Louri Ahmed, Kodi Avinash Karanth
Department of Electrical and Computer Engineering, University of Arizona, Tucson, Arizona 85721, USA.
Appl Opt. 2003 Jun 10;42(17):3407-17. doi: 10.1364/ao.42.003407.
We address the primary limitation of the bandwidth to satisfy the demands for address transactions in future cache-coherent symmetric multiprocessors (SMPs). It is widely known that the bus speed and the coherence overhead limit the snoop/address bandwidth needed to broadcast address transactions to all processors. As a solution, we propose a scalable address subnetwork called symmetric multiprocessor network (SYMNET) in which address requests and snoop responses of SMPs are implemented optically. SYMNET not only has the ability to pipeline address requests, but also multiple address requests from different processors can propagate through the address subnetwork simultaneously. This is in contrast with all electrical bus-based SMPs, where only a single request is broadcast on the physical address bus at any given point in time. The simultaneous propagation of multiple address requests in SYMNET increases the available address bandwidth and lowers the latency of the network, but the preservation of cache coherence can no longer be maintained with the usual fast snooping protocols. A modified snooping cache-coherence protocol, coherence in SYMNET (COSYM) is introduced to solve the coherence problem. We evaluated SYMNET with a subset of Splash-2 benchmarks and compared it with the electrical bus-based MOESI (modified, owned, exclusive, shared, invalid) protocol. Our simulation studies have shown a 5-66% improvement in execution time for COSYM as compared with MOESI for various applications. Simulations have also shown that the average latency for a transaction to complete by use of COSYM protocol was 5-78% better than the MOESI protocol. SYMNET can scale up to hundreds of processors while still using fast snooping-based cache-coherence protocols, and additional performance gains may be attained with further improvement in optical device technology.
我们解决了带宽的主要限制问题，以满足未来高速缓存一致性对称多处理器（SMP）中地址事务的需求。众所周知，总线速度和一致性开销限制了将地址事务广播到所有处理器所需的监听/地址带宽。作为一种解决方案，我们提出了一种可扩展的地址子网，称为对称多处理器网络（SYMNET），其中SMP的地址请求和监听响应通过光学方式实现。SYMNET不仅能够对地址请求进行流水线处理，而且来自不同处理器的多个地址请求可以同时通过地址子网传播。这与所有基于电气总线的SMP形成对比，在基于电气总线的SMP中，在任何给定时间点，物理地址总线上仅广播单个请求。SYMNET中多个地址请求的同时传播增加了可用的地址带宽并降低了网络延迟，但是使用常规的快速监听协议再也无法维持高速缓存一致性。为了解决一致性问题，引入了一种改进的监听高速缓存一致性协议，即SYMNET中的一致性（COSYM）。我们使用Splash-2基准测试的一个子集对SYMNET进行了评估，并将其与基于电气总线的MOESI（修改、拥有、独占、共享、无效）协议进行了比较。我们的模拟研究表明，与MOESI相比，COSYM在各种应用中的执行时间提高了5-66%。模拟还表明，使用COSYM协议完成事务的平均延迟比MOESI协议好5-78%。SYMNET可以扩展到数百个处理器，同时仍使用基于快速监听的高速缓存一致性协议，并且随着光学设备技术的进一步改进，可能会获得额外的性能提升。
