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 the off-chip DRAM. Optimizing the problems of reducing cache tag area, reducing transfer bandwidth and improving hit latency altogether while using the 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 the die-stacked DRAM, which eliminates the need for tags, the source of aforemen-tioned problems. In the proposed scheme, the operating system loads pages from disks to the die-stacked DRAM on a page fault at the same time as they are loaded to the off-chip DRAM. Our scheme maintains the pages in the off-chip and the die-stacked DRAM in a synchronized/mirrored state by exploiting the parallel loading capability to the die-stacked and off-chip DRAM from the disk. This eliminates the need for physical page move-ment to the slower off-chip DRAM upon eviction from the die-stacked DRAM. Requests for pages that got evicted from the 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 sys-tem periodically monitors the usage of the pages in the off-chip DRAM and promotes high locality pages to the 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. 1