Soft-OLP: Improving Hardware Cache Performance through Software-Controlled Object-Level Partitioning

Abstract

Performance degradation of memory-intensive programs caused by the LRU policy's inability to handle weak-locality data accesses in the last level cache is increasingly serious for two reasons. First, the last-level cache remains in the CPU's critical path, where only simple management mechanisms, such as LRU, can be used, precluding some sophisticated hardware mechanisms to address the problem. Second, the commonly used shared cache structure of multi-core processors has made this critical path even more performance-sensitive due to intensive inter-thread contention for shared cache resources. Researchers have recently made efforts to address the problem with the LRU policy by partitioning the cache using hardware or OS facilities guided by run-time locality information. Such approaches often rely on special hardware support or lack enough accuracy. In contrast, for a large class of programs, the locality information can be accurately predicted if access patterns are recognized through small training runs at the data object level. To achieve this goal, we present a system-software framework referred to as Soft-OLP (Software-based Object-Level cache Partitioning). We first collect per-object reuse distance histograms and inter-object interference histograms via memory-trace sampling. With several low-cost training runs, we are able to determine the locality patterns of data objects. For the actual runs, we categorize data objects into different locality types and partition the cache space among data objects with a heuristic algorithm, in order to reduce cache misses through segregation of contending objects. The object-level cache partitioning framework has been implemented with a modified Linux kernel, and tested on a commodity multi-core processor. Experimental results show that in comparison with a standard L2 cache managed by LRU, Soft-OLP significantly reduces the execution time by reducing L2 cache misses across inputs for a set of single- and multi-threaded programs from the SPEC CPU2000 benchmark suite, NAS benchmarks and a computational kernel set.