1. A memory comprising: a plurality of physical garbage collection units divided into different usage groups with each of the different usage groups comprising one or more different ones of the plurality of garbage collection units; and a controller configured to perform just in time garbage collection of the plurality of physical garbage collection units by: monitoring erased garbage collection unit usage in the different usage groups; scheduling garbage collection requests for submission to a particular one of the different usage groups based on the erased garbage collection unit usage within the particular usage group; assigning a higher garbage collection priority to the particular usage group if a demand for garbage collection units within the particular usage group increases relative to a demand for garbage collection units within other ones of the usage groups; and if erased garbage collection units are unavailable in the particular usage group after the particular usage group is assigned the higher garbage collection priority, increasing a variable threshold number of erased garbage collection units for the particular usage group, and wherein the controller is further configured to alter the variable threshold number of erased garbage collection units for the particular usage group in response an erased garbage collection unit count in the particular usage group passing the variable threshold number of erased garbage collection units for the particular usage group.