the default value for param "mapreduce.reduce.shuffle.input.buffer.percent"

[wzc]

Hi,
   I find the defualt value for
"mapreduce.reduce.shuffle.input.buffer.percent" is 0.7
in mapred-default.xml but  0.9 in MergeManagerImpl.java. I wonder which
default value should I choose ? I think 0.9 maybe too high ?
Any help is appreciated.

Thanks.