I was wondering what scheduling algorithm is used in Hadoop (version
0.20.2 in particular), for a ReduceTask to determine in what order it is
supposed to read the map outputs from the various mappers that have been
run? In particular, suppose we have 10maps called map1, map2,....,
map10. and say 2 reducers r1 and r2. Which map's output does r1/r2 read
Also, suppose that the mapred.reduce.parallel.copies is set to 5. Then
do both r1 and r2 read from 5 map outputs concurrently?