Once upon a time, computers just had one type memory, so memory within
a given system was interchangeable. The arrival of non-uniform
memory access (NUMA) systems complicated the situation significantly; now
some memory was faster to access than the rest, and memory-management
algorithms had to adapt or performance would suffer. But NUMA was just the
start; today’s tiered-memory systems, which may include several tiers of
memory with different performance characteristics, are adding new
challenges. A couple of relevant patch sets currently under review help to
illustrate the types of problems that will have to be solved.

Source: LWN.net – [$] Two memory-tiering patch sets