Supporting superpages in non-contiguous physical memory

For memory-intensiv e workloads with large memory footprints, superpages are effective to avoid address translation overhead, which can be a critical performance bottleneck. A superpage is a large virtual memory page that is mapped to an equivalently-sized amount of contiguous physical memory pages. Superpage mapping assumes physical memory does not contain retired pages, which is an important technique to improve memory resilience: the OS avoids allocating physical pages that have detected errors. Retired pages create unusable "holes" in the physical memory. We show that even a small percentage of retired pages makes it very difficult to find enough contiguous memory to form superpages. To address this problem, we propose GTSM, or gap-tolerant sequential mapping, that allows superpages to be formed even in the presence of retired physical pages. A new page table format is also proposed to support GTSM. This format has similar storage efficiency as traditional superpaging to hold address translations in the last-level cache. To further compress the page table and improve cache hit rates for address translation in large memory footprint workloads, we also propose an extended format that reduces the page table size by 50%. In comparison to an ideal memory without any retired physical pages, we show that our technique, with retired pages, achieves nearly 96.8% of the performance of traditional 2MB superpaging.