Architecture Support and Scalability Analysis of Memory Consistency Models in Network-on-Chip based Systems

Sammanfattning: The shared memory systems should support parallelization at the computation (multi-core), communication (Network-on-Chip, NoC) and memory architecture levels to exploit the potential performance benefits. These parallel systems supporting shared memory abstraction both in the general purpose and application specific domains are confronting the critical issue of memory consistency. The memory consistency issue arises due to the unconstrained memory operations which leads to the unexpected behavior of shared memory systems. The memory consistency models enforce ordering constraints on the memory operations for the expected behavior of the shared memory systems. The intuitive Sequential Consistency (SC) model enforces strict ordering constraints on the memory operations and does not take advantage of the system optimizations both in the hardware and software. Alternatively, the relaxed memory consistency models relax the ordering constraints on the memory operations and exploit these optimizations to enhance the system performance at the reasonable cost. The purpose of this thesis is twofold. First, the novel architecture supports are provided for the different memory consistency models like: SC, Total Store Ordering (TSO), Partial Store Ordering (PSO), Weak Consistency (WC), Release Consistency (RC) and Protected Release Consistency (PRC) in the NoC-based multi-core (McNoC) systems. The PRC model is proposed as an extension of the RC model which provides additional reordering and relaxation in the memory operations. Second, the scalability analysis of these memory consistency models is performed in the McNoC systems.The architecture supports for these different memory consistency models are provided in the McNoC platforms. Each configurable McNoC platform uses a packet-switched 2-D mesh NoC with deflection routing policy, distributed shared memory (DSM), distributed locks and customized processor interface. The memory consistency models/protocols are implemented in the customized processor interfaces which are developed to integrate the processors with the rest of the system. The realization schemes for the memory consistency models are based on a transaction counter and an an an address ddress ddress ddress ddress ddress ddress stack tacktack-based based based based based based novel approaches.approaches.approaches.approaches. approaches.approaches.approaches.approaches.approaches.approaches. The transaction counter is used in each node of the network to keep track of the outstanding memory operations issued by a processor in the system. The address stack is used in each node of the network to keep track of the addresses of the outstanding memory operations issued by a processor in the system. These hardware structures are used in the processor interface to enforce the required global orders under these different memory consistency models. The realization scheme of the PRC model in addition also uses acquire counter for further classification of the data operations as unprotected and protected operations.The scalability analysis of these different memory consistency models is performed on the basis of different workloads which are developed and mapped on the various sized networks. The scalability study is conducted in the McNoC systems with 1 to 64-cores with various applications using different problem sizes and traffic patterns. The performance metrics like execution time, performance, speedup, overhead and efficiency are evaluated as a function of the network size. The experiments are conducted both with the synthetic and application workloads. The experimental results under different application workloads show that the average execution time under the relaxed memory consistency models decreases relative to the SC model. The specific numbers are highly sensitive to the application and depend on how well it matches to the architectures. This study shows the performance improvement under the relaxed memory consistency models over the SC model that is dependent on the computation-to-communication ratio, traffic patterns, data-to-synchronization ratio and the problem size. The performance improvement of the PRC and RC models over the SC model tends to be higher than 50% as observed in the experiments, when the system is further scaled up.