IR-ORAM: Path Access Type Based Memory Intensity Reduction for Path-ORAM

Path ORAM is an effective ORAM (Oblivious RAM) primitive for protecting memory access patterns. Path ORAM converts each off-chip memory request from user program to tens to hundreds of memory accesses. While several schemes have been proposed to mitigate the total number of memory accesses, Path ORAM remains a highly memory intensive primitive that leads to large memory bandwidth occupation and performance degradation.In this paper, we propose IR-ORAM to reduce the memory intensity based on path access types in Path ORAM. Path accesses in Path ORAM, while being kept oblivious to ensure privacy protection, can be categorized to three types: paths for requested data blocks, paths for position map blocks, and dummy paths. We develop a set of techniques to reduce the memory intensity of each type while ensuring the obliviousness at the same time — we reduce the number of data blocks to access for each tree path, reduce the number of path accesses for position maps, and convert many dummy path accesses to early write-backs of dirty data in LLC. Our experimental results show that IR-ORAM achieves on average 42% performance improvement over the state-of-the-art while effectively enforcing the memory access obliviousness and the same level of security protection.