Two commits, MAPREDUCE-5465 and HADOOP-12107 are making Terasort on YARN 10% slower.
Reduce phase with those commits ~5 mins
Reduce phase without ~3.5 mins
Average Reduce is taking 4mins, 16sec with those commits compared to 3mins, 48sec without.
long running apps may have a huge number of StatisticsData instances under FileSystem
Tasks are often killed before they exit on their own