[
https://issues.apache.org/jira/browse/HIVE-3997?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13577920#comment-13577920
]
Gopal V commented on HIVE-3997:
-------------------------------
Tested with 6x12 slots for the tasks, the results remain relatively the same
With the old code, it took 44504 milliseconds, the dist-cache run took 52265
milliseconds.
|| client-hash || dist-cache ||
|GC time elapsed (ms)=25444 | GC time elapsed (ms)=95839 |
|CPU time spent (ms)=399890 | CPU time spent (ms)=894940 |
The advantages of implementing this seem to be rather slim to potentially
negative.
> Use distributed cache to cache/localize dimension table & filter it in map
> task setup
> -------------------------------------------------------------------------------------
>
> Key: HIVE-3997
> URL: https://issues.apache.org/jira/browse/HIVE-3997
> Project: Hive
> Issue Type: Improvement
> Reporter: Gopal V
> Assignee: Gopal V
>
> The hive clients are not always co-located with the hadoop/hdfs cluster.
> This means that the dimension table filtering, when done on the client side
> becomes very slow. Not only that, the conversion of the small tables into
> hashtables has to be done every single time a query is run with different
> filters on the big table.
> That entire hashtable has to be part of the job, which involves even more
> HDFS writes from the far client side.
> Using the distributed cache also has the advantage that the localized files
> can be kept between jobs instead of firing off an HDFS read for every query.
> Moving the operator pipeline for the hash generation into the map task itself
> has perhaps a few cons.
> The map task might OOM due to this change, but it will take longer to recover
> until all the map attempts fail, instead of being conditional on the client.
> The client has no idea how much memory the hashtable needs and has to rely on
> the disk sizes (compressed sizes, perhaps) to determine if it needs to fall
> back onto a reduce-join instead.
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators
For more information on JIRA, see: http://www.atlassian.com/software/jira
