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courses:mapreduce-tutorial:step-8 [2012/01/25 15:00] straka |
courses:mapreduce-tutorial:step-8 [2012/01/31 15:55] (current) straka |
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====== MapReduce Tutorial : Multiple mappers, reducers and partitioning ====== | ====== MapReduce Tutorial : Multiple mappers, reducers and partitioning ====== | ||
- | In order to achieve parallelism, mappers and reducers | + | A Hadoop job, which is expected |
===== Multiple mappers ===== | ===== Multiple mappers ===== | ||
The number of mappers is determined automatically according to input files sizes. Every input file is divided into //splits//. The default split size is 32MB. Every file split is then executed by a different mapper. | The number of mappers is determined automatically according to input files sizes. Every input file is divided into //splits//. The default split size is 32MB. Every file split is then executed by a different mapper. | ||
- | The size of file split can be overridden by '' | + | The size of file split can be overridden by '' |
===== Multiple reducers ===== | ===== Multiple reducers ===== | ||
- | Then number of reducers is specified by the job, default number is one. As the outputs of reducers are not merged, there are as many output files as reducers. | + | The number of reducers is specified by the job, defaulting to one if unspecified. As the outputs of reducers are not merged, there are as many output files as reducers. |
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 [-jt cluster_master | -c cluster_size [-w sec_to_wait]] [-r number_of_reducers] |
+ | |||
+ | Optimal number of reducers is the same as the number of machines in the cluster, so that all the reducers can run in parallel at the same time. | ||
==== Partitioning ==== | ==== Partitioning ==== | ||
When there are multiple reducers, it is important how the (key, value) pairs are distributed between the reducers. | When there are multiple reducers, it is important how the (key, value) pairs are distributed between the reducers. | ||
- | By default, (key, value) pair is sent to reducer number //hash(key) modulo number_of_reducers// | + | By default, (key, value) pair is sent to a reducer number //hash(key) modulo number_of_reducers// |
+ | |||
+ | To override the default behaviour, MR job can specify a // | ||
+ | |||
+ | A partitioner should be provided if | ||
+ | * the default partitioner fails to distribute the data between reducers equally, i.e., some of the reducers operate on much more data than others. | ||
+ | * you need an explicit control of (key, value) placement. This can happen for example when [[.: | ||
+ | |||
+ | <code perl> | ||
+ | package My:: | ||
+ | use Moose; | ||
+ | with ' | ||
+ | |||
+ | sub getPartition { | ||
+ | my ($self, $key, $value, $partitions) = @_; | ||
+ | |||
+ | return $key % $partitions; | ||
+ | } | ||
+ | |||
+ | ... | ||
+ | package main; | ||
+ | use Hadoop:: | ||
+ | |||
+ | my $runner = Hadoop:: | ||
+ | ... | ||
+ | partitioner => My:: | ||
+ | ...); | ||
+ | ... | ||
+ | </ | ||
+ | |||
+ | 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 lexicographic order//. | ||
+ | |||
+ | On the other hand, the order of values belonging to one key is undefined. | ||
+ | |||
+ | ===== Exercise ===== | ||
+ | |||
+ | Run one MR job on '/ | ||
+ | wget --no-check-certificate ' | ||
+ | # NOW EDIT THE FILE | ||
+ | # $EDITOR step-8-exercise.pl | ||
+ | rm -rf step-8-out-ex; | ||
+ | less step-8-out-ex/ | ||
+ | |||
+ | ==== Solution ==== | ||
+ | You can also download the solution {{: | ||
+ | wget --no-check-certificate ' | ||
+ | # NOW VIEW THE FILE | ||
+ | # $EDITOR step-8-solution.pl | ||
+ | rm -rf step-8-out-sol; | ||
+ | less step-8-out-sol/ | ||
- | To override the default behaviour, MR job can specify a // | + | ---- |
+ | < | ||
+ | <table style=" | ||
+ | <tr> | ||
+ | <td style=" | ||
+ | <td style=" | ||
+ | <td style=" | ||
+ | </tr> | ||
+ | </ | ||
+ | </ |