From Genome Analysis Wiki
=== MaCH-Admix ===
If you are doing imputation only once (think twice if this is really true) or under an immediate time pressure, you can use MaCH-Admix, which does not require pre-phased data and takes ~1/7 of the computing time of
the original MaCH. For large dataset, we recommend breaking the genome into small overlapping segments ( [http://genome.sph.umich.edu/wiki/MaCH_FAQ#Divide_and_Conquer Divide-and-Conquer]). Details see [http://www.unc.edu/~yunmli/MaCH-Admix/ MaCH-Admix].
=== Divide and Conquer ===