IBM Power 770, Technical Overview And Introduction

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Chapter 2. Architecture and technical overview
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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 called 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 from 10 - 15% of power
consumption in the processor core.
򐂰 Processor core sleep mode
To be able to save even more energy, the processor has an even lower power
mode referred to as
sleep
. Before a core and its associated L2 and L3 caches enter sleep
mode, caches are flushed, transition lookaside buffers (TLB) are invalidated, and the
hardware clock is turned off in the core and in the caches. Voltage is reduced to minimize
leakage current. Processor cores inactive in the system (such as CoD processor cores)
are kept in sleep mode. Sleep mode saves about 35% power consumption in the
processor core and associated L2 and L3 caches.
򐂰 Fan control and altitude input
System firmware dynamically adjusts fan speed based on energy consumption, altitude,
ambient temperature, and energy savings modes. Power Systems are designed to
operate in worst-case environments, in hot ambient temperatures, at high altitudes, and
with high power components. In a typical case, one or more of these constraints are not
valid. When no power savings setting is enabled, fan speed is based on ambient
temperature and assumes a high-altitude environment. When a power savings setting is
enforced (either Power Energy Saver Mode or Dynamic Power Saver Mode), fan speed
will vary based on power consumption, ambient temperature, and altitude available.
System altitude can be set in IBM Director Active Energy Manager. If no altitude is set, the
system will assume a default value of 350 meters above sea level.
򐂰 Processor folding
Processor folding is a consolidation technique that dynamically adjusts, over the short
term, the number of processors available for dispatch to match the number of processors
demanded by the workload. As the workload increases, the number of processors made
available increases. As the workload decreases, the number of processors that are made
available decreases. Processor folding increases energy savings during periods of low to
moderate workload because unavailable processors remain in low-power idle states (nap
or sleep) longer.
򐂰 EnergyScale for I/O
IBM POWER7 and processor-based systems automatically power off hot
pluggable PCI adapter slots that are empty or not being used. System firmware