Currently io.sort.record.percent is a fairly obscure, per-job configurable, expert-level parameter which controls how much accounting space is available for records in the map-side sort buffer (io.sort.mb). Typically values for io.sort.mb (100) and io.sort.record.percent (0.05) imply that we can store ~350,000 records in the buffer before necessitating a sort/combine/spill.
However for many applications which deal with small records e.g. the world-famous wordcount and it's family this implies we can only use 5-10% of io.sort.mb i.e. (5-10M) before we spill inspite of having much more memory available in the sort-buffer. The word-count for e.g. results in ~12 spills (given hdfs block size of 64M). The presence of a combiner exacerbates the problem by piling serialization/deserialization of records too...
Sure, jobs can configure io.sort.record.percent, but it's tedious and obscure; we really can do better by getting the framework to automagically pick it by using all available memory (upto io.sort.mb) for either the data or accounting.