Datasheet

Datasheet, Volume 1 45
Power Management
4.2.4.5 C-State Auto-Demotion
In general, deeper C-states, such as C6, have long latencies and have higher energy
entry/exit costs. The resulting performance and energy penalties become significant
when the entry/exit frequency of a deeper C-state is high.
Therefore, incorrect or inefficient usage of deeper C-states may have a negative impact
on power consumption. To increase residency and improve power consumption in
deeper C-states, the processor supports C-state auto-demotion.
There are two C-State auto-demotion options:
•C6 to C3
• C6/C3 To C1
The decision to demote a core from C6 to C3 or C3/C6 to C1 is based on each core’s
residency history. Requests to deeper C-states are demoted to shallower C-states when
the original request doesn't make sense from a performance or energy perspective.
This feature is disabled by default. BIOS must enable it in the
PMG_CST_CONFIG_CONTROL register. The auto-demotion policy is also configured by
this register.
4.2.5 Package C-States
The processor supports C0, C1/C1E, C3, and C6 power states. The following is a
summary of the general rules for package C-state entry. These apply to all package C-
states unless specified otherwise:
• A package C-state request is determined by the lowest numerical core C-state
amongst all cores.
• A package C-state is automatically resolved by the processor depending on the
core idle power states and the status of the platform components.
— Each core can be at a lower idle power state than the package if the platform
does not grant the processor permission to enter a requested package C-state.
— The platform may allow additional power savings to be realized in the
processor.
• For package C-states, the processor is not required to enter C0 before entering any
other C-state.
The processor exits a package C-state when a break event is detected. Depending on
the type of break event, the processor does the following:
• If a core break event is received, the target core is activated and the break event
message is forwarded to the target core.
— If the break event is not masked, the target core enters the core C0 state and
the processor enters package C0.
— If the break event is masked, the processor attempts to re-enter its previous
package state.
• If the break event was due to a memory access or snoop request.
— But the platform did not request to keep the processor in a higher package C-
state, the package returns to its previous C-state.
— And the platform requests a higher power C-state, the memory access or snoop
request is serviced and the package remains in the higher power C-state.