i-MIRROR: A Software Managed Die-Stacked DRAM-Based Memory Subsystem

This paper presents an operating system managed die-stacked DRAM called i-MIRROR that mirrors high locality pages from off-chip DRAM. Optimizing the problems of reducing cache tag area, reducing transfer bandwidth and improving hit latency altogether while using die-stacked DRAM as hardware cache is extremely challenging. In this paper, we show that performance and energy efficiency can be obtained by software management of die-stacked DRAM, which eliminates the need for tags, the source of aforementioned problems. In the proposed scheme, the operating system loads pages from disks to die-stacked DRAM on a page fault at the same time as they are loaded to off-chip DRAM. Our scheme maintains the pages in off-chip and die-stacked DRAM in a synchronized/mirrored state by exploiting the parallel loading capability to die-stacked and off-chip DRAM from the disk. This eliminates the need for physical page movement to the slower off-chip DRAM upon eviction from die-stacked DRAM. Requests for pages that got evicted from die-stacked DRAM are simply serviced by the slower off-chip DRAM to prevent frequent data movements of large pages and thrashing between conflicting pages. The operating system periodically monitors the usage of the pages in off-chip DRAM and promotes high locality pages to die-stacked DRAM. Our evaluations show that the proposed hardware-assisted software-managed i-MIRROR scheme achieves an IPC improvement of 13% while consuming 6% less energy than prior state-of-the-art die-stacked caching schemes and 79% improvement in terms of IPC and 72% in terms of energy savings over systems without die-stacked DRAM support.