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From "Chris Douglas (JIRA)" <j...@apache.org>
Subject [jira] Updated: (MAPREDUCE-64) Map-side sort is hampered by io.sort.record.percent
Date Wed, 09 Dec 2009 08:19:18 GMT

     [ https://issues.apache.org/jira/browse/MAPREDUCE-64?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Chris Douglas updated MAPREDUCE-64:
-----------------------------------

    Attachment: M64-4.patch

Merged with trunk.

Thanks for running the coverage tool, Todd.

bq. Since we're using the Local Runner for these tests, it's all a single partition

TestMiniMRDFSSort is the only test I know of that uses multiple partitions. I hope that will
change after MAPREDUCE-1050.

bq. Line 1097 ("if (bufindex + headbytelen < avail) {" in void reset()) is always true
in our tests. We should get a test case to exercise the other half of this branch.

This is awkward to write reliably, the patch already adds a lot of very specific test code,
and the tested path is the more aggressive one. I'm OK leaving this for now.

bq. We don't current run any tests with job.getCompressMapOutput returning true
bq.  Line 1365 (kvstart >= kvend ternary in sortAndSpill) is always true.

Added cases to TestMapCollection.

bq. can you put in a small comment describing the synchronization policy for the various offsets?
Those used to be volatile and now they're under a lock, so it should be good to note that
in the code.

The patch- and this issue- adds a fair amount of documentation, including calling out some
of the places where locking and visibility are non-trivial. The {{volatile}} modifier in the
existing code was paranoid, if not redundant.

> Map-side sort is hampered by io.sort.record.percent
> ---------------------------------------------------
>
>                 Key: MAPREDUCE-64
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-64
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>            Reporter: Arun C Murthy
>            Assignee: Chris Douglas
>         Attachments: M64-0.patch, M64-0i.png, M64-1.patch, M64-1i.png, M64-2.patch, M64-2i.png,
M64-3.patch, M64-4.patch
>
>
> Currently io.sort.record.percent is a fairly obscure, per-job configurable, expert-level
parameter which controls how much accounting space is available for records in the map-side
sort buffer (io.sort.mb). Typically values for io.sort.mb (100) and io.sort.record.percent
(0.05) imply that we can store ~350,000 records in the buffer before necessitating a sort/combine/spill.
> However for many applications which deal with small records e.g. the world-famous wordcount
and it's family this implies we can only use 5-10% of io.sort.mb i.e. (5-10M) before we spill
inspite of having _much_ more memory available in the sort-buffer. The word-count for e.g.
results in ~12 spills (given hdfs block size of 64M). The presence of a combiner exacerbates
the problem by piling serialization/deserialization of records too...
> Sure, jobs can configure io.sort.record.percent, but it's tedious and obscure; we really
can do better by getting the framework to automagically pick it by using all available memory
(upto io.sort.mb) for either the data or accounting.

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