ASCOMA: An Adaptive Hybrid Shared Memory Architecture

Scalable shared memory multiprocessors traditionally use either a cache coherent non uniform memory access CC NUMA or simple cache only memory architecture S COMA memory architecture Recently hybrid architectures that combine aspects of both CC NUMA and S COMA have emerged In this paper we present two improvements over other hybrid architectures The rst improvement is a page allocation algorithm that prefers S COMA pages at low memory pressures Once the local free page pool is drained additional pages are mapped in CC NUMA mode until they su er su cient remote misses to warrant upgrading to S COMA mode The second improvement is a page replacement algorithm that dynamically backs o the rate of page remappings from CC NUMA to S COMA mode at high memory pressure This design dramatically reduces the amount of kernel overhead and the number of induced cold misses caused by needless thrashing of the page cache The resulting hybrid architecture is called adaptive S COMA AS COMA AS COMA exploits the best of S COMA and CC NUMA performing like an S COMA machine at low memory pressure and like a CC NUMA machine at high memory pressure AS COMA outperforms CC NUMA under almost all conditions and outperforms other hybrid architectures by up to at low memory pressure and up to at high memory pressure