cube buiding wait in map sort stage

[??????]

Hi,
my cube contains sigle table,it's size about 70M,
but it contains 2 TopN measures.
when buiding cube,it alway wait in 
      #8 Step Name: Build N-Dimension Cuboid Data : 1-Dimension
 wait in this step's map sort stage for 1 hour until map task timeout,then 
container kill it.
excuse me, how to deal with this problem..