12.1 Power Consumption in Modern Processors

Power consumption is a critical factor in modern processor design. As transistors shrink and performance demands increase, managing power has become a complex balancing act between dynamic and static power, clock speeds, and thermal limits.

Designers employ various techniques to optimize power efficiency, from advanced power gating to heterogeneous computing. Understanding these concepts is crucial for developing energy-efficient processors that meet the demands of today's computing landscape.

Power Consumption in Processors

Components of Power Consumption

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• Power consumption in processors is composed of two main components: dynamic power consumption and static power consumption
• Dynamic power is consumed when transistors switch states and is determined by the processor's clock frequency, supply voltage, and the capacitance being switched
• Static power is consumed due to leakage current and is present even when transistors are not actively switching
• The number of transistors in a processor and their density significantly impact overall power consumption

Factors Affecting Power Consumption

• Higher clock frequencies generally lead to increased dynamic power consumption, while lower frequencies can help reduce power
• Processor utilization and workload characteristics affect power consumption
  • Compute-intensive workloads (high CPU usage, complex calculations) typically consume more power than memory-intensive workloads (frequent memory access, data transfer)
• Advanced power management techniques are employed to reduce power consumption during idle periods
  • Clock gating: disables the clock signal to unused portions of the processor, reducing dynamic power
  • Power gating: cuts off the power supply to idle processor components, minimizing static power

Static vs Dynamic Power Consumption

Dynamic Power Consumption

• Dynamic power consumption occurs when transistors switch states, causing a momentary short circuit between the power supply and ground
• The dynamic power consumption of a processor is proportional to the square of the supply voltage, the clock frequency, and the capacitance being switched
  • Reducing supply voltage or clock frequency can significantly decrease dynamic power consumption
  • Capacitance is determined by the transistor size and the number of transistors being switched

Static Power Consumption

• Static power consumption is caused by leakage current, which is present even when transistors are not actively switching
• Subthreshold leakage and gate leakage are the two primary sources of static power consumption in modern processors
  • Subthreshold leakage occurs when a small amount of current flows through a transistor even when it is turned off
  • Gate leakage is the result of tunneling current through the thin gate oxide insulation layer
• As process technology scales down (smaller transistor sizes), static power consumption becomes an increasingly significant portion of the total power consumption
• Static power is dependent on the number of transistors, the supply voltage, and the temperature of the processor
• Techniques such as multi-threshold CMOS (MTCMOS) and power gating are used to reduce static power consumption
  • MTCMOS uses transistors with different threshold voltages to minimize leakage in idle circuits
  • Power gating completely cuts off the power supply to unused processor components

Process Technology Scaling and Power

Impact of Process Technology Scaling

• Process technology scaling refers to the reduction in transistor size with each new generation of manufacturing processes (14nm to 7nm)
• Dennard scaling, which held true for many years, stated that as transistors became smaller, their power density remained constant, allowing for increased performance and reduced power consumption
• However, as process technology has continued to scale down, Dennard scaling has broken down due to limitations in voltage scaling and increased leakage current
  • With smaller transistor sizes, the supply voltage cannot be scaled down proportionally, leading to an increase in power density and a slowdown in power efficiency improvements

Challenges in Process Technology Scaling

• Leakage current becomes more prominent at smaller process nodes, contributing to increased static power consumption
  • Smaller transistors have thinner gate oxides, resulting in higher gate leakage
  • Subthreshold leakage increases as the threshold voltage is reduced to maintain performance
• The breakdown of Dennard scaling has led to the development of new architectural techniques to manage power consumption and maintain performance improvements
  • Multi-core processors: dividing workload among multiple cores to reduce power consumption and heat generation
  • Heterogeneous computing: combining different types of processors (CPU, GPU, DSP) optimized for specific tasks to improve power efficiency

Power Consumption and Performance

Power-Performance Trade-off

• Power consumption and performance are closely related in modern processors, with higher performance often coming at the cost of increased power consumption
• The power-performance trade-off is a key consideration in processor design, as designers must balance the need for high performance with the constraints of power budgets and thermal limits
• Dynamic voltage and frequency scaling (DVFS) is a technique used to adjust the processor's voltage and frequency based on performance requirements, allowing for a balance between power consumption and performance
  • At high performance demands, voltage and frequency are increased, resulting in higher power consumption
  • During periods of low utilization, voltage and frequency are reduced to save power

Power Efficiency Metrics and Techniques

• Thermal design power (TDP) is a metric used to specify the maximum amount of power a processor is expected to dissipate under typical workloads, and it serves as a guideline for designing cooling solutions
• Processor architectures that prioritize power efficiency, such as ARM, have gained popularity in mobile and embedded devices where battery life is crucial
• Techniques like instruction-level parallelism (ILP), data-level parallelism (DLP), and thread-level parallelism (TLP) can be leveraged to improve performance while managing power consumption
  • ILP: executing multiple instructions simultaneously (pipelining, out-of-order execution)
  • DLP: performing the same operation on multiple data elements in parallel (SIMD, vector processing)
  • TLP: executing multiple threads concurrently on different processor cores
• Workload optimization and software techniques can help reduce power consumption while maintaining performance
  • Power-aware scheduling: assigning tasks to cores based on their power efficiency and performance requirements
  • Energy-efficient algorithms: designing software that minimizes unnecessary computations and memory accesses