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--- courses:mapreduce-tutorial:step-11 [2012/01/25 19:38]
straka
+++ courses:mapreduce-tutorial:step-11 [2012/01/30 16:45]
dusek
@@ Line 1: / Line 1: @@
-====== MapReduce Tutorial : Initialization and cleanup of MR tasks ======
+====== MapReduce Tutorial : Initialization and cleanup of MR tasks, performance of combiners ======
 During the mapper or reducer task execution the following steps take place:
@@ Line 15: / Line 15: @@
 ===== Exercise =====
-Improve the {{:courses:mapreduce-tutorial:step-5-solution1.txt|wc-without-combiner.pl}} script by manually combining the results in the Mapper -- create a hash of word occurrences, fill it during the ''map'' calls and output the (key, value) pairs in ''cleanup'' method.
+Improve the {{:courses:mapreduce-tutorial:step-5-solution1.txt|step-11-wc-without-combiner.pl}} script by manually combining the results in the Mapper -- create a hash of word occurrences, populate it during the ''map'' calls without outputting results and finally output all (key, value) pairs in the ''cleanup'' method.
-Then measure the improvement.
+  wget --no-check-certificate 'https://wiki.ufal.ms.mff.cuni.cz/_media/courses:mapreduce-tutorial:step-5-solution1.txt' -O 'step-11-wc-without-combiner.pl'
+  # NOW EDIT THE FILE
+  # $EDITOR step-11-exercise.pl
+  rm -rf step-11-out-wout; time perl step-11-wc-without-combiner.pl run /home/straka/wiki/cs-text-medium/ step-11-out-wout
+  less step-11-out-wout/part-*
+Measure the improvement.
-{{:courses:mapreduce-tutorial:step-11-solution.txt|Solution.pl}}
+==== Solution ====
+You can also download the solution {{:courses:mapreduce-tutorial:step-11-solution.txt|step-11-wc-with-perl-hash.pl}} and check the correct output.
-===== Combiners and Perl API =====
+  wget --no-check-certificate 'https://wiki.ufal.ms.mff.cuni.cz/_media/courses:mapreduce-tutorial:step-11-solution.txt' -O 'step-11-wc-with-perl-hash.pl'
+  # NOW VIEW THE FILE
+  # $EDITOR step-11-solution.pl
+  rm -rf step-11-out-with-hash; time perl step-11-wc-with-perl-hash.pl run /home/straka/wiki/cs-text-medium/ step-11-out-with-hash
+  less step-11-out-with-hash/part-*
-As you have seen, the combiners are not efficient when using the Perl API. This is a problem of Perl -- reading and writing the (key, value) pairs is very slow and a combiner does not help -- it in fact increases the number of (key, value) pairs that need to be read/written.
+===== Combiners and Perl API performance =====
+As you have seen, the combiners are not very efficient when using the Perl API. This is a problem of the Perl API -- reading and writing the (key, value) pairs is relatively slow and a combiner does not help -- it in fact increases the number of (key, value) pairs that need to be read/written.
 This is even more obvious with larger input data:
-^ Script ^ Seconds to complete on ''/home/straka/wiki/cs-text'' ^
+^ Script ^ Time to complete on ''/home/straka/wiki/cs-text'' ^ Commands ^
-| {{:courses:mapreduce-tutorial:step-5-solution1.txt|wc-without-combiner.pl}} | |
+| {{:courses:mapreduce-tutorial:step-5-solution1.txt|step-11-wc-without-combiner.pl}} | 5mins, 4sec | <html><pre>wget --no-check-certificate 'https://wiki.ufal.ms.mff.cuni.cz/_media/courses:mapreduce-tutorial:step-5-solution1.txt' -O 'step-11-wc-without-combiner.pl'<br>rm -rf step-11-out-wout; time perl step-11-wc-without-combiner.pl run /home/straka/wiki/cs-text/ step-11-out-wout</pre></html> |
-| {{:courses:mapreduce-tutorial:step-10.txt|wc-with-combiner.pl}} | |
+| {{:courses:mapreduce-tutorial:step-10.txt|step-11-wc-with-combiner.pl}} | 5mins, 33sec  | <html><pre>wget --no-check-certificate 'https://wiki.ufal.ms.mff.cuni.cz/_media/courses:mapreduce-tutorial:step-10.txt' -O 'step-11-wc-with-combiner.pl'<br>rm -rf step-11-out-with-combiner; time perl step-11-wc-with-combiner.pl run /home/straka/wiki/cs-text/ step-11-out-with-combiner</pre></html>|
-| | |
+| {{:courses:mapreduce-tutorial:step-11-solution.txt|step-11-wc-with-perl-hash.pl}} | 2mins, 24sec | <html><pre>wget --no-check-certificate 'https://wiki.ufal.ms.mff.cuni.cz/_media/courses:mapreduce-tutorial:step-11-solution.txt' -O 'step-11-wc-with-perl-hash.pl'<br>rm -rf step-11-out-with-perl-hash; time perl step-11-wc-with-perl-hash.pl run /home/straka/wiki/cs-text/ step-11-out-with-perl-hash</pre></html>|
+For comparison, here are times of Java solutions:
+^ Program ^ Time to complete on ''/home/straka/wiki/cs-text'' ^ Size of map output ^
+| Wordcount without combiner | 2mins, 26sec | 367MB |
+| Wordcount with combiner | 1min, 51sec | 51MB |
+| Wordcount with hash in mapper | 1min, 14sec | 51MB |
+Using the combiner is beneficial, although combining the word occurrences in mapper manually is still faster.
+----
+<html>
+<table style="width:100%">
+<tr>
+<td style="text-align:left; width: 33%; "></html>[[step-10|Step 10]]: Combiners.<html></td>
+<td style="text-align:center; width: 33%; "></html>[[.|Overview]]<html></td>
+<td style="text-align:right; width: 33%; "></html>[[step-12|Step 12]]: Additional output from mappers and reducers.<html></td>
+</tr>
+</table>
+</html>

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