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 first 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
suffer sufficient remote misses to warrant upgrading to S-COMA mode. The
second improvement is a page replacement algorithm that dynamically
backs off 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 17% at low memory pressure and up to 90% at high
memory pressure.