Every memory-tiering paper and every vendor deck reports a number of the same shape: this much slow-tier traffic was avoided, against this baseline policy, on these workloads. The question underneath it — given this trace and this fast-tier capacity, what is the least slow-tier traffic any placement policy could achieve? — has no standard instrument. So nobody computes it, and a gain of 30 % over LRU is reported without anyone knowing whether the ceiling was 32 % or 300 %.

The paper What Limits Memory Tiering? An Exact Oracle and the Workload-Intrinsic Floor on Slow-Tier Traffic builds that instrument, then tests what it implies. Three registered hypotheses about it are all refuted, and the reason they are all refuted turns out to be a structural result with a sharp practical consequence.…