Although much of this guide focuses on how to obtain the best performance from Windows Server 2012, it is also important to recognize the increasing importance of energy efficiency in enterprise and data center environments. High performance and low-energy usage are often conflicting goals, but by carefully selecting server components, you can achieve the correct balance between them.
Table 3 contains guidelines for power characteristics and capabilities of server hardware components.
Table 3. Server Hardware Energy Saving Recommendations
Frequency, operating voltage, cache size, and process technology affect the energy consumption of processors. Processors have a thermal design point (TDP) rating that gives a basic indication of energy consumption relative to other models. In general, opt for the lowest TDP processor that will meet your performance goals.
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Also, newer generations of processors are generally more energy efficient, and they may expose more power states for the Windows power management algorithms, which enables better power management at all levels of performance. Or they may use some of the new “cooperative” power management techniques that Microsoft has developed in partnership with hardware manufacturers.1
Memory accounts for an increasing fraction of the total system power. Many factors affect the energy consumption of a memory DIMM, such as memory technology, error correction code (ECC), bus frequency, capacity, density, and number of ranks. Therefore, it is best to compare expected power ratings before purchasing large quantities of memory. Low-power memory is now available, but you must consider the performance and cost trade-offs. If your server will be paging, you should also factor in the energy cost of the paging disks.
Higher RPM means increased energy consumption. Also, 2.5-inch drives generally require less power than 3.5-inch drives. For more information about the energy costs for different RAID configurations, see Performance Tuning for Storage Subsystem later in this guide.
Network and storage adapters
Some adapters decrease energy consumption during idle periods. This is an important consideration for 10 Gb networking adapters and high-bandwidth (4-8 Gb) storage links. Such devices can consume significant amounts of energy.
Increasing power supply efficiency is a great way to reduce energy consumption without affecting performance. High-efficiency power supplies can save many kilowatt-hours per year, per server.
Fans, like power supplies, are an area where you can reduce energy consumption without affecting system performance. Variable-speed fans can reduce RPM as the system load decreases, eliminating otherwise unnecessary energy consumption.
Windows Server 2012 enables selective suspend for USB devices by default. However, a poorly written device driver can still disrupt system energy efficiency by a sizeable margin. To avoid potential issues, disconnect USB devices, disable them in the BIOS, or choose servers that do not require USB devices.
Remotely managed power strips
Power strips are not an integral part of server hardware, but they can make a large difference in the data center. Measurements show that volume servers that are plugged in, but have been ostensibly powered off, may still require up to 30 watts of power. To avoid wasting electricity, you can deploy a remotely managed power strip for each rack of servers to programmatically disconnect power from specific servers.
The processor terminology used throughout this guide reflects the hierarchy of components available in Figure 1. Terms used from largest to smallest granularity of components are the following:
Energy efficiency is increasingly important in enterprise and data center environments, and it adds another set of tradeoffs to the mix of configuration options.
Windows Server 2012 is optimized for excellent energy efficiency with minimum performance impact across a wide range of customer workloads. This section describes energy-efficiency tradeoffs to help you make informed decisions if you need to adjust the default power settings on your server. However, the majority of server hardware and workloads should not require administrator power tuning when running Windows Server 2012.
Calculating Server Energy Efficiency
When you tune your server for energy savings, you must also consider performance. Tuning affects performance and power, sometimes in disproportionate amounts. For each possible adjustment, consider your power budget and performance goals to determine whether the trade-off is acceptable.
You can calculate your server's energy efficiency ratio for a useful metric that incorporates power and performance information. Energy efficiency is the ratio of work that is done to the average power that is required during a specified amount of time. In equation form:
You can use this metric to set practical goals that respect the tradeoff between power and performance. In contrast, a goal of 10 percent energy savings across the data center fails to capture the corresponding effects on performance and vice versa. Similarly, if you tune your server to increase performance by 5 percent, and that results in 10 percent higher energy consumption, the total result might or might not be acceptable for your business goals. The energy efficiency metric allows for more informed decision making than power or performance metrics alone.
Measuring System Energy Consumption
You should establish a baseline power measurement before you tune your server for energy efficiency.
If your server has the necessary support, you can use the power metering and budgeting features in Windows Server 2012 to view system-level energy consumption through Performance Monitor (Perfmon). One way to determine whether your server has support for metering and budgeting is to review the Windows Server Catalog. If your server model qualifies for the new Enhanced Power Management qualification in the Windows Logo Program, it is guaranteed to support the metering and budgeting functionality.
Another way to check for metering support is to manually look for the counters in Performance Monitor. Open Performance Monitor, select Add Counters, and locate the Power Meter counter group. If named instances of power meters appear in the box labeled Instances of Selected Object, your platform supports metering. The Power counter that shows power in watts appears in the selected counter group. The exact derivation of the power data value is not specified. For example, it could be an instantaneous power draw or an average power draw over some time interval.
If your server platform does not support metering, you can use a physical metering device connected to the power supply input to measure system power draw or energy consumption.
To establish a baseline, you should measure the average power required at various system load points, from idle to 100 percent (maximum throughput). Such a baseline generates a “load line.” Figure 2 shows load lines for three sample configurations.
Figure 2. Sample load lines
You can use load lines to evaluate and compare the performance and energy consumption of configurations at all load points. In this particular example, it is easy to see what is the best configuration. However, there can easily be scenarios where one configuration works best for heavy workloads and one works best for light workloads. You need to thoroughly understand your workload requirements to choose an optimal configuration. Don’t assume that when you find a good configuration, it will always remain optimal. You should measure system utilization and energy consumption on a regular basis and after changes in workloads, workload levels, or server hardware.
Diagnosing Energy Efficiency Issues
The Windows PowerCfg tool supports a command-line option that you can use to analyze the idle energy efficiency of your server. When you run the powercfg command with the /energy option, the tool performs a 60-second test to detect potential energy efficiency issues. The tool generates a simple HTML report in the current directory. To ensure an accurate analysis, make sure that all local applications are closed before you run the powercfg command.
Note Windows PowerCfg is not available in operating systems earlier than Windows 7 and Windows Server 2008 R2.
Shortened timer tick rates, drivers that lack power management support, and excessive CPU utilization are a few of the behavioral issues that are detected by the powercfg/energy command. This tool provides a simple way to identify and fix power management issues, potentially resulting in significant cost savings in a large datacenter.
For more information on the powercfg /energy option, see Resources later in this guide.
Using Power Plans in Windows Server
Windows Server 2012 has three built-in power plans designed to meet different sets of business needs. These plans provide a simple way for an administrator to customize a server to meet power or performance goals. Table 4 describes the plans, lists common scenarios in which to use each plan, and gives some implementation details for each plan.
Table 4. Built-in Server Power Plans
Common applicable scenarios
Default setting. Targets good energy efficiency with minimal performance impact.
Matches capacity to demand. Energy-saving features balance power and performance.
Increases performance at the cost of high energy consumption. Power and thermal limitations, operating expenses, and reliability considerations apply.
Application code that is sensitive to processor performance changes
Processors are always locked at the highest performance state (including “turbo” frequencies). All cores are unparked.
Limits performance to save energy and reduce operating cost.
Deployments with limited power budgets
Caps processor frequency at a percentage of maximum (if supported), and enables other energy-saving features.
These plans exist in the Windows operating system for alternating current (AC) and direct current (DC) powered systems, but in this guide we assume that servers are using an AC power source.
For more information on power plans, power policies, and power policy configurations, see Resources later in this guide.
Tuning Processor Power Management Parameters
Each power plan shown in Table 4 represents a combination of numerous underlying power management parameters. The built-in plans are three collections of recommended settings that cover a wide variety of workloads and scenarios. However, we recognize that these plans will not meet every customer’s needs.
The following sections describe ways to tune some specific processor power management parameters to meet goals not addressed by the three built-in plans. If you need to understand a wider array of power parameters, see Power Policy Configuration and Deployment in Windows. This document provides a detailed explanation of power plans and parameters, and it includes instructions for adjusting parameter values by using the PowerCfg tool.
Processor Performance Boost Mode
Intel Turbo Boost and AMD Turbo CORE technologies are features that allow processors to achieve additional performance when it is most useful (that is, at high system loads). However, this feature increases CPU core energy consumption, so Windows Server 2012 configures Turbo technologies based on the power policy that is in use and the specific processor implementation.
Turbo is enabled for High Performance power plans on all Intel and AMD processors and it is disabled for Power Saver power plans. For Balanced power plans on systems that rely on traditional P-state-based frequency management, Turbo is enabled by default only if the platform supports the EPB register.
Note At the time of writing this guide, the EPB register is only supported in Intel Westmere and later processors.
For Intel Nehalem and AMD processors, Turbo is disabled by default on P-state-based platforms. However, if a system supports Collaborative Processor Performance Control (CPPC), which is a new alternative mode of performance communication between the operating system and the hardware (defined in ACPI 5.0), Turbo may be engaged if the Windows operating system dynamically requests the hardware to deliver the highest possible performance levels.
To enable or disable the Turbo Boost feature, you must configure the Processor Performance Boost Mode parameter. Processor Performance Boost Mode has five allowable values, as shown in Table 5. For P-state-based control, the choices are Disabled, Enabled (Turbo is available to the hardware whenever nominal performance is requested), and Efficient (Turbo is available only if the EPB register is implemented). For CPPC-based control, the choices are Disabled, Efficient Enabled (Windows specifies the exact amount of Turbo to provide), and Aggressive (Windows asks for “maximum performance” to enable Turbo). In Windows Server 2012, the default value for Boost Mode is 3.
Powercfg -setactive scheme_current
Note You must run the powercfg -setactive command to enable the new settings. You do not need to reboot the server.
To set this value for power plans other than the currently selected plan, you can use aliases such as SCHEME_MAX (Power Saver), SCHEME_MIN (High Performance), and SCHEME_BALANCED (Balanced) in place of SCHEME_CURRENT. Replace “scheme current” in the powercfg -setactive commands previously shown with the desired alias to enable that power plan. For example, to adjust the Boost Mode in the Power Saver plan and make Power Saver the current plan, run the following commands:
Processors change between performance states (“P-states”) very quickly to match supply to demand, delivering performance where necessary and saving energy when possible. If your server has specific high-performance or minimum-power-consumption requirements, you might consider configuring the Minimum Processor Performance State parameter or the Maximum Processor Performance State parameter.
The values for the Minimum and Maximum Processor Performance State parameters are expressed as a percentage of maximum processor frequency, with a value in the range 0 – 100.
If your server requires ultra-low latency, invariant CPU frequency, or the highest performance levels, you might not want the processors switching to lower-performance states. For such a server, you can cap the minimum processor performance state at 100 percent by using the following commands:
Powercfg -setactive scheme_current
If your server requires lower energy consumption, you might want to cap the processor performance state at a percentage of maximum. For example, you can restrict the processor to 75 percent of its maximum frequency by using the following commands:
Powercfg -setactive scheme_current
Note Capping processor performance at a percentage of maximum requires processor support. Check the processor documentation to determine whether such support exists, or view the Perfmon counter “% of maximum frequency” in the Processor group to see if any frequency caps were applied.
Processor Performance Core Parking Maximum and Minimum Cores
Core parking is a feature that was introduced in Windows Server 2008 R2. The processor power management (PPM) engine and the scheduler work together to dynamically adjust the number of cores that are available to run threads. The PPM engine chooses a minimum number of cores for the threads that will be scheduled. Cores that are chosen to “park” generally do not have any threads scheduled, and they will drop into very low power states when they are not processing interrupts, DPCs, or other strictly affinitized work. The remaining set of “unparked” cores are responsible for the remainder of the workload. Core parking can potentially increase energy efficiency during lower usage periods on the server because parked cores can drop into deep low-power states.
For most servers, the default core-parking behavior provides a reasonable balance of throughput and energy efficiency. On processors where core parking may not show as much benefit on generic workloads, it can be disabled by default. If your server has specific core parking requirements, you can control the number of cores that are available to park by using the Processor Performance Core Parking Maximum Cores parameter or the Processor Performance Core Parking Minimum Cores parameter in Windows Server 2012.
One scenario that core parking has difficulty with is when there are one or more active threads affinitized to a non-trivial subset of CPUs in a NUMA node (that is, more than 1 CPU, but less than the entire set of CPUs on the node). When the core parking algorithm is picking cores to unpark (assuming an increase in workload intensity occurs), it does not know to pick the cores within the active affinitized subset (or subsets) to unpark, and thus may end up unparking cores that won’t actually be utilized.
The values for these parameters are percentages in the range 0 – 100. The Processor Performance Core Parking Maximum Cores parameter controls the maximum percentage of cores that can be unparked (available to run threads) at any time, while the Processor Performance Core Parking Minimum Cores parameter controls the minimum percentage of cores that can be unparked. To turn off core parking, set the Processor Performance Core Parking Minimum Cores parameter to 100 percent by using the following commands:
Processor Performance Core Parking Utility Distribution
Utility Distribution is an algorithmic optimization in Windows Server 2012 that is designed to improve power efficiency for some workloads. It tracks “unmovable” CPU activity (that is, DPCs, interrupts, or strictly affinitized threads), and it predicts the future work on each processor based on the assumption that any movable work can be distributed equally across all unparked cores. Utility Distribution is enabled by default for the Balanced power plans for some processors. It can reduce processor power consumption by lowering the requested CPU frequencies of workloads that are in a reasonably steady state. However, Utility Distribution is not necessarily a good algorithmic choice for workloads that are subject to high activity bursts or for programs where the workload quickly and randomly shifts across processors. For such workloads, we recommend disabling Utility Distribution by using the following commands: