Flat Memory

configname: CONFIG_FLATMEM_MANUAL

Memory model
└─>Flat Memory
In linux kernel since version 2.6.13 (release Date: 2005-08-28)  
This option is best suited for non-NUMA systems with
flat address space. The FLATMEM is the most efficient
system in terms of performance and resource consumption
and it is the best option for smaller systems.

For systems that have holes in their physical address
spaces and for features like NUMA and memory hotplug,
choose "Sparse Memory".

If unsure, choose this option (Flat Memory) over any other.