hadoop - Execuation time of MapReduce with Combiner -


i have mapreduce job reads file collect words 5 characters or less , start upper case letter using first letter key. ran job twice once without using combiner , second time combiner. compared execution times , noticed using combiner increased execution time. know causes increase in time , case when using combiner?

thank you

as name suggests combiners should used when there possibility combine. generally, shall applied on functions commutative(a.b = b.a) , associative {a.(b.c) = (a.b).c} . caution, there no hard , fast rule has commutative , associative. combiners may operate on subset of keys , values or may not execute @ all. if there less amount of duplicate keys in mapper output @ times using combiners may backfire , instead become useless burden. use combiners when there enough scope of combining.

quoting chuck lam's 'hadoop in action':

"a combiner doesn't improve performance. should monitor job's behavior see if number of records outputted combiner meaningfully less number of records going in. reduction must justify execution time of running combiner. "

hence, in case possible number of subsets can combined less in ratio, hence overhead of running combiner increases execution time.

read more article here.


Comments

Post a Comment

Popular posts from this blog

java.util.scanner - How to read and add only numbers to array from a text file -

i'm learning java , have question regarding reading file want read numbers file contains strings well. here example of file: 66.56 "3 java 3-43 5-42 2.1 1 and here coding: public class test { public static void main (string [] args){ if (0 < args.length) { file x = new file(args[0]); try{ scanner in = new scanner( new fileinputstream(x)); arraylist<double> test = new arraylist<>(); while(in.hasnext()){ if(in.hasnextdouble()){ double f=in.nextdouble(); test.add(f);} else {in.next();} } catch(ioexception e) { system.err.println("exception during reading: " + e); } } my problem add 66.56,2.1 , 1 doesn't add 3 after "3 or ignores 3-43 , 5-42 can tell me how skip strings , add doubles here? thanks all said 3 ie; "3, 3-43 , 4-42 strings either u read string , split , check number @ " , - or put in space between characters , integers. jvm after compilation tre...
Read more

rewrite - Trouble with Wordpress multiple custom querystrings -

i'm working on adding multiple querystring , doesn't seem work. i've found codes seems work others might i'm missing something. i want like: http://mydomain.com/board/?getyear=2013&getsometext2=sometext to http://mydomain.com/board/2013/sometext/ i added code below functions.php file. doesn't seem work should though. this works. echoing querystring gets both variable without problem. http://mydomain.com/board/2013/sometext/ but doesn't. tested querystring vis isset() seems nothing being set/get http://mydomain.com/board/2013/ function add_query_vars($vars) { $vars[] = "getyear"; $vars[] = "getsometext2"; return $vars; } add_filter('query_vars', 'add_query_vars'); function add_rewrite_rules($arules) { $anewrules = array('board/([^/]+)/([^/]+)/?$' => 'index.php?pagename=board&getyear=$matches[1]&getsometext2=$matches[2]'); $arules = $a...
Read more