GitHub user RadixSeven opened a pull request:
https://github.com/apache/spark/pull/3936
Document that groupByKey will OOM for large keys
This pull request is my own work and I license it under Spark's open-source
license.
This contribution is an improvement to the documentation. I documented that
the maximum number of values per key for groupByKey is limited by available RAM
(see [Datablox][datablox link] and [the spark mailing list][list link]).
Just saying that better performance is available is not sufficient.
Sometimes you need to do a group-by - your operation needs all the items
available in order to complete. This warning explains the problem.
[datablox link]:
http://databricks.gitbooks.io/databricks-spark-knowledge-base/content/best_practices/prefer_reducebykey_over_groupbykey.html
[list link]:
http://apache-spark-user-list.1001560.n3.nabble.com/Understanding-RDD-GroupBy-OutOfMemory-Exceptions-tp11427p11466.html
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/RadixSeven/spark better-group-by-docs
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/spark/pull/3936.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #3936
----
commit 238e81b926779ccf788d97c8a78306811dfbf2e9
Author: Eric Moyer <[email protected]>
Date: 2015-01-07T20:49:33Z
Doc that groupByKey will OOM for large keys
Documented that maximum number of values per key for groupByKey is limited
by available RAM (see [Datablox][datablox link] and [the spark mailing
list][list link]).
Just saying that better performance is available is not sufficient.
Sometimes you need to do a group-by - your operation needs all the items
available in order to complete. This warning explains the problem.
[datablox link]:
http://databricks.gitbooks.io/databricks-spark-knowledge-base/content/best_practices/prefer_reducebykey_over_groupbykey.html
[list link]:
http://apache-spark-user-list.1001560.n3.nabble.com/Understanding-RDD-GroupBy-OutOfMemory-Exceptions-tp11427p11466.html
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