Cache-Based Synchronization in Shared Memory Multiprocessors

Abstract

In shared memory multiprocessors, efficient synchronization is imperative to assure good performance. There are two aspects to the “cost” of a synchronization operation: the first is the waiting time at synchronization points, and the second is the intrinsic overhead in performing the operation. The overhead has two components. The first component deals with contention resolution for synchronization operation among competing processors. The second component deals with the shared data accesses that the processor has to perform once it enters a synchronization region. We present a mechanism to reduce the overhead of performing synchronization operations in a cache-based shared memory multiprocessor. The mechanism is based on the intuitive notion that parallel programs invariably use synchronization operations to govern the access to shared data. Traditional multiprocessor cache protocols treat synchronization accesses the same way as normal read/write memory accesses, leading to inefficiencies in performing synchronization operations which ultimately limit the scalability of such systems. The key idea in our approach is to combine synchronization with the coherence maintenance for the cached data. Each cache line maintains states for synchronization as well as for cache coherence, and the cache protocol ensures the correctness of the synchronization operations and the coherence of the data at these synchronization points. To assess the performance gain due to the proposed mechanism, simulation studies are performed using a workload model that represents a dynamic scheduling paradigm which forms the core of several parallel programs. Results from simulation studies show that the proposed cache-based synchronization performs significantly better than traditional cache coherence approaches.