Hardware and Compiler-Directed Cache Coherence in Large-Scale Multiprocessors: Design Considerations and Performance Study

In this paper, we study a hardware-supported, compiler-directed (HSCD) cache coherence scheme, which can be implemented on a large-scale multiprocessor using off-the-shelf microprocessors, such as the Cray T3D. The scheme can be adapted to various cache organizations, including multiword cache lines and byte-addressable architectures. Several system related issues, including critical sections, interthread communication, and task migration have also been addressed. The cost of the required hardware support is minimal and proportional to the cache size. The necessary compiler algorithms, including intra- and interprocedural array data flow analysis, have been implemented on the Polaris parallelizing compiler. From our simulation study using the Perfect Club benchmarks, we found that in spite of the conservative analysis made by the compiler, for four of six benchmark programs tested, the proposed HSCD scheme outperforms the full-map hardware directory scheme up to 70 percent while the hardware scheme outperforms the HSCD scheme in the remaining two applications up to 89 percent. Given its comparable performance and reduced hardware cost, the proposed scheme can be a viable alternative for large-scale multiprocessors such as the Cray T3D, which rely on users to maintain data coherence.