[$] Two memory-tiering patch sets

Once upon a time, computers just had one type memory, so memory withina given system was interchangeable. The arrival of non-uniformmemory access (NUMA) systems complicated the situation significantly; nowsome memory was faster to access than the rest, and memory-managementalgorithms had to adapt or performance would suffer. But NUMA was just thestart; today's tiered-memory systems, which may include several tiers ofmemory with different performance characteristics, are adding newchallenges. A couple of relevant patch sets currently under review help toillustrate the types of problems that will have to be solved.