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courses:mapreduce-tutorial:step-8 [2012/01/25 15:29] straka |
courses:mapreduce-tutorial:step-8 [2012/01/25 18:37] straka |
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To use multiple reducers, the MR job must be executed by a cluster (even with one computer), not locally. The number of reducers is specified by '' | To use multiple reducers, the MR job must be executed by a cluster (even with one computer), not locally. The number of reducers is specified by '' | ||
- | perl script.pl [-j cluster_master | -c cluster_size [-w sec_to_wait]] [-r number_of_reducers] | + | perl script.pl |
==== Partitioning ==== | ==== Partitioning ==== | ||
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A MR job must have a reducer if it specifies a partitioner. Also, the partitioner is not called if there is only one reducer. | A MR job must have a reducer if it specifies a partitioner. Also, the partitioner is not called if there is only one reducer. | ||
+ | |||
+ | ===== The order of keys during reduce ===== | ||
+ | It is guaranteed that every reducer processes the keys in //ascending order//. | ||
+ | |||
+ | On the other hand, when processing one key, the order of its values is undefined. | ||
===== Example ===== | ===== Example ===== | ||
- | Run a MR job on '/ | + | Run a MR job on '/ |
{{: | {{: |