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From Adeel Qureshi <adeelmahm...@gmail.com>
Subject Re: secondary sort - number of reducers
Date Fri, 30 Aug 2013 00:55:46 GMT
okay so when i specify the number of reducers e.g. in my example i m using
4 (for a much smaller data set) it works if I use a single column in my
composite key .. but if I add multiple columns in the composite key
separated by a delimi .. it then throws the illegal partition error (keys
before the pipe are group keys and after the pipe are the sort keys and my
partioner only uses the group keys

java.io.IOException: Illegal partition for *Atlanta:GA|Atlanta:GA:1:Adeel*(-1)
        at
org.apache.hadoop.mapred.MapTask$MapOutputBuffer.collect(MapTask.java:1073)
        at
org.apache.hadoop.mapred.MapTask$NewOutputCollector.write(MapTask.java:691)
        at
org.apache.hadoop.mapreduce.TaskInputOutputContext.write(TaskInputOutputContext.java:80)
        at com.att.hadoop.hivesort.HSMapper.map(HSMapper.java:39)
        at com.att.hadoop.hivesort.HSMapper.map(HSMapper.java:1)
        at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:144)
        at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:764)
        at org.apache.hadoop.mapred.MapTask.run(MapTask.java:370)
        at org.apache.hadoop.mapred.Child$4.run(Child.java:255)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.security.auth.Subject.doAs(Subject.java:396)
        at
org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1136)
        at org.apache.hadoop.mapred.Child.main(Child.java:249)


public int getPartition(Text key, HCatRecord record, int numParts) {
//extract the group key from composite key
String groupKey = key.toString().split("\\|")[0];
return groupKey.hashCode() % numParts;
}


On Thu, Aug 29, 2013 at 8:31 PM, Shekhar Sharma <shekhar2581@gmail.com>wrote:

> No...partitionr decides which keys should go to which reducer...and
> number of reducers you need to decide...No of reducers depends on
> factors like number of key value pair, use case etc
> Regards,
> Som Shekhar Sharma
> +91-8197243810
>
>
> On Fri, Aug 30, 2013 at 5:54 AM, Adeel Qureshi <adeelmahmood@gmail.com>
> wrote:
> > so it cant figure out an appropriate number of reducers as it does for
> > mappers .. in my case hadoop is using 2100+ mappers and then only 1
> reducer
> > .. since im overriding the partitioner class shouldnt that decide how
> > manyredeucers there should be based on how many different partition
> values
> > being returned by the custom partiotioner
> >
> >
> > On Thu, Aug 29, 2013 at 7:38 PM, Ian Wrigley <ian@cloudera.com> wrote:
> >>
> >> If you don't specify the number of Reducers, Hadoop will use the default
> >> -- which, unless you've changed it, is 1.
> >>
> >> Regards
> >>
> >> Ian.
> >>
> >> On Aug 29, 2013, at 4:23 PM, Adeel Qureshi <adeelmahmood@gmail.com>
> wrote:
> >>
> >> I have implemented secondary sort in my MR job and for some reason if i
> >> dont specify the number of reducers it uses 1 which doesnt seems right
> >> because im working with 800M+ records and one reducer slows things down
> >> significantly. Is this some kind of limitation with the secondary sort
> that
> >> it has to use a single reducer .. that kind of would defeat the purpose
> of
> >> having a scalable solution such as secondary sort. I would appreciate
> any
> >> help.
> >>
> >> Thanks
> >> Adeel
> >>
> >>
> >>
> >> ---
> >> Ian Wrigley
> >> Sr. Curriculum Manager
> >> Cloudera, Inc
> >> Cell: (323) 819 4075
> >>
> >
>

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