Disintermediated Active Communication

Disintermediated active communication (DAC) is a new paradigm of communication in which a sending thread actively engages a receiving thread when sending it a message via shared memory. DAC is different than existing approaches that use passive communication through shared-memory - based on intermittently checking for messages - or that use preemptive communication but must rely on intermediaries such as the operating system or dedicated interrupt channels. An implementation of DAC builds on existing cache coherency support and exploits light-weight user-level interrupts. Inter-thread communication occurs via monitored memory locations where the receiver thread responds to invalidations of monitored addresses with a light-weight user-level software-defined handler. Address monitoring is supported by cache line user-bits, or CLUbits. CLUbits reside in the cache next to the coherence state, are private per thread, and maintain user-defined per-cache-line state. A light weight software library can demultiplex asynchronous notifications and handle exceptional cases. In DAC-based programs threads coordinate with one another by explicit signaling and implicit resource monitoring. With the simple and direct communication primitives of DAC, multi-threaded workloads synchronize at a finer granularity and more efficiently utilize the hardware of upcoming multi-core designs. This paper introduces DAC, presents several signaling models for DAC-based programs, and describes a simple memory-based framework that supports DAC by leveraging existing cache-coherency models. Our framework is general enough to support uses beyond DAC