IBM Power Systems S822LC Technical Overview And Introduction page 72

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 ensured in all configurations
and conditions. If the energy management goal is to meet a particular consumption limit,
soft power capping is the mechanism to use.
Processor core nap mode
nap
The POWER8 processor uses a low-power mode that is called 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 that are 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 off the clock on most of
the circuits in 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 and causes a cumulative effect. Nap mode saves 10 - 15% of power consumption
in the processor core.
Processor core sleep mode
To save even more energy, the POWER8 processor has an even lower power mode
sleep
referred to as . Before a core and its associated private L2 cache enter sleep mode,
the cache is flushed, transition lookaside buffers (TLB) are invalidated, and the hardware
clock is turned off in the core and in the cache. Voltage is reduced to minimize leakage
current. Processor cores that are inactive in the system (such as capacity on demand
(CoD) processor cores) are kept in sleep mode. Sleep mode saves about 80% of the
power consumption in the processor core and its associated private L2 cache.
Processor chip winkle mode
winkle
The most energy can be saved when a whole POWER8 chiplet enters the mode. In
this mode, the entire chiplet is turned off, including the L3 cache. This mode can save
more than 95% power consumption.
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), the fan
speed varies based on power consumption and ambient temperature.
Processor folding
Processor folding is a consolidation technique that dynamically adjusts, over the short
term, the number of processors that are available for dispatch to match the number of
processors that are 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.
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IBM Power Systems S822LC for High Performance Computing