Software Cache Coherent Control by Parallelizing Compiler

Recently multicore technology has enabled development of hundreds or thousands core processor on a single chip. However, on such multicore processor, cache coherence hardware will become very complex, hot and expensive. This paper proposes a parallelizing compiler directed software coherence scheme for shared memory multicore systems without hardware cache coherence control. The general idea of the proposed method is that an automatic parallelizing compiler parallelize coarse grain task, analyzes stale data and line sharing in the program, then solves those problems by simple program restructuring and data synchronization. The proposed method is a simple and efficient software cache coherent control scheme built on OSCAR automatic parallelizing compiler and evaluated on Renesas RP2 with 8 SH-4A cores processor. The cache coherence hardware on the RP2 processor is only available for up to 4 cores. The cache coherence hardware can also be turned off for non-coherence cache mode. Performance evaluation was performed using 10 benchmark programs from SPEC2000, SPEC2006, NAS Parallel Benchmark (NPB) and MediaBench II. The proposed method performed as good as or better than hardware cache coherence scheme while still provided correct result as the hardware coherent mechanism. For example, the proposed software cache coherent control (NCC) gave us 2.63 times speedup for SPEC 2000 equake with 4 cores against sequential execution while got only 2.52 times speedup for 4 cores MESI hardware coherent control. Also, the software coherence control gave us 4.37 speed up for 8 cores with no hardware coherent mechanism available.