ACPI Power Management States
These are the power management states as defined in the ACPI specification. I’ll go through them briefly but let's get a big picture of it first. The details of each and every state will be described in the following sections.
Global system states
- The entire system state which is visible to user.
- Divided into 4 states – G0, G1, G2, G3
- They are the sleeping states resides in global system state G1 (except S5)
- Divided into 5 states – S1, S2, S3, S4, S5
Device Power states
- This power state is usually invisible to user.
- When a device is at ‘on’ state, another one might be in ‘off’ state.
- Divided into 4 states – D0, D1, D2, and D3.
CPU Power states
- Also known as CPU sleep states.
- It is within the global system state G0
- So far divided into 5 states – C0, C1, C2, C3, and C4.
- In future, there will be up to C6 states (Penryn).
CPU / Device Performance states
- CPU / Device power management when it is still active.
- Usually clock speed and voltage varies depend on workload.
- The amount of P-state is CPU / Device specific
- Eg. Higher clock ratio CPU will have more P-states than a lower clock ratio CPU
CPU Thermal Monitor
- It throttles the CPU to a lower performance state when temperature exceeds the threshold.
- In TM1, throttling is done via changing its duty cycle.
- In TM2, throttling is done via changing its clock speed and core voltage (P-state).
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