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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-5907) Improve getSplits() performance for fs implementations that can utilize performance gains from recursive listing
Date Mon, 12 Feb 2018 12:48:00 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-5907?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16360698#comment-16360698
] 

Steve Loughran commented on MAPREDUCE-5907:
-------------------------------------------

Catching up on this after spending many weeks in HADOOP-13786

* Performance boosts with s3 are moot because both the v1 and v2 algorithms are unreliable
with raw s3, and slow even with a consistency layer; it's obsolete.
* none of the other stores *currently* do anything with the listFiles

accordingly, we don't see any immediate benefit in moving to the API, But if/when someone
does implement the listFiles call in a store which also offers consistent metadata and O(1)
renames, then this will help, especially in v1, where the listing is done in the serialized
phase of job commit

> Improve getSplits() performance for fs implementations that can utilize performance gains
from recursive listing
> ----------------------------------------------------------------------------------------------------------------
>
>                 Key: MAPREDUCE-5907
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-5907
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>          Components: client
>    Affects Versions: 2.4.0
>            Reporter: Sumit Kumar
>            Assignee: Sumit Kumar
>            Priority: Major
>              Labels: BB2015-05-TBR
>         Attachments: MAPREDUCE-5907-2.patch, MAPREDUCE-5907-3.patch, MAPREDUCE-5907.patch
>
>
> FileInputFormat (both mapreduce and mapred implementations) use recursive listing while
calculating splits. They however do this by doing listing level by level. That means to discover
files in /foo/bar means they do listing at /foo/bar first to get the immediate children, then
make the same call on all immediate children for /foo/bar to discover their immediate children
and so on. This doesn't scale well for object store based fs implementations like s3 and swift
because every listStatus call ends up being a webservice call to backend. In cases where large
number of files are considered for input, this makes getSplits() call slow. 
> This patch adds a new set of recursive list apis that gives opportunity to the fs implementations
to optimize. The behavior remains the same for other implementations (that is a default implementation
is provided for other fs so they don't have to implement anything new). However for objectstore
based fs implementations it provides a simple change to include recursive flag as true (as
shown in the patch) to improve listing performance.



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