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--- courses:mapreduce-tutorial:step-26 [2012/01/28 20:22]
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+++ courses:mapreduce-tutorial:step-26 [2012/01/31 11:29]
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@@ Line 1: / Line 1: @@
-====== MapReduce Tutorial : Counters, compression and job configuration ======
+====== MapReduce Tutorial : Compression and job configuration ======
-===== Counters =====
-As in the Perl API, a mapper or a reducer can increment various counters by using ''context.getCounter("Group", "Name").increment(value)'':
-<code java>
-public void map(Text key, Text value, Context context) throws IOException, InterruptedException {
-  ...
-  context.getCounter("Group", "Name").increment(value);
-  ...
-}
-</code>
-The ''getCounter'' method returns a [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/mapreduce/Counter.html|Counter]] object, so if a counter is incremented frequently, the ''getCounter'' method can be called only once:
-<code java>
-public void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
-  ...
-  Counter values = context.getCounter("Reducer", "Number of values");
-  for (IntWritable value : values) {
-    ...
-    values.increment(1);
-  }
-}
-</code>
 ===== Compression =====
@@ Line 30: / Line 10: @@
 </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 =====
@@ Line 45: / Line 35: @@
 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]]: Custom data types.<html></td>
+</tr>
+</table>
+</html>

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