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From "Lefty Leverenz (JIRA)" <>
Subject [jira] [Commented] (HIVE-15796) HoS: poor reducer parallelism when operator stats are not accurate
Date Thu, 23 Feb 2017 08:26:44 GMT


Lefty Leverenz commented on HIVE-15796:

Doc note:  This adds *hive.spark.use.op.stats* to for release 2.2.0, so it needs
to be documented in the wiki.

* [Configuration Properties -- Spark |]

Added a TODOC2.2 label.

> HoS: poor reducer parallelism when operator stats are not accurate
> ------------------------------------------------------------------
>                 Key: HIVE-15796
>                 URL:
>             Project: Hive
>          Issue Type: Improvement
>          Components: Statistics
>    Affects Versions: 2.2.0
>            Reporter: Chao Sun
>            Assignee: Chao Sun
>              Labels: TODOC2.2
>             Fix For: 2.2.0
>         Attachments: HIVE-15796.1.patch, HIVE-15796.2.patch, HIVE-15796.3.patch, HIVE-15796.4.patch,
HIVE-15796.5.patch, HIVE-15796.6.patch, HIVE-15796.wip.1.patch, HIVE-15796.wip.2.patch, HIVE-15796.wip.patch
> In HoS we use currently use operator stats to determine reducer parallelism. However,
it is often the case that operator stats are not accurate, especially if column stats are
not available. This sometimes will generate extremely poor reducer parallelism, and cause
HoS query to run forever. 
> This JIRA tries to offer an alternative way to compute reducer parallelism, similar to
how MR does. Here's the approach we are suggesting:
> 1. when computing the parallelism for a MapWork, use stats associated with the TableScan
> 2. when computing the parallelism for a ReduceWork, use the *maximum* parallelism from
all its parents.

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