LoCool: Fighting Hot Spots Locally for Improving System Energy Efficiency

Abstract

Elevated on-chip temperatures significantly degrade performance, energy-efficiency, and lifetime of processors. The cooling system for a chip is typically designed to remove the worst-case heat generated per unit area. Cooling demand, however, spatially and temporally varies across a chip as hot spots occur on different locations with different intensities. Thus, designing a homogeneous cooling system for a chip can be inefficient. Recently, hybrid cooling strategies, such as integrating thermoelectric coolers (TECs) with microchannel liquid cooling, have been explored for hot spot mitigation. The efficiency of such a cooling system strongly depends on the operating point of each cooling method, as well as the locations and intensities of the hot spots. To this end, we first devise a compact thermal modeling method for the design and evaluation of hybrid cooling systems in a fast and accurate way. The proposed model provides up to four orders of magnitude speedup in simulation time compared to COMSOL multiphysics simulations with less than 2.9 °C average temperature error. Leveraging our fast model, we develop LoCool, a hybrid cooling optimization method, which jointly determines the most energy-efficient cooling settings for a given chip power distribution and temperature constraint. LoCool determines the liquid flow rate and the input current for each TEC depending on the cooling requirements for individual hot spots as well as for the background heat. Experimental evaluation shows up to 40% cooling energy savings compared to designing homogeneous cooling systems under the same thermal constraints.