1506011089   |
> | 3f3a13bb45618542b5ac9d9536704d3a  | 1105719049   |
> | 6935afd0c693c67bba482cedb7a2919b  | -18137557    |
> | ca2a938d6d7e57bda40501578f98c2a8  | -1372666789  |
> | fab7f08402c8836563b0a5c94dbf0aec  | -1930778239  |
> | 9eb4620dcb68a84d17209da279236431  | -970026001   |
> | 16eed4a4e801b98550b4ff504242961e  | 356133757    |
> | a46f7935fea578ce61d8dd45bfbc2b3d  | -94010449    |
> | 7fdf5344536080c15deb2b5a2975a2b7  | -141361507   |
> | b82560a06e2e51b461c9fe134a8211bd  | -375376717   |
> +-----------------------------------+--------------+
> {noformat}
> This indicates an underlying issue with the XXHash64 java implementation, which is Drill's
implementation of the C version.  One of the key difference as pointed out by [~jnadeau] was
the use of unsigned int64 in the C version compared to the Java version which uses (signed)
long.  I created an XXHash version using com.google.common.primitives.UnsignedLong.  However,
UnsignedLong does not have bit-wise operations that are needed for XXHash such as rotateLeft(),
 XOR etc.  One could write wrappers for these but at this point, the question is: should we
think of an alternative hash function ? 
> The alternative approach could be the murmur hash for numeric data types that we were
using earlier and the Mahout version of hash function for string types (https://github.com/apache/drill/blob/master/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/impl/HashHelper.java#L28).
 As a test, I reverted to this function and was getting good hash distribution for the test
> I could not find any performance comparisons of our perf tests (TPC-H or DS) with the
original and newer (XXHash) hash functions.  If performance is comparable, should we revert
to the original function ?  
> As an aside, I would like to remove the hash64 versions of the functions since these
are not used anywhere. 

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