[jira] [Commented] (HIVE-7074) The reducer parallelism should be a prime number for better stride protection

["Gopal V (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-7074?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13999580#comment-13999580 ] 

Gopal V commented on HIVE-7074:
-------------------------------

To bring up tpc-ds 200-scale inventory table as a comparison.

{code}
hive> select inv_item_sk%200 as bucket, count(1) as total from inventory group by inv_item_sk % 200 order by total desc limit 1;

199     225120
Time taken: 11.112 seconds, Fetched: 1 row(s)
{code}

versus 199 buckets.

{code}
hive> select inv_item_sk%199 as bucket, count(1) as total from inventory group by inv_item_sk % 199 order by total desc limit 1;
31      190428
Time taken: 10.377 seconds, Fetched: 1 row(s)
{code}

That's 18% more for the slowest bucket while needing an extra reducer.

> The reducer parallelism should be a prime number for better stride protection
> -----------------------------------------------------------------------------
>
>                 Key: HIVE-7074
>                 URL: https://issues.apache.org/jira/browse/HIVE-7074
>             Project: Hive
>          Issue Type: Improvement
>          Components: Statistics
>            Reporter: Gopal V
>            Assignee: Gopal V
>         Attachments: HIVE-7074.1.patch
>
>
> The current hive reducer parallelism results in stride issues with key distribution.
> a JOIN generating even numbers will get strided onto only some of the reducers.
> The probability of distribution skew is controlled by the number of common factors shared by the hashcode of the key and the number of buckets.
> Using a prime number within the reducer estimation will cut that probability down by a significant amount.



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