How zone_balane_ratio is decided?

["amey d inamdar" <iamey@rediffmail.com>]

Hello there,
   In a code of zone allocator free_area_init_core() minimum pages, low pages and high pages for any zone are decided using

mask = realpages/zone_balance_ratio
minpages=mask
low pages=2*mask 
high pages=3*mask

how zone balance ratio is decided for each zone? What is its significance? Please tell as soon as possible. I am unable to go further reading code. 
 Thanking you in anticipation

- Amey 

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