Chapter 2. Architecture and technical overview
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Figure 2-32, from the Active Energy Manager, shows the dynamic CPU frequency change
in a system using the dynamic power saver mode.
Figure 2-32 Example of a system using dynamic power saver mode
Power capping
Power capping enforces a user-specified limit on power usage. Power capping is not a
power-saving mechanism. It enforces power caps by throttling the processors in the
system, degrading performance significantly. The idea of a power cap is to set a limit that
must never be reached but that frees up extra power never used in the data center. The
margined
power is this amount of extra power that is allocated to a server during its
installation in a data center. It is based on the server environmental specifications that
usually are never reached because server specifications are always based on maximum
configurations and worst-case scenarios. The user must set and enable an energy cap
from the IBM Director Active Energy Manager user interface.
Soft power capping
There are two power ranges into which the power cap can be set: power capping, as
described previously, and soft power capping. Soft power capping extends the allowed
energy capping range further, beyond a region that can be guaranteed in all configurations
and conditions. If the energy management goal is to meet a particular consumption limit,
then soft power capping is the mechanism to use.
Processor core nap mode
The IBM POWER7 and processor uses a low-power mode referred to as
nap
that stops processor execution when there is no work to do on that processor core. The
latency of exiting nap mode is small, typically not generating any impact on applications
running. Therefore, the IBM POWER Hypervisor™ can use nap mode as a
general-purpose idle state. When the operating system detects that a processor thread is
idle, it yields control of a hardware thread to the POWER Hypervisor. The POWER
Hypervisor immediately puts the thread into nap mode. Nap mode allows the hardware to
turn the clock off on most of the circuits inside the processor core. Reducing active energy
consumption by turning off the clocks allows the temperature to fall, which further reduces
leakage (static) power of the circuits causing a cumulative effect. Nap mode saves 10 -
15% of power consumption in the processor core.
Overclocking
Power saving
