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From "Abdul Qadeer (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4010) Chaging LineRecordReader algo so that it does not need to skip backwards in the stream
Date Tue, 02 Sep 2008 05:42:46 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4010?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12627581#action_12627581
] 

Abdul Qadeer commented on HADOOP-4010:
--------------------------------------

(1)  In TestLineInputFormat, as you mentioned, equal number of lines
are placed in a split, except the last one.  Due to new LineRecordReader
algorithm, the first split will process one more line as compared to other
mappers.  Due to this reason I am leaving the first split as well.

(2) About the caching test failure, I am not really sure what is happening.
I tried the LineRecordReader in isolation, for the same kind of test and it
works.  Something is going wrong in symlink stuff.  I want to debug
the test case but doing so in Eclipse gives error that WebApps are not
not classpath, when infact I have put them on the eclipse classpath.
Any suggestion to debug this test case?

Thanks,
Abdul Qadeer






> Chaging LineRecordReader algo so that it does not need to skip backwards in the stream
> --------------------------------------------------------------------------------------
>
>                 Key: HADOOP-4010
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4010
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: mapred
>    Affects Versions: 0.19.0
>            Reporter: Abdul Qadeer
>            Assignee: Abdul Qadeer
>             Fix For: 0.19.0
>
>         Attachments: Hadoop-4010.patch, Hadoop-4010_version2.patch
>
>
> The current algorithm of the LineRecordReader needs to move backwards in the stream (in
its constructor) to correctly position itself in the stream.  So it moves back one byte from
the start of its split and try to read a record (i.e. a line) and throws that away.  This
is so because it is sure that, this line would be taken care of by some other mapper.  This
algorithm is difficult and in-efficient if used for compressed stream where data is coming
to the LineRecordReader via some codecs. (Although in the current implementation, Hadoop does
not split a compressed file and only makes one split from the start to the end of the file
and so only one mapper handles it.  We are currently working on BZip2 codecs where splitting
is possible to work with Hadoop.  So this proposed change will make it possible to uniformly
handle plain as well as compressed stream.)
> In the new algorithm, each mapper always skips its first line because it is sure that,
that line would have been read by some other mapper.  So now each mapper must finish its reading
at a record boundary which is always beyond its upper split limit.  Due to this change, LineRecordReader
does not need to move backwards in the stream.

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