Hi, this 0-33-66-100% phases are really confusing to beginners. We see that in our training classes. The output should be more verbose, such as breaking down the phases into seperate progress numbers.
Does that make sense? Am 19.09.2011 um 06:17 schrieb Arun C Murthy: > Nan, > > The 'phase' is implicitly understood by the 'progress' (value) made by the > map/reduce tasks (see o.a.h.mapred.TaskStatus.Phase). > > For e.g. > Reduce: > 0-33% -> Shuffle > 34-66% -> Sort (actually, just 'merge', there is no sort in the reduce since > all map-outputs are sorted) > 67-100% -> Reduce > > With 0.23 onwards the Map has phases too: > 0-90% -> Map > 91-100% -> Final Sort/merge > > Now,about starting reduces early - this is done to ensure shuffle can proceed > for completed maps while rest of the maps run, there-by pipelining shuffle > and map completion. There is a 'reduce slowstart' feature to control this - > by default, reduces aren't started until 5% of maps are complete. Users can > set this higher. > > Arun > > On Sep 18, 2011, at 7:24 PM, Nan Zhu wrote: > >> Hi, all >> >> recently, I was hit by a question, "how is a hadoop job divided into 2 >> phases?", >> >> In textbooks, we are told that the mapreduce jobs are divided into 2 phases, >> map and reduce, and for reduce, we further divided it into 3 stages, >> shuffle, sort, and reduce, but in hadoop codes, I never think about >> this question, I didn't see any variable members in JobInProgress class >> to indicate this information, >> >> and according to my understanding on the source code of hadoop, the reduce >> tasks are unnecessarily started until all mappers are finished, in >> constract, we can see the reduce tasks are in shuffle stage while there are >> mappers which are still in running, >> So how can I indicate the phase which the job is belonging to? >> >> Thanks >> -- >> Nan Zhu >> School of Electronic, Information and Electrical Engineering,229 >> Shanghai Jiao Tong University >> 800,Dongchuan Road,Shanghai,China >> E-Mail: [email protected] > > -- Kai Voigt [email protected]
