Modern enterprise servers are increasingly embracing tiered memory systems with a combination of low latency DRAMs and large capacity but high latency non-volatile main memories (NVMMs) such as Intel's Optane DC PMM. Prior works have focused on efficient placement and migration of data on a tiered memory system, but have not studied the optimal placement of page tables. Explicit and efficient placement of page tables is crucial for large memory footprint applications with high TLB miss rates because they incur dramatically higher page walk latency when page table pages are placed in NVMM. We show that (i) page table pages can end up on NVMM even when enough DRAM memory is available and (ii) page table pages that spill over to NVMM due to DRAM memory pressure are not migrated back later when memory is available in DRAM. We study the performance impact of page table placement in a tiered memory system and propose an efficient and transparent page table management technique that (i) applies different placement policies for data and page table pages, (ii) introduces a differentiating policy for page table pages by placing a small but critical part of the page table in DRAM, and (iii) dynamically and judiciously manages the rest of the page table by transparently migrating the page table pages between DRAM and NVMM. Our implementation on a real system equipped with Intel's Optane NVMM running Linux reduces the page table walk cycles by 12% and total cycles by 20% on an average. This improves the runtime by 20% on an average for a set of synthetic and real-world large memory footprint applications when compared with various default Linux kernel techniques.

中文翻译：

---

异构内存系统的页表管理

现代企业服务器越来越多地采用低延迟DRAM和大容量，高延迟非易失性主存储器（NVMM）（例如英特尔的Optane DC PMM）相结合的分层存储系统。先前的工作集中在分层存储系统上数据的有效放置和迁移上，但是还没有研究页表的最佳放置。对于具有高TLB丢失率的大型内存占用应用程序，页表的显式和有效放置至关重要，因为当将页表页放入NVMM时，它们会导致更高的页遍历等待时间。我们显示，即使有足够的DRAM内存可用，（i）页表页面也可以最终在NVMM上显示；并且（ii）由于DRAM内存压力而溢出到NVMM的页表页面在DRAM中可用内存时不会稍后迁移回去。我们研究了分层内存系统中页表放置的性能影响，并提出了一种有效且透明的页表管理技术，该技术（i）为数据和页表页面应用不同的放置策略，（ii）为页表页面引入差异化策略通过将页面表的一小部分但至关重要的部分放在DRAM中，以及（iii）通过透明地在DRAM和NVMM之间迁移页面表页面来动态，明智地管理页面表的其余部分。我们在装有运行Linux的英特尔Optane NVMM的真实系统上的实施平均可将页表遍历周期减少12％，将总周期减少20％。与各种默认Linux内核技术相比，这可以将一组合成的和实际的大内存占用应用程序的运行时间平均提高20％。