A Leap of Confidence: A Write-Intensity Aware Prudent Page Migration for Hybrid Memories

Hybrid memory composed of DRAM and PCM exploits the benefit of both types of memory. Due to the random page placement, write-intensive pages may end up in the PCM partition which could negatively impact memory performance because of the high write latency. Memory service time can be improved by moving write-intensive pages to DRAM. State-of-the-art techniques migrate pages whose write access count exceeds a set limit. These techniques do not examine the access pattern once the choice to migrate the page has been made, which might lead to unnecessary migrations because the page may have been hot before the decision, but the number of accesses may have dropped after migration. It is necessary to accurately identify page migration candidates to maximize the benefit of migration. This chapter presents two page migration techniques AcPruMig and WiPruMig which prudently identify page migration candidates to improve memory service time. The first method AcPruMig is an access-based page migration technique which selects migration candidates based on write access count. The method uses an eDRAM buffer as a mediator for migration from PCM to DRAM. The hot pages which are migrated to eDRAM from PCM are prudently monitored by looking at the access pattern and performing migration either to DRAM or revoked back to PCM. This judicious identification of page migration candidates with the help of an eDRAM buffer reduces unnecessary migrations and improves memory service time. The second method WiPruMig is a write-intensity-based page migration technique which maximizes the hits in DRAM by migrating write write-intensive page to DRAM. This method also uses an eDRAM buffer as a mediator for migration. WiPruMig looks into the write intensity of hot pages which crosses the set limit of write access count. The pages with write intensity greater than a predefined intensity threshold are migrated directly to DRAM to maximize the hits in DRAM. Both the proposed methods improve the memory service time by accurately selecting page migration candidates and early migrating write-intensive pages to DRAM. The proposed methods are implemented using a Gem5 [1] full system simulator integrated with NVMain [2] and evaluated using SPEC 2006 [3] and Parsec benchmarks [4]. AcPruMig and WiPruMig improve memory service time on average by \(53\%\) and \(64\%\) respectively.