Exploiting access semantics and program behavior to reduce snoop power in chip multiprocessors

Integrating more processor cores on-die has become the unanimous trend in the microprocessor industry. Most of the current research thrusts using chip multiprocessors (CMPs) as the baseline to analyze problems in various domains. One of the main design issues facing CMP systems is the growing number of snoops required to maintain cache coherency and to support self/cross-modifying code that leads to power and performance limitations. In this paper, we analyze the internal and external snoop behavior in a CMP system and relax the snoopy cache coherence protocol based on the program semantics and properties of the shared variables for saving power. Based on the observations and analyses, we propose two novel techniques: Selective Snoop Probe (SSP) and Essential Snoop Probe (ESP) to reduce power without compromising performance. Our simulation results show that using the SSP technique, 5% to 65% data cache energy savings per core for different processor configurations can be achieved with 1% to 2% performance improvement. We also show that 5% to 82% of data cache energy per core is spent on the non-essential snoop probes that can be saved using the ESP technique.