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courses:mapreduce-tutorial:step-26 [2012/01/28 20:26]
straka
courses:mapreduce-tutorial:step-26 [2012/01/31 14:41] (current)
straka
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 ===== ===== Compression =====
Line 55: 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: 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''.   * **''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>

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