How to Save Idle Energy in Computer Servers

How often is your computer server in a low or zero energy utilization state? The answer may surprise you. On average, utilization is only about 10%-15%. Business patterns vary throughout the day, week, month, and/or year, but long periods of inactivity are typically unavoidable.  

Server utilization is highly dependent on workload as well as the level of virtualization achieved on a group of computer servers, ranging anywhere from 5% to 30% on average in an enterprise environment. Products in a cloud service environment can hit higher utilizations.  

Take Full Advantage of Power Management in Servers 

Most data center operators focus on workload and utilization optimization, but effectively using power management can lower energy use up to 58% for unavoidable server downtimes, thereby saving your organization money with no impact on performance. A bonus is that other data center IT hardware likely also has power management features that can further increase your cost savings. ENERGY STAR certified computer servers are shipped with power management enabled, which is the critical feature when it comes to savings from the server. Having your IT team keep this feature turned on, or alternatively enabling it tuned to the needs of your organization, will save money and help you reach your corporate sustainability goals.  
Here is what you need to know when considering the power management settings of your computer server. 

1)  The Use of C states and P states in Computer Server Configuration  

Processor performance states (P-states) control process power when work is being performed and inactive states (C-states) control the process power when there is a shortage of work. Both variables are typically controlled by the server’s operating system. The figure below shows average process power across processor speed for a typical server CPU with traditional P and C states: 

Plotted graph of Example Processor Power States.

P-state tuning can be used to reduce the voltage and frequency of CPU cores or groups of cores depending on the utilization of the current workload. ENERGY STAR certified servers implement dynamic voltage and frequency scaling (DVFS), allowing the server to transition between these P-states on the order of micro-seconds, constantly optimizing for the workload as it changes.  

C-states further reduce power demand for a core or group of cores below the minimum operating voltage of the cores when work is being done. C-states directly impact measured idle power in a computer server, with a server using a C6 setting in the example above having a notably lower power demand than a server using the C1 setting. More aggressive C-states do come with a performance penalty in the form of increased latency. Therefore, it is important for your data center operator to understand how aggressively C-states can be set for specific workloads to achieve the best balance of energy savings and performance in typical operation. 

2.)  Impact of Component Level Idle Power Management  

Beyond energy savings from the CPU highlighted above, power management functions can also be applied in memory, storage and network components within a server. These features are especially helpful in reducing energy use when the CPU is in various C-state conditions, typically by isolating and turning off component parts not needed to maintain a given level of capacity or latency. Like P-state and C-state tuning above, the most effective results will balance performance needs with sufficiently aggressive power management for a specific user workload or set of workloads.  

Maximize the Benefits of Power Management  

Most servers offer pre-set power management profiles which optimize for performance and energy. For smaller data center operators who do not have complicated workloads, selecting the energy operating profile may be enough for solid savings while maintaining necessary performance. For large data center operators, using tools such as SERT to investigate how tuning P-states, C-states, and component settings at different server utilization levels for your specific workloads can allow for a truly optimized environment where maximum energy is saved while maintaining IT performance requirements.  
Looking for more detailed information?

The Green Grid has provided a more in-depth discussion of how to use SERT to identify ideal power management settings for your computer servers. Interested in more information on SERT and how to best utilize it? Visit the SERT Overview and Deployed Power pages for more information.

Author: Ryan Fogle