Hello Andrzej, Thanks for the response. I am using 3 machines to fetch right now. NumFetchers=36, #map tasks=12, #reduce tasks=6 and mapred.tasktracker.tasks.maximum=4
I will try changing #map tasks to 30 and # reduce tasks to 18 (1.5*3*4) for the next fetch. I am not sure about what value I should set the numfetchers parameter to. I believe generate process will create 36 different fetchlists here, since numFetchers is 36. Now, even though the #maps was set to 12 in my mapred-default.xml, it is overridden by the numFetchers param while fetching, so I have 36 map tasks. Finally, the # of reduce tasks determines the number of output partitions for the fetcher. What is the best way to figure out what I should set numFetchers to? Thanks, -vishal. -----Original Message----- From: Andrzej Bialecki [mailto:[EMAIL PROTECTED] Sent: Thursday, October 12, 2006 6:14 PM To: [email protected] Subject: Re: IdentityReducer while fetching Vishal Shah wrote: > Hi, > > While running a fetch on 3M urls with -noParsing option set, I noticed > that the reduce is taking too long. Since the reducer class is the > IdentityReducer class in this case, couldn't hadoop handle it internally > by setting the output path of map directly to the final output path? Or, > do a simple rename of the temp output directory to the final output > directory? > > For the reduce phase, it seems that the copy is unnecessary in this > case. I am unfamiliar with the details of Hadoop, so maybe there is a > strong reason to do things the way they are done right now, or maybe I > am mistaken about how they are done. Can the experts please throw some > light on this? > > Unfortunately, it's not possible - partial outputs from map tasks are basically in random key order, and need to be sorted in order to produce output MapFile-s. IdentityReducer indeed does nothing, but the ReduceTask framework is doing the sorting ... Do you think the copy/sort phase is a performance bottleneck in your case? You could try changing the number of reduce tasks, or perhaps the number of tasks per tasktracker - please see http://wiki.apache.org/lucene-hadoop/HowManyMapsAndReduces for more details. -- Best regards, Andrzej Bialecki <>< ___. ___ ___ ___ _ _ __________________________________ [__ || __|__/|__||\/| Information Retrieval, Semantic Web ___|||__|| \| || | Embedded Unix, System Integration http://www.sigram.com Contact: info at sigram dot com ------------------------------------------------------------------------- Using Tomcat but need to do more? Need to support web services, security? Get stuff done quickly with pre-integrated technology to make your job easier Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642 _______________________________________________ Nutch-general mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/nutch-general
