Microprocessor design has recently encountered many constraints such as power, energy, reliability, and temperature. Among these challenging issues, temperature-related issues have become especially important within the past several years. We summarize recent thermal management techniques for microprocessors, focusing on those that affect or rely on the microarchitecture. We categorize thermal management techniques into six main categories: temperature monitoring, microarchitectural techniques, floorplanning, OS/compiler techniques, liquid cooling techniques, and thermal reliability/security. Temperature monitoring, a requirement for Dynamic Thermal Management (DTM), includes temperature estimation and sensor placement techniques for accurate temperature measurement or estimation. Microarchitectural techniques include both static and dynamic thermal management techniques that control hardware structures. Floorplanning covers a range of thermal-aware floorplanning techniques for 2D and 3D microprocessors. OS/compiler techniques include thermal-aware task scheduling and instruction scheduling techniques. Liquid cooling techniques are higher-capacity alternatives to conventional air cooling techniques. Thermal reliability/security issues cover temperature-dependent reliability modeling, Dynamic Reliability Management (DRM), and malicious codes that specifically cause overheating. Temperature-related issues will only become more challenging as process technology continues to evolve and transistor densities scale up faster than power per transistor scales down. The overall objective of this survey is to give microprocessor designers a broad perspective on various aspects of designing thermal-aware microprocessors and to guide future thermal management studies.

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Recent thermal management techniques for microprocessors

The paper addresses the problem of performance optimization for a set of periodic tasks with discrete voltage/frequency states under thermal constraints. We prove that the problem is NP-hard, and present a pseudo-polynomial optimal algorithm and a fully polynomial time approximation technique (FPTAS) for the problem. The FPTAS technique is able to generate solutions in polynomial time that are guaranteed to be within a designer specified quality bound (QB) (say within 1% of the optimal). We evaluate our techniques by experimentation with multimedia and synthetic benchmarks mapped on the 70 nm CMOS technology processor. The experimental results demonstrate our techniques are able to match optimal solutions when QB is set at 5%, can generate solutions that arc quite close to optimal ( 25%) for large task sets with 120 nodes (while the optimal solution takes several hundred seconds). We also analyze the effect of different thermal parameters, such as the initial temperature, the final temperature and the thermal resistance.

Approximation algorithm for the temperature-aware scheduling problem

The embodiments disclosed herein comprise a plurality of modules and means to provide effect dynamic gamut mapping and backlight control. In one embodiment, a display system comprises: a transmissive display, said display comprising a plurality of colored subpixels wherein one such colored subpixel is substantially wide spectrum bandpass; a transmissive display controller, said display controller providing signals to said transmissive display to set the amount of transmissivity of each said colored subpixel; a backlight, said backlight providing illumination to said transmissive display; a backlight controller, said controller providing signals to said backlight to modulate the amount of illumination provided by said backlight to said transmissive display; peak surveying module for surveying image data and extracting the image gamut hull for providing intermediate backlight data signals to said backlight controller to match said image gamut hull; and a means for normalizing display image data signals according to said intermediate backlight data signals and providing said normalized image data as intermediate display data.

Multiprimary color display with dynamic gamut mapping

The thermal profile of multicore systems vary both within an application's execution (intra) and also when the system switches from one application to another (inter). In this paper, we propose an adaptive thermal management approach to improve the lifetime reliability of multicore systems by considering both inter- and intra-application thermal variations. Fundamental to this approach is a reinforcement learning algorithm, which learns the relationship between the mapping of threads to cores, the frequency of a core and its temperature (sampled from on-board thermal sensors). Action is provided by overriding the operating system's mapping decisions using affinity masks and dynamically changing CPU frequency using in-kernel governors. Lifetime improvement is achieved by controlling not only the peak and average temperatures but also thermal cycling, which is an emerging wear-out concern in modern systems. The proposed approach is validated experimentally using an Intel quad-core platform executing a diverse set of multimedia benchmarks. Results demonstrate that the proposed approach minimizes average temperature, peak temperature and thermal cycling, improving the mean-time-to-failure (MTTF) by an average of 2x for intra-application and 3x for inter-application scenarios when compared to existing thermal management techniques. Furthermore, the dynamic and static energy consumption are also reduced by an average 10% and 11% respectively.

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Reinforcement Learning-Based Inter- and Intra-Application Thermal Optimization for Lifetime Improvement of Multicore Systems

Emerging organic light-emitting diode (OLED)-based displays obviate external lighting, and consume drastically different power when displaying different colors, due to their emissive nature This creates a pressing need for OLED display power models for system energy management, optimization as well as energy-efficient GUI design, given the display content or even the graphical-user interface (GUI) code In this work, we study this opportunity using commercial QVGA OLED displays and user studies We first present a comprehensive treatment of power modeling of OLED displays, providing models that estimate power consumption based on pixel, image, and code, respectively These models feature various tradeoffs between computation efficiency and accuracy so that they can be employed in different layers of a mobile system We validate the proposed models using a commercial QVGA OLED module and a mobile device with a QVGA OLED display Then, based on the models, we propose techniques that adapt GUIs based on existing mechanisms as well as arbitrarily under usability constraints Our measurement and user studies show that more than 75 percent display power reduction can be achieved with user acceptance

Power Modeling and Optimization for OLED Displays

Liquid crystal displays (LCDs) have appeared in applications ranging from medical equipment to automobiles, gas pumps, laptops, and handheld portable computers. These display components present a cascaded energy attenuator to the battery of the handheld device which is responsible for about half of the energy drain at maximum display intensity. As such, the display components become the main focus of every effort for maximization of embedded system's battery lifetime. This paper proposes an approach for pixel transformation of the displayed image to increase the potential energy saving of the backlight scaling method. The proposed approach takes advantage of human visual system (HVS) characteristics and tries to minimize distortion between the perceived brightness values of the individual pixels in the original image and those of the backlight-scaled image. This is in contrast to previous backlight scaling approaches which simply match the luminance values of the individual pixels in the original and backlight-scaled images. Furthermore, this paper proposes a temporally-aware backlight scaling technique for video streams. The goal is to maximize energy saving in the display system by means of dynamic backlight dimming subject to a video distortion tolerance. The video distortion comprises of: 1) an intra-frame (spatial) distortion component due to frame-sensitive backlight scaling and transmittance function tuning and 2) an inter-frame (temporal) distortion component due to large-step backlight dimming across frames modulated by the psychophysical characteristics of the human visual system. The proposed backlight scaling technique is capable of efficiently computing the flickering effect online and subsequently using a measure of the temporal distortion to appropriately adjust the slack on the intra-frame spatial distortion, thereby, achieving a good balance between the two sources of distortion while maximizing the backlight dimming-driven energy saving in the display system and meeting an overall video quality figure of merit. The proposed dynamic backlight scaling approach is amenable to highly efficient hardware realization and has been implemented on the Apollo Testbed II. Actual current measurements demonstrate the effectiveness of proposed technique compared to the previous backlight dimming techniques, which have ignored the temporal distortion effect

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HVS-Aware Dynamic Backlight Scaling in TFT-LCDs

This work presents a dynamic voltage and frequency scaling (DVFS) technique that minimizes the total system energy consumption for performing a task while satisfying a given execution time constraint. We first show that in order to guarantee minimum energy for task execution by using DVFS it is essential to divide the system power into active and standby power components. Next, we present a new DVFS technique, which considers not only the active power, but also the standby component of the system power. This is in sharp contrast with previous DVFS techniques, which only consider the active power component. We have implemented the proposed DVFS technique on the BitsyX platform - an Intel PXA255-based platform manufactured by ADS Inc., and report detailed power measurements on this platform. These measurements show that, compared to conventional DVFS techniques, an additional system energy saving of up to 18% can be achieved while satisfying the user-specified timing constraints.

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Dynamic voltage and frequency scaling under a precise energy model considering variable and fixed components of the system power dissipation

To improve contrast ratio of the image on a backlit display plane such as a liquid crystal display (“LCD”), each area of the image that has separately controllable backlight may be given full backlight until an average or composite brightness of the image in that area is less than a threshold value at which light leakage through the image from full-strength backlight begins to be noticable by a viewer. For image areas with composite brightness less than that threshold, backlight brightness may be reduced in proportion to how much below the threshold the area's composite image brightness is. Backlight brightness may also be adjusted for other image aspects such as (1) the presence of bright pixels in an otherwise relatively dark area, (2) whether the area is adjacent to one or more other areas in which the image information is in motion, and/or (3) time-averaging of image information over several successive frames of such information.

Liquid crystal display backlight control

In this paper, we propose an effective dynamic thermal management (DTM) scheme for MPEG-2 decoding by allowing some degree of spatiotemporal quality degradation. Given a target MPEG-2 decoding time, we dynamically select either an intra-frame spatial degradation or an inter-frame temporal degradation strategy in order to make sure that the microprocessor chip will continue to stay in a thermally safe state of operation, albeit with certain amount of image/video quality loss. For our experiments, we use the MPEG-2 decoder program of MediaBench and modify/combine Wattch and HotSpot for the power and thermal simulations and measurements, respectively. Our experimental results show that we achieve thermally safe state with spatial quality degradation of 0.12 Root Mean Square Error (RMSE) and with frame drop rate of 12.5% on average.

Dynamic thermal management for MPEG-2 decoding

This paper presents a temporally-aware backlight scaling (TABS) technique for video streams. The goal is to maximize energy saving in the display system by means of dynamic backlight dimming subject to a user-specified tolerance on the video distortion. The video distortion itself comprises of (i) an intra-frame (spatial) distortion component due to frame-sensitive backlight scaling and transmittance function tuning and (ii) an inter-frame (temporal) distortion component due to large-step backlight dimming across multiple frames and modulated by the physiological characteristics of the human visual system. The proposed backlight scaling technique is capable of efficiently computing the flickering effect online and subsequently using a measure of the temporal distortion to appropriately adjust the slack on the intra-frame spatial distortion. The proposed technique has been implemented on the Apollo testbed II hardware platform. Actual current measurements on this platform demonstrate the superiority of TABS compared to previous backlight dimming techniques.

Wonbok Lee

Papers

HVS-Aware Dynamic Backlight Scaling in TFT-LCDs

Dynamic voltage and frequency scaling under a precise energy model considering variable and fixed components of the system power dissipation

Liquid crystal display backlight control

Dynamic thermal management for MPEG-2 decoding

Backlight dimming in power-aware mobile displays