一句话：提出以页访问关键性为核心的分层内存在线迁移设计，替代仅按访问热度放置页面。
分层内存系统通常按访问频率（热度）放置页面，但热度无法反映页面对应用性能的真实影响。作者提出 PACT：一种在线、页粒度的分层内存设计，将性能关键性提升为首要设计原则。其核心是每页访问关键性（PAC），量化每个页面对应用性能的贡献，而非只统计访问次数。PACT 利用硬件队列占用计数器与轻量分析模型在线剖析 PAC，分解各层内存级并行，将 CPU 停顿直接归因到页面。针对 PAC 分布高度倾斜，它采用急降级与自适应提升等以 PAC 为中心的迁移策略，将关键页面放入 DRAM。在 13 个负载上，PACT 相比 7 种最新分层方案中的最佳者性能提升最高 61%，迁移次数减少最高 50 倍。
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.