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courses:mapreduce-tutorial:step-26 [2012/01/27 20:11]
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| ====== MapReduce Tutorial : ====== | ====== MapReduce Tutorial : Compression and job configuration ====== |
| | |
| | |
| | |
| | ===== Compression ===== |
| | |
| | The output files can be compressed using |
| | <code java> |
| | FileOutputFormat.setCompressOutput(job, true); |
| | </code> |
| | |
| | The default compression format is ''deflate'' -- raw Zlib compression. Several other compression formats can be selected: |
| | <code java> |
| | import org.apache.hadoop.io.compress.*; |
| | |
| | ... |
| | FileOutputFormat.setOutputCompressorClass(GzipCodec.class); //.gz |
| | FileOutputFormat.setOutputCompressorClass(BZip2Codec.class); //.bz2 |
| | </code> |
| | |
| | Of course, any of these formats is decompressed transparently when the file is being read. |
| | |
| | ===== Job configuration ===== |
| | |
| | The job properties can be set: |
| | * on the command line -- the [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/util/ToolRunner.html|ToolRunner]] parses options in format ''-Dname=value''. See the [[.:step-24#running-the-job|syntax of the hadoop script]]. |
| | * using the [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/mapreduce/Job.html|Job]]''.getConfiguration()'' a [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/conf/Configuration.html|Configuration]] object is retrieved. It provides following methods: |
| | * ''String get(String name)'' -- get the value of the ''name'' property, ''null'' if it does not exist. |
| | * ''String get(String name, String defaultValue)'' -- get the value of the ''name'' property |
| | * ''getBoolean'', ''getClass'', ''getFile'', ''getFloat'', ''getInt'', ''getLong'', ''getStrings'' -- return a typed value of the ''name'' property (i.e., number, file name, class name, ...). |
| | * ''set(String name, String value)'' -- set the value of the ''name'' property to ''value''. |
| | * ''setBoolean'', ''setClass'', ''setFile'', ''setFloat'', ''setInt'', ''setLong'', ''setStrings'' -- set the typed value of the ''name'' property (i.e., number, file name, class name, ...). |
| | * in a mapper or a reducer, the ''context'' object also provides the ''getConfiguration()'' method, so the job properties can be accessed in the mappers and reducers too. |
| | |
| | Apart from already mentioned [[.:step-9#a-brief-list-of-hadoop-options|brief list of Hadoop properties]], there is one important Java-specific property: |
| | * **''mapred.child.java.opts''** with default value **''-Xmx200m''**. This property sets the Java options used for every task attempt (mapper, reducer, combiner, partitioner). The default value ''-Xmx200m'' specifies the maximum size of memory allocation pool. If your mappers and reducers need 1GB memory, use ''-Xmx1024m''. Other Java options can be found in ''man java''. |
| | |
| | |
| | ---- |
| | |
| | <html> |
| | <table style="width:100%"> |
| | <tr> |
| | <td style="text-align:left; width: 33%; "></html>[[step-25|Step 25]]: Reducers, combiners and partitioners.<html></td> |
| | <td style="text-align:center; width: 33%; "></html>[[.|Overview]]<html></td> |
| | <td style="text-align:right; width: 33%; "></html>[[step-27|Step 27]]: Running multiple Hadoop jobs in one source file.<html></td> |
| | </tr> |
| | </table> |
| | </html> |
