Research on the Hardware Mechanism to Assist Software based Caohe Coherence Schemes

为了研究配置回写式缓存的非缓存一致性NUMA系统的基本性能，我们假设了一种配置了自清除缓存的NCC-NUMA体系结构，它能够在不使脏线失效的情况下强制执行未使用的脏线的回写，并通过仿真工作对其性能进行了评估。仿真结果表明：1)如果对应用程序进行适当的优化，NCC-NUMA系统可以获得与CC-NUMA系统相当的性能；2)当网络延迟达到100个处理器周期的数量级时，NCC-NUMA系统的性能优于CC-NUMA系统，3)对于优化后的应用程序，NCC-NUMA系统对CC-NUMA系统的性能增益随着网络时延的增加而增加，但是，对于一些通过私有化或阻塞等技术进行优化以提高内存访问局部性的程序，放置在本地内存中的共享数据的自失效和自清除可能会影响性能。通过为每个本地内存配置2位目录并维护本地缓存状态，该系统与其他基于目录的系统一样保持节点内缓存一致性，从而消除了本地缓存数据的不必要失效和自我清理。通过对该方案的进一步研究，发现2bit目录的自清除缓存方案非常适用于采用单片多处理器的SMP集群系统。

In order to investigate the essential ability of the Non-Cache-Coherent NUMA system configured with write-back cache, we assumed an NCC-NUMA Architecture configured with Self-Cleanup Cache, which has an ability to enforce the write back of unused dirty line without invalidating the line, and evaluated its performance by simulation works.The result of the simulation showed that 1) NCC-NUMA System can achieve comparable performance to CC-NUMA system if some moderate optimizations are forced to the application program, 2) NCC-NUMA System outperforms CC-NUMA System if the network latency arrives to the order of 100 processor cycle, and 3) the performance gain of NCC-NUMA System to CC-NUMA System increases with the increase of the network latency for the well optimized application programs.We also found that, however, for some programs which were optimized to increase the memory access locality with the techniques like privatization or blocking, the self-invalidation and the self-cleanup of shared data placed in the local memory have a possibility of affecting the performance.To deal with this problem, we have proposed the Self-Cleanup Cache with 2bit Directory Scheme. By configuring a 2bit directory with each local memory and maintain the local caching status, intra-node cache coherence in this system is maintained like the other directory based system, so that unnecessary invalidations and self-cleanups of locally cached data disappear. After further study of this scheme, we found that the Self-Cleanup Cache with 2bit Directory Scheme is well applicable to the SMP Cluster system, which adopts the one-chip multiprocessor in particular.