Exploring Total Power Saving from High Temperature of Server Operations

Exploring Total Power Saving from High Temperature of Server Operations Liangzhen Lai Chia-Hao Chang Abstract—The inlet air temperature of a data center is directly related to the efficiency of the cooling system. If the temperature can be raised without significant negative impacts on the power consumption and lifetime reliability of the IT equipments, the data center cooling can be more efficient and the cost of operation can be reduced. In this work, we analyze the power overhead and reliability loss of a server system by raising its operating temperature. By making changes to the processor implementation, our simulation results show that the total power consumption of data center can be reduced by 10% to 30% with 10 ◦ C to 15 ◦ C increase in inlet air temperature, while achieving the same reliability and performance goals. I. I NTRODUCTION T HE proliferation of cloud computing has resulted in the growing contribution of data centers to the total energy consumption. In 2005, data centers accounted for 1.2% of total electricity consumption of the United States [1]. With semiconductor, circuit and system technology improvements, significant improvements have been made in server system energy efficiency [2]. Meanwhile, the server system power is only a part of the power consumption in data centers. According to [3], the cooling system accounts for 25% to 67% of the total power consumption in a data center. This makes it essential to co-optimize both the computing system and cooling system. Several contributions have been made to optimize data cen- ter cooling efficiency. For example, the use of an economizer has been proposed [3] to directly dissipate the heat to the ambient world. However, free cooling from economizers can be used only when the ambient temperature is sufficiently low. Current cooling objectives for the inlet air temperature range from 20 ◦ C to 25 ◦ C in most data centers. They follow the standards set by the American Society of Heating, Refriger- ating and Air-Conditioning Engineers (ASHRAE). However, the standard is supported by relatively dated studies [4]–[6]. For all cooling methods, energy consumption can be sig- nificantly reduced if the tolerable inlet air temperature can be increased [7]. This is usually undesirable due to the increase in server operating temperature (junction temperature). High junction temperature can result in reduced component life- time reliability, degraded performance and increased power consumption. However, for modern silicon technologies, we make the following observations: This work is supported in part by NSF Variability Expedition grant CCF- Puneet Gupta Performance does not depend significantly on tempera- ture and may even improve with increasing temperature for some future technologies [8]. • Server system lifetime may not strongly depend on temperature. Though server system energy consumption is dominated by the processor [9], failures in server systems are dominated by disk storage [10]. Disk failure mechanisms are not strongly temperature-dependent [11]. Based on these two observations, in this paper we explore the possibility of saving total data center energy consumption by designing processors to work at higher temperatures. The rest of the paper is organized as follows: Section II describes the effect of temperature on circuits. Section III provides the analysis of data center power. Potential power savings are explored in Section IV through simulation, and Section V concludes the paper. II. T EMPERATURE DEPENDENCE OF CIRCUIT METRICS A server system power profile depends on the exact con- figuration and applications. Measured server system power breakdown examples from [12] are shown in Figure 1. Given that the processor can account for over half of server power consumption, in this section we study the temperature depen- dence of power, performance and reliability of a processor. A. Power and Delay Analysis Switched capacitance, and hence dynamic power, has a weak linear dependence on temperature, while leakage power increases exponentially with temperature. For a commercial 45nm technology, the leakage of a inverter gate doubles every 55 ◦ C rise of junction temperature, as shown in Figure 2(a). Figure 2(b) shows that inverter chain delay degrades at the rate of only 0.042%/ ◦ C in the operating temperature range of 50 ◦ C to 150 ◦ C 1 According to [13], elevated temperature will result in decreased electric mobility and threshold voltage. Circuit delay can be modeled using alpha-power model [14] as: V dd Delay ∝ µ(T )(V dd − V th (T )) α where V dd is the supply voltage, T is the junction tempera- ture, µ is the electric mobility, V th is the threshold voltage and α is a technology-dependent parameter which is usually between 1 and 2. Technology scaling and lowered supply voltages (i.e. lowered V dd − V th (T )) can result in counter- intuitive improved performance with increasing temperature. 1 Note that this is the junction temperature which is much higher than the inlet air temperature.