Tiered-memory systems are built with multiple types of memory, each of
which has different performance characteristics.  In addition to the usual
DRAM, a tiered system might also provide faster high-bandwidth memory or
slower CXL memory.  On these systems, the placement of memory allocations
has a significant effect on the performance that a workload will obtain.
While work on tiered-memory improvements has been ongoing for years, it
feels like the pace has slowed a bit recently.  Even so, there are a few
efforts underway, but they are facing questions about whether the tiering
design makes sense.

https://lwn.net/Articles/1092001/
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