[ 
https://issues.apache.org/jira/browse/MAHOUT-937?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
 ]

Mat Kelcey updated MAHOUT-937:
------------------------------

    Description: 
The first pass of the collocations discovery job (as described by 
CollocDriver.generateCollocations) uses the 
org.apache.mahout.vectorizer.collocations.llr.GramKeyPartitioner partitioner. 

This partitoner has an instance variable offset that is supposed to be set by a 
call to setOffsets() but this call is never made (not sure why? is this method 
expected to be called by the Hadoop framework itself?) 

The offset not being set results in getPartition always returning 0 and so all 
intermediate data is sent to the one reducer. 

I couldn't quite understand what this partitioning was meant to be doing, but 
simply hashing the Grams primary string representation (ie without the leading 
'type' byte) does what is required...

{code}
public class GramKeyPartitioner extends Partitioner<GramKey, Gram> {

  @Override
  public int getPartition(GramKey key, Gram value, int numPartitions) {
    // exclude first byte which is the key type 
    byte[] keyBytesWithoutTypeByte = new byte[key.getPrimaryLength()-1]; 
    System.arraycopy(key.getBytes(), 1, keyBytesWithoutTypeByte, 0, 
keyBytesWithoutTypeByte.length); 
    int hash = WritableComparator.hashBytes(keyBytesWithoutTypeByte, 
keyBytesWithoutTypeByte.length);
    return (hash & Integer.MAX_VALUE) % numPartitions;    
  }
  
}
{code}




  was:
The first pass of the collocations discovery job (as described by 
CollocDriver.generateCollocations) uses the 
org.apache.mahout.vectorizer.collocations.llr.GramKeyPartitioner partitioner. 

This partitoner has an instance variable offset that is supposed to be set by a 
call to setOffsets() but this call is never made (not sure why? is this method 
expected to be called by the Hadoop framework itself?) 

The offset not being set results in getPartition always returning 0 and so all 
intermediate data is sent to the one reducer. 

I couldn't quite understand what this partitioning was meant to be doing, but 
simply hashing the Grams primary string representation (ie without the leading 
'type' byte) does what is required...

public class GramKeyPartitioner extends Partitioner<GramKey, Gram> {

  @Override
  public int getPartition(GramKey key, Gram value, int numPartitions) {
    // exclude first byte which is the key type 
    byte[] keyBytesWithoutTypeByte = new byte[key.getPrimaryLength()-1]; 
    System.arraycopy(key.getBytes(), 1, keyBytesWithoutTypeByte, 0, 
keyBytesWithoutTypeByte.length); 
    int hash = WritableComparator.hashBytes(keyBytesWithoutTypeByte, 
keyBytesWithoutTypeByte.length);
    return (hash & Integer.MAX_VALUE) % numPartitions;    
  }
  
}




    
> Collocations Job Partitioner not being configured properly
> ----------------------------------------------------------
>
>                 Key: MAHOUT-937
>                 URL: https://issues.apache.org/jira/browse/MAHOUT-937
>             Project: Mahout
>          Issue Type: Bug
>            Reporter: Mat Kelcey
>            Priority: Minor
>         Attachments: GramKeyPartitioner.java
>
>
> The first pass of the collocations discovery job (as described by 
> CollocDriver.generateCollocations) uses the 
> org.apache.mahout.vectorizer.collocations.llr.GramKeyPartitioner partitioner. 
> This partitoner has an instance variable offset that is supposed to be set by 
> a call to setOffsets() but this call is never made (not sure why? is this 
> method expected to be called by the Hadoop framework itself?) 
> The offset not being set results in getPartition always returning 0 and so 
> all intermediate data is sent to the one reducer. 
> I couldn't quite understand what this partitioning was meant to be doing, but 
> simply hashing the Grams primary string representation (ie without the 
> leading 'type' byte) does what is required...
> {code}
> public class GramKeyPartitioner extends Partitioner<GramKey, Gram> {
>   @Override
>   public int getPartition(GramKey key, Gram value, int numPartitions) {
>     // exclude first byte which is the key type 
>     byte[] keyBytesWithoutTypeByte = new byte[key.getPrimaryLength()-1]; 
>     System.arraycopy(key.getBytes(), 1, keyBytesWithoutTypeByte, 0, 
> keyBytesWithoutTypeByte.length); 
>     int hash = WritableComparator.hashBytes(keyBytesWithoutTypeByte, 
> keyBytesWithoutTypeByte.length);
>     return (hash & Integer.MAX_VALUE) % numPartitions;    
>   }
>   
> }
> {code}

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