mapreduce.reduce.shuffle.input.buffer.percent：
mapreduce.reduce.shuffle.input.buffer.percent tells about the percentage of the reducer's heap memory to be allocated for the circular buffer to store the intermediate outputs copied from multiple mappers.
mapreduce.reduce.shuffle.memory.limit.percent：
默認爲0.25可以設置小一點0.1，這個和shuffle階段fetcher的線程個數相關，線程變多了索要佔用的資源就多了。