C-states

The processor power states, or C-states, are low-power modes that can be used when the CPU is idle. The idea is not new: already in the 80486 processor a specific instruction (HLT) was used for this purpose. This was later accompanied by a pair of other instructions (MONITOR, MWAIT). By default, NetBSD may use either one; see the machdep.idle-mechanism sysctl(8) variable. ACPI provides the latest amendment.

The following C-states are typically available. Additional processor or vendor specific states (C4, ..., Cn) are handled internally by acpicpu.

C0

This is the normal state of a processor; the CPU is busy executing instructions.

C1

This is the state that is typically reached via the mentioned x86 instructions. On a typical processor, C1 turns off the main internal CPU clock, leaving APIC running at full speed. The CPU is free to temporarily leave the state to deal with important requests.

C2

The main difference between C1 and C2 lies in the internal hardware entry method of the processor. While less power is expected to be consumed than in C1, the bus interface unit is still running. But depending on the processor, the local APIC timer may be stopped. Like with C1, entering and exiting the state are expected to be fast operations.

C3

This is the deepest conventional state. Parts of the CPU are actively powered down. The internal CPU clock is stopped. The local APIC timer is stopped. Depending on the processor, additional timers such as x86/tsc(9) may be stopped. Processor caches may be flushed. Entry and exit latencies are expected to be high; the CPU can no longer “quickly” respond to bus activity or other interruptions.

Each state has a latency associated with entry and exit. The higher the state, the lower the power consumption, and the higher the potential performance costs.

The acpicpu driver tries to balance the latency constraints when choosing the appropriate state. One of the checks involves bus master activity; if such activity is detected, a lower state is used. It is known that particularly usb(4) may cause high activity even when not in use. If maximum power savings are desirable, it may be necessary to use a custom kernel without USB support. And generally: to save power with C-states, one should avoid polling, both in userland and in the kernel.

P-states

The processor performance states, or P-states, are used to control the clock frequencies and voltages of a CPU. Underneath the abstractions of ACPI, P-states are associated with such technologies as “SpeedStep” (Intel), “PowerNow!” (AMD), and “PowerSaver” (VIA).

The P0-state is always the highest operating frequency supported by the processor. The number of additional P-states may vary across processors and vendors. Each higher numbered P-state represents lower clock frequencies and hence lower power consumption. Note that while acpicpu always uses the exact frequencies internally, the user-visible values reported by ACPI may be rounded or approximated by the vendor.

Unlike conventional CPU frequency management, ACPI provides support for Dynamic Voltage and Frequency Scaling (DVFS). Among other things, this means that the firmware may request the implementation to dynamically scale the presently supported maximum or minimum clock frequency. For example, if acpiacad(4) is disconnected, the maximum available frequency may be lowered. By default, the NetBSD implementation may manipulate the frequencies according to the notifications from the firmware.

T-states

Processor T-states, or “throttling states”, can be used to actively modulate the time a processor is allowed to execute. Outside the ACPI nomenclature, throttling and T-states may be known as “on-demand clock modulation” (ODCM).

The concept of “duty cycle” is relevant to T-states. It is generally defined to be a fraction of time that a system is in an “active” state. The T0-state has always a duty cycle of 100 %, and thus, comparable to the C0-state, the processor is fully active. Each additional higher-numbered T-state indicates lower duty cycles. At most eight T-states may be available, although also T-states use DVFS.

The duty cycle does not refer to the actual clock signal, but to the time period in which the clock signal is allowed to drive the processor chip. For instance, if a T-state has a duty cycle of 75 %, the CPU runs at the same clock frequency and uses the same voltage, but 25 % of the time the CPU is forced to idle. Because of this, the use of T-states may severely affect system performance.

There are two typical situations for throttling: power management and thermal control. As a technique to save power, T-states are largely an artifact from the past. There was a short period in the x86 lineage when P-states were not yet available and throttling was considered as an option to modulate the processor power consumption. The approach was however quickly abandoned. In modern x86 systems P-states should be preferred in all circumstances. It is also more beneficial to move from the C0-state to deeper C-states than it is to actively force down the duty cycle of a processor.

But T-states have retained their use as a last line of defense against critical thermal conditions. Many x86 processors include a catastrophic shutdown detector. When the processor core temperature reaches this factory defined trip-point, the processor execution is halted without any software control. Before this fatal condition, it is possible to use throttling for a short period of time in order to force the temperatures to lower levels. The thermal control modulation is typically started only when the system is in the highest-power P-state and a high temperature situation exists. After the temperatures have returned to non-critical levels, the modulation ceases.

System Control Variables

The acpicpu driver uses the same sysctl(8) controls for P-states as the ones provided by est(4) and powernow(4). Depending on the processor, the second-level node is either machdep.est or machdep.powernow. Please note that future versions of acpicpu may however remove these system control variables without further notice.

In addition, the following two variables are available.

hw.acpi.cpu.dynamic

A boolean that controls whether the states are allowed to change dynamically. When enabled, C-, P-, and T-states may all change at runtime, and acpicpu may also take actions based on requests from the firmware.

hw.acpi.cpu.passive

A boolean that enables or disables automatic processor thermal management via acpitz(4).

Statistics

The acpicpu driver uses event counters to track the times a processor has entered a given state. It is possible to view the statistics by using vmstat(1) (with the -e flag).