[jira] [Resolved] (SPARK-3526) Docs section on data locality

["Patrick Wendell (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SPARK-3526?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Patrick Wendell resolved SPARK-3526.
------------------------------------
       Resolution: Fixed
    Fix Version/s: 1.2.0

Thanks [~aash] for contributing.

> Docs section on data locality
> -----------------------------
>
>                 Key: SPARK-3526
>                 URL: https://issues.apache.org/jira/browse/SPARK-3526
>             Project: Spark
>          Issue Type: Documentation
>          Components: Documentation
>    Affects Versions: 1.0.2
>            Reporter: Andrew Ash
>            Assignee: Andrew Ash
>             Fix For: 1.2.0
>
>
> Several threads on the mailing list have been about data locality and how to interpret PROCESS_LOCAL, NODE_LOCAL, RACK_LOCAL, etc.  Let's get some more details in the docs on this concept so we can point future questions there.
> A couple people appreciated the below description of locality so it could be a good starting point:
> {quote}
> The locality is how close the data is to the code that's processing it.  PROCESS_LOCAL means data is in the same JVM as the code that's running, so it's really fast.  NODE_LOCAL might mean that the data is in HDFS on the same node, or in another executor on the same node, so is a little slower because the data has to travel across an IPC connection.  RACK_LOCAL is even slower -- data is on a different server so needs to be sent over the network.
> Spark switches to lower locality levels when there's no unprocessed data on a node that has idle CPUs.  In that situation you have two options: wait until the busy CPUs free up so you can start another task that uses data on that server, or start a new task on a farther away server that needs to bring data from that remote place.  What Spark typically does is wait a bit in the hopes that a busy CPU frees up.  Once that timeout expires, it starts moving the data from far away to the free CPU.
> {quote}



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