Modern computing systems can feature multiple types of memory that differ in
their performance characteristics. The most common example is NUMA
architectures, where memory attached to the local node is faster to access
than memory on other nodes. Recently, persistent memory has
started appearing in deployed systems as well; this type of memory is
byte-addressable like DRAM, but it is available in larger sizes and is slower to
access, especially for
writes. This new memory type makes memory allocation even more complicated
for the kernel, driving the need for a method to better manage multiple types of
memory in one system.

Source: LWN.net – [$] Top-tier memory management