3
Introduction
Server performance-per-watt continues to increase steadily. However, the number of watts-per-server also continues
to climb steadily. These increases, combined with the increasing number of servers and density in modern data
centers, make planning and managing facility power and cooling resources critically important. HP Power Capping
and HP Dynamic Power Capping are ProLiant power management tools that assist the data center administrator in
these critical tasks.
HP implements both Power Capping and HP Dynamic Power Capping in system hardware and firmware. Therefore,
they are not dependent on the operating system or applications. Power capping uses the power monitoring and
control mechanisms built into ProLiant servers. These mechanisms allow an administrator to limit, or cap, the power
consumption of a server or group of servers. Power capping lets you manage the data center parameters that server
power consumption directly influences, including data center cooling requirements and electrical provisioning.
Power capping also lets you control server power consumption in emergencies such as loss of primary AC power.
It is important to understand that power capping does not reduce the total energy consumption required for a server
to accomplish a given computational workload. Power capping simply limits the amount of power that a server can
use at any point in time. This lets you allocate data center power and cooling resources more efficiently. In general,
if a given power cap restricts the amount of power that a server would normally use to perform a task, that task will
take longer. Over time, the server will consume about the same total energy to execute the same computational
workload.
This paper outlines the use of power capping as part of a planning and provisioning strategy in the data center. It
also describes the relationship between power capping and other power management tools such as HP Power
Regulator.
Basics of server power control
The processor complex is one of the single greatest power consumers in ProLiant servers. In many common
configurations it is responsible for one-third of the power a server consumes (Figure 1). It also indirectly drives the
power consumption of other server components. A busy processor naturally increases the workload in both the
memory and peripherals. The heat generated by the increased workload causes the fans to work harder.
All HP power management technologies use this processor-driven model to control the processor’s power
consumption directly and to control overall server power consumption indirectly. The power management system
accomplishes this control using two separate mechanisms: changing the processor P-state and throttling the
processor clock.
