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From "Chris Douglas (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 11:17:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4010?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12627626#action_12627626
] 

Chris Douglas commented on HADOOP-4010:
---------------------------------------

bq. Due to new LineRecordReader algorithm, the first split will process one more line as compared
to other mappers

That's probably not going to be acceptable to users of NLineInputFormat. Users employing N
formatted lines to initialize and run a mapper may find their jobs no longer work if the input
is offset or if a map receives N+1 lines. If this is necessary for the new algorithm, rewriting
or somehow accommodating this case may be required.

bq. Something is going wrong in symlink stuff. I want to debug the test case but doing so
in Eclipse gives error[...]

Sorry, I don't use eclipse. It looks like the symlink resolution is working; both cache files
are picked up as arguments from the input file. At a glance, what appears to be going wrong
is newline detection or propagation between invocations of cat from xargs, a bad interaction
with streaming (it also uses LineRecordReader, IIRC), or input exercising an edge case for
LineRecordReader. Since it sounds like you've ruled out the latter, have you tried running
a streaming job like the one in the testcase? I suspect the cache isn't necessary to reproduce
this.

> 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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