[ https://issues.jboss.org/browse/ISPN-4372?page=com.atlassian.jira.plugin.... ] Dan Berindei commented on ISPN-4372: ------------------------------------ I don't think it's fair to say that M/R performance depends on the input cache's value size. There is also another factor involved: {{WordCountMapperEmitPerValue}} coalesces all the occurrences of the same word in the value, so the number of intermediary values {{emit()}}ed by the mapper decreases a lot as the cache value size increases. We should confirm that the same behaviour occurs with the basic {{WordCountMapper}}, otherwise it would be more fair to say that M/R performance is depended on the number of the number of intermediary entries emitted by the Mapper, which is to be expected. However, it is surprising that M/R throughput doesn't continue increasing as the cache value size increases (and the number of intermediary values decreases) past 32KB. This is definitely worth investigating. -- This message was sent by Atlassian JIRA (v6.2.3#6260)