Intel G2120 CM8063701095801 User Manual

Datasheet, Volume 1

45

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.

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.