PACT: A Criticality-First Design for Tiered Memory

Tiered memory systems typically place pages based on access frequency (hotness), yet frequency alone fails to capture the true performance impact. We present PACT, an online, page-granular tiered memory design that elevates performance criticality to a first-class design principle. At its core is Per-page Access Criticality (PAC), a fine-grained metric that quantifies each page's contribution to application performance rather than merely counting accesses. PACT profiles PAC online using a lightweight analytical model that uniquely decomposes per-tier memory-level parallelism via hardware queue occupancy counters, enabling direct CPU stall attribution to individual pages. To handle highly skewed PAC distributions, PACT employs PAC-centric migration policies: eager demotion and adaptive promotion, to dynamically place performance-critical pages in DRAM. Across 13 workloads, PACT achieves up to 61% performance improvement over the best of 7 state-of-the-art tiering designs with up to 50× fewer migrations.