Intel ACPI Guide - C/G/S/P states and OCs

SQLinsert · Jul 5, 2011

Intel ACPI Guide - C/G/S/P states and OCs

There are quite a array of power modification settings (

) in the BIOS and in Windows that will change how a CPU operates concerning stability, multiplier throttling, power consumption, sleep states and processor voltage/multipliers. Let me know if I forgot anything and I will add it to the list. If possible I will update this later on with more details for each setting.

THE BASICS

P-States are responsible for lowering the CPU multiplier and CPU voltage when there is no work load. These are configured in the BIOS as PPM or Speed Step. They are passed onto the OS and configured in Power Options control panel (in Windows).
S-States are sleep states. These are set in the BIOS and then configured in the power options control panel (timeout in minutes). There are various sleep technologies such as S1 sleep, S3 sleep, Hybrid sleep and Hibernation (which is a laptop sleep technology).
G-States are global operating states and are not configurable by the user. These are just used in documentation to specify certain system states such as on, off, sleeping.
C-States are advanced CPU current lowering technologies. These are configured in the BIOS and are automatically used by the OS.

VARIOUS TERMINOLOGY

ACPI (Advanced Configuration & Power Interface) - ACPI is basically a umbrella under which most modern power reduction technologies exist.
C-STATE
C1E (Enhanced C1 state)
Hybrid Sleep
Intelppm.sys registry key
Processor Power Management kernel driver (handling EIST & Hibernation?)
SpeedStep (also known as Performance State or "PPM P-States")
Turbo Boost (becomes active at P-State 0 or P0)
Windows Power Options Control Panel

Other guides
About how to troubleshoot power plans in Windows 7
ACPI APIC Function
Advanced Configuration & Power Interface (ACPI)
C-states, C-states and even more C-states
C-states and P-states are very different
CPU Idle States
Powercfg Command-Line Options
PPM in Windows 7 and Windows Server 2008 R2
What exactly is a P-state? (Pt. 1)
What is difference between deep and deeper sleep states? (mobile procs only)
What's the difference between sleep, hibernate, and hybrid sleep?

Global power management scheme models

Sleep States

Useful Data

Quote:

2.4 Sleeping State Definitions

Sleeping states (Sx states) are types of sleeping states within the global sleeping state, G1. The Sx states are briefly defined below. For a detailed definition of the system behavior within each Sx state, see section 7.3.4, "System \_Sx States." For a detailed definition of the transitions between each of the Sx states, see section 15.1, "Sleeping States."

S1 Sleeping State
The S1 sleeping state is a low wake latency sleeping state. In this state, no system context is lost (CPU or chip set) and hardware maintains all system context.

S2 Sleeping State
The S2 sleeping state is a low wake latency sleeping state. This state is similar to the S1 sleeping state except that the CPU and system cache context is lost (the OS is responsible for maintaining the caches and CPU context). Control starts from the processor's reset vector after the wake event.

S3 Sleeping State
The S3 sleeping state is a low wake latency sleeping state where all system context is lost except system memory. CPU, cache, and chip set context are lost in this state. Hardware maintains memory context and restores some CPU and L2 configuration context. Control starts from the processor's reset vector after the wake event.

S4 Sleeping State
The S4 sleeping state is the lowest power, longest wake latency sleeping state supported by ACPI. In order to reduce power to a minimum, it is assumed that the hardware platform has powered off all devices. Platform context is maintained.

S5 Soft Off State
The S5 state is similar to the S4 state except that the OS does not save any context. The system is in the "soft" off state and requires a complete boot when it wakes. Software uses a different state value to distinguish between the S5 state and the S4 state to allow for initial boot operations within the BIOS to distinguish whether or not the boot is going to wake from a saved memory image.

2.5 Processor Power State Definitions

Processor power states (Cx states) are processor power consumption and thermal management states within the global working state, G0. The Cx states possess specific entry and exit semantics and are briefly defined below. For a more detailed definition of each Cx state, see section 8.1, "Processor Power States."

C0 Processor Power State
While the processor is in this state, it executes instructions.

C1 Processor Power State
This processor power state has the lowest latency. The hardware latency in this state must be low enough that the operating software does not consider the latency aspect of the state when deciding whether to use it. Aside from putting the processor in a non-executing power state, this state has no other software-visible effects.

C2 Processor Power State
The C2 state offers improved power savings over the C1 state. The worst-case hardware latency for this state is provided via the ACPI system firmware and the operating software can use this information to determine when the C1 state should be used instead of the C2 state. Aside from putting the processor in a non-executing power state, this state has no other software-visible effects.

C3 Processor Power State
The C3 state offers improved power savings over the C1 and C2 states. The worst-case hardware latency for this state is provided via the ACPI system firmware and the operating software can use this information to determine when the C2 state should be used instead of the C3 state. While in the C3 state, the processor's caches maintain state but ignore any snoops. The operating software is responsible for ensuring that the caches maintain coherency.

8.1.5 Additional Processor Power States
ACPI introduced optional processor power states beyond C3 starting in ACPI 2.0. These power states, C4… Cn, are conveyed to OSPM through the _CST object defined in section 8.4.2.1, "_CST (C-States)." These additional power states are characterized by equivalent operational semantics to the C1 through C3 power states, as defined in the previous sections, but with different entry/exit latencies and power savings. See section 8.4.2.1, "_CST (C-States)," for more information

2.6 Device and Processor Performance State Definitions

Device and Processor performance states (Px states) are power consumption and capability states within the active/executing states, C0 for processors and D0 for devices. The Px states are briefly defined below. For a more detailed definition of each Px state from a processor perspective, see section 8.4.4, "Processor Performance Control." For a more detailed definition of each Px state from a device perspective see section 3.6, "Device and Processor Performance States," and the device class specifications in Appendix A.

P0 Performance State
While a device or processor is in this state, it uses its maximum performance capability and may consume maximum power.

P1 Performance State
In this performance power state, the performance capability of a device or processor is limited below its maximum and consumes less than maximum power.

Pn Performance State
In this performance state, the performance capability of a device or processor is at its minimum level and consumes minimal power while remaining in an active state. State n is a maximum number and is processor or device dependent. Processors and devices may define support for an arbitrary number of performance states not to exceed 16.

Quoted from: Advanced Configuration and Power Interface Specification. Revision 4.0a: April 5, 2010.

Quote:

8 Processor Configuration and Control

ACPI defines the power state of system processors while in the G0 working state as being either active (executing) or sleeping (not executing). Processor power states include are designated C0, C1, C2, C3, …Cn. The C0 power state is an active power state where the CPU executes instructions. The C1 through Cn power states are processor sleeping states where the processor consumes less power and dissipates less heat than leaving the processor in the C0 state. While in a sleeping state, the processor does not execute any instructions. Each processor sleeping state has a latency associated with entering and exiting that corresponds to the power savings. In general, the longer the entry/exit latency, the greater the power savings when in the state. To conserve power, OSPM places the processor into one of its supported sleeping states when idle. While in the C0 state, ACPI allows the performance of the processor to be altered through a defined "throttling" process and through transitions into multiple performance states (P-states). A diagram of processor power states is provided below.

Notice that these CPU states map into the G0 working state. The state of the CPU is undefined in the G3
sleeping state, the Cx states only apply to the G0 state.

Quote:

3.5 Processor Power Management aka PPM

To further save power in the Working state, the OS puts the CPU into low-power states (C1, C2, and C3) when the OS is idle. In these low-power states, the CPU does not run any instructions, and wakes when an interrupt, such as the OS scheduler's timer interrupt, occurs. The OS determines how much time is being spent in its idle loop by reading the ACPI Power Management Timer. This timer runs at a known, fixed frequency and allows the OS to precisely determine idle time. Depending on this idle time estimate, the OS will put the CPU into different quality low-power states (which vary in power and latency) when it enters its idle loop. The CPU states are defined in detail in section 8, "Processor Configuration and Control."

Running Pwrtest to evaluate system PPM and P-States

DL LINK

Code:

Code:

usage: pwrtest /scenario [/scenario_options] [/common_options]

scenario indicates the test scenario                                 Min OS Req
  sleep          for sleep/resume transition testing                 Vista
  battery        for battery information testing                     Vista
  info           for system capabilities information                 Vista
  es             for thread execution state changes                  Vista
  idle           for power idle statistics                           Vista
  ppm            for processor power management testing              Vista
  timer          for system timer resolution statistics              Vista
  disk           for disk idle statistics                            Vista SP1
  device         for device idle statistics                          Vista SP1
  monitor        for monitor dimming and blanking statistics         Win7
  requests       for showing power requests                          Win7
  thermal        for ACPI thermal zone monitoring                    Win7
  processidle    for forcing idle/background tasks to be run         Win7

common_options can be any of the following options
  /session:name      name indicates the ETW session name.
                     Default session name is PwrTest
  /xmllogname:path   path indicates the XML log file path.
                     For example, c:\slprsm.xml or \\server\share\slprsm.xml
                     Default XML log, PwrTestLog.xml, is created in the same
                     directory as pwrtest.exe
  /l:path            same as /xmllogname option
  /wttlogname:path   path indicates the WTT log file path.
                     For example, c:\pwrtest.wtl or \\server\share\pwrtest.wtl
                     Default WTT log, pwrtest.wtl, is created in the same
                     directory as pwrtest.exe
  /etwbuffersize:n   n indicates ETW buffer size in KB if larger than default.
                     Default is current page size or 256KB (whichever is
                     greater).
  /etwminbuffers:n   n indicates the minimum number of buffers allocated for
                     the ETW session if larger than the minimum of 2 per
                     logical processor.
                     Default is 2 per logical processor
  /etwmaxbuffers:n   n indicates the maximum number of buffers allocated for
                     the ETW session if larger than the minimum of 2 per
                     logical processor and larger than etwminbuffers.
                     Default is etwminbuffers + 20

For detailed scenario_options information, type pwrtest.exe /scenario /?. For
example, to display detailed information about the /sleep scenario, type:

  pwrtest.exe /sleep /?

This useful tool can be used to verify if the OS sees C-State Enabled/Disabled or if you are using C-State in conjunction with Speed Step you can use the second command.

Verify C-State Disabled in OS >> C0

Code:

Code:

...  pwrtest.exe /ppm
Info: the CPU doesn't support P-states.
Info: the CPU doesn't support C-states.
The CPU doesn't support P/C states.

To verify PPM states (example) - it should be noted that in most of the documentation from Intel the "Performance States" are basically just steps the processor takes to reduce its power consumption and overall temperature. These states are apparently configured in the Windows Power Options control panel under "max" and "min" CPU performance percentage (%). As mentioned below, it is apparently a option to alter the OS registry settings and create custom non-linear profiles. The States begin at 0/100% and then go down to a higher number and lower percentage. These are the "P-States" and are part of Speed Step technology, PPM and your BIOS configuration. If you want to use these you need to think about how you're going to set up all the configuration points (Auto voltage, Speed Step on, Multiplier Auto, etc.).

Code:

Code:

C:\Program Files\Microsoft PwrTest>pwrtest /info:ppm
PROCESSOR_POWER_INFORMATION
         CPU Number             = 0
         MaxMhz                 = 1833
         CurrentMhz             = 1833
         MhzLimit               = 1833
         MaxIdleState           = 3
         CurrentIdleState       = 3

InstanceName: ACPI\GenuineIntel_-_x86_Family_6_Model_14\_0_0

Processor Performance States
  PerfStates:
    Max Transition Latency:  10 us
    Number of States:        11

    State  Speed (Mhz)    Type
    -----  ------------   ----
      0     1833 (100%)   Performance
      1     1333 ( 72%)   Performance
      2     1000 ( 54%)   Performance
      3     1000 ( 54%)   Throttle
      4      880 ( 48%)   Throttle
      5      750 ( 40%)   Throttle
      6      630 ( 34%)   Throttle
      7      500 ( 27%)   Throttle
      8      380 ( 20%)   Throttle
      9      250 ( 13%)   Throttle
      10     130 (  7%)   Throttle

Even if you have C-States off in BIOS you can still verify your S-States by using (below). I believe some motherboards allow you to specify whether or not these settings are available to Windows. ASUS motherboards have the settings "Power > Suspend Mode S1/S3; Power > ACPI 2.0 On/Off; Power > EuP Ready S5; APM > Power On By PCI Devices S5." The dump from my system (shown below) shows S1 & S3-5 available even though my power options are all set to leave the machine on and not go into sleep. Meaning, the OS handles the configuration of sleep and the BIOS tells the OS what S-levels are available.

Code:

Code:

... pwrtest.exe /info:all
SYSTEM_POWER_CAPABILITIES
         SystemS1StateSupported = 1
         SystemS2StateSupported = 0
         SystemS3StateSupported = 1
         SystemS4StateSupported = 1
         SystemS5StateSupported = 1
         RtcWakeSupported       = S4
         FastSystemS4           = 1
         VideoDimPresent        = 0

SYSTEM_POWER_INFORMATION
         MaxIdlenessAllowed     = 0
         Idleness               = 99
         TimeRemaining          = -1
         CoolingMode            = 0

SYSTEM_BATTERY_STATE
         AcOnLine               = 1
         BatteryPresent         = 0
         Charging               = 0
         Discharging            = 0
         MaxCapacity            = 0
         RemainingCapacity      = 0
         RateOfDrain            = 0
         EstimatedTime          = 0
         DefaultAlert1          = 0
         DefaultAlert2          = 0
Warn: Warning: There is no Battery in this machine!
    File: _:\5359\base\power\tools\pwrtest\exe\sleep.cpp
    Line: 4830

There are registry listings in the document "Processor Power Management in Windows 7 and Windows Server 2008 R2" that explain how to adjust manually the Processor Power Policy Settings (pages 27 to 39) in the registry. This may be useful for extreme modders.

LINK

It should be noted that the P-States according to this intel page will both change the input voltage and input multiplier (hence frequency) of your processor. If you are OC'ing this may be a bad thing and you would want to disable SpeedStep in the BIOS and do the ppm.sys driver tweak disable in Windows. Intel also calls P-States "processor power state levels."

Can I OC while using P-States aka Speed Step?

Why yes you can. It's very simple. Establish your OC. Set your multiplier, voltages and other BIOS settings. Stability check your OC and work out any bugs. Then, finally go into your BIOS and enable Speed Step. Then you can enter your OS and set your performance levels in Power Options. Notice in the following two images that the minimum processor state (%) directly changes the lowest multiplier in use as well as altering the final processor clock as well as the power consumption (lower watts/amps in use). You will need to stability test your machine to see how well it transitions from your low % to your high %. That's on you. Some people have complained that it's not possible to lower the processor voltage as well as the cpu multiplier to have the added benefit of lower temperatures. Well, in order to get your OC working you probably manually entered a BIOS voltage for your processor and that overrides your OS P-State. Some people have had offset usage success, but you'll need to experiment with that as it seems to be working or not working on a case-by-case basis.

4GHz OC'd machine operating P-State 5% minimum performance

4GHz OC'd machine operating P-State 75% minimum performance

You will also need to enable your intelppm.sys registry key for "start = 1" and reboot. This driver can cause BSOD issues and is normally disabled on my machine. Registry levels: start= . Obviously since P-States only work in the OS system/1 is best.

Speed Step ramping up under load (below) - as you can see it will ramp up to your highest multiplier under load. My score on this run was 6.72 and normally on full power with Speed Step off it is 6.86. So, with the power saving (but not really temperature saving) P-States there is a slight reduction in performance by about 2.041%. I also notice that the WDK pwrtest.exe module would not report accurate values when OC'd with Speed Step enabled. This is probably a limitation of the software. It only reported the highest frequency state as the OEM listed rating. Example, enable switch /info_:_ppmperf (without _) highest speed listed for my i7 950 is "100%/3068 MHz."

This above mini-section /\ was written with P-States on and C-States off. C-states are additional power saving controls that can be configured in your BIOS. Experiment and get Speed Step running first and then experiment with your C-State settings. All the C-State details are explained above. Different BIOS manufactures implement this setting differently and you can play with them on your own time.

During testing I discovered that there is a instability introduced into a system when the ramp up to full multiplier is large compared to the minimum processor % set in power options. A 5% change in processor delta will change your cpu multiplier 1 unit (at least on my board). So say your minimum processor percentage is 0 percent and that equates to a multiplier of 1; increasing to 5 percent results in a minimum CPU multiplier of 2, and so on. During testing a 124 bsod hit my system when the min ppm in power options was set to 50% (12x multiplier). The error happened in internet explorer unfortunately as a HAL (hardware abstraction layer) problem. After increasing the minimum processor % in power options (ppm p-state) to 65% (15x multi.) there haven't been any issues since, even during stability benching. So basically don't set the minimum processor state too low and if you have issues try increasing it by 5% at a time (if you want) to increase the minimum CPU multiplier by 1x. It is possible to Speed Step your OC you just have to dial it in.

C-States and Amperage Savings - ymmv. My bios also has a C6 setting, but since that isn't mentioned in the official Intel, et al industry specification I only allowed C3 as a minimum. C-States also have the unfortunate side effect of jacking with Razer mice really bad. If you are a gamer make sure all of your C-State stuff is OFF. CPU Spread Spectrum is also a bad choice when using gaming mice. Turn that off too. If you have a moderate overclock you may be able to use C-States but otherwise turn them off if you are a high speed OCer. I have noticed they cause instability with high OCs and Windows crashing errors (not BSODs but "not responding" problems). As you can see there is over a 50% savings in terms of current draw, although the max with C-States on went up to 19.31 compared to 15.69. C-States really aren't that big of a deal; if they work on your system and you like them go ahead. If they cause interference then just turn them off. Speed Step is a much more reliable power saving mechanism at least in my testing. If you are this interested in these power saving C-States then download the official ACPI specification and read pages 309-312 where you can clearly see there are additional "internal speed steps" available. They are posted here for your review.

|

Hybrid Sleep Mode explained

In Windows the hybrid sleep setting is located in the Power Options control panel. It's either On or Off. Notice how various drivers if not programmed correctly will not awaken from this sleep mode. Entering hybrid sleep is possible by letting a idle timer expire or entering sleep from the start menu. If hybrid sleep is off the OS will default to whatever S-level is configured in the BIOS. Btw, if your BIOS is set to disable S3 or in other words set to only run S1 max (in sleep state) the power options control panel will not display Hybrid Sleep as a option (as expected) -- fyi.

More on sleep states
Quote:

Sleep is a power-saving state that allows a computer to quickly resume full-power operation (typically within several seconds) when you want to start working again. Putting your computer into the sleep state is like pausing a DVD player-the computer immediately stops what it's doing and is ready to start again when you want to resume working.

Hibernation is a power-saving state designed primarily for laptops. While sleep puts your work and settings in memory and draws a small amount of power, hibernation puts your open documents and programs on your hard disk, and then turns off your computer. Of all the power-saving states in Windows, hibernation uses the least amount of power. On a laptop, use hibernation when you know that you won't use your laptop for an extended period and won't have an opportunity to charge the battery during that time.

Hybrid sleep is designed primarily for desktop computers. Hybrid sleep is a combination of sleep and hibernate-it puts any open documents and programs in memory and on your hard disk, and then puts your computer into a low-power state so that you can quickly resume your work. That way, if a power failure occurs, Windows can restore your work from your hard disk. When hybrid sleep is turned on, putting your computer into sleep automatically puts your computer into hybrid sleep. Hybrid sleep is typically turned on by default on desktop computers.

Turbo Boost

Most people are probably already familiar with Turbo Boost technology, but here you go anyway. I assume that if you wanted to use Turbo Boost you would also need to make sure your BIOS is configured for auto voltage control. Meaning, if you specify a particular manual voltage and turbo boost attempts to increase your multiplier at P0 -- you may crash. Similar to Speed Step, I think this requires both a voltage and multiplier change. Obviously if you OC you would want both turbo boost and speed step off. At least disable Turbo Boost if you are using Speed Step aka P-States with your OC. It's more of a consumer-level technology that will give a slight bump for a mild, temporary OC.
Quote:

Intel Turbo Boost Technology 2.0 is activated when the Operating System (OS) requests the highest processor performance state (P0).

Other Pages of interest
Intel Turbo Boost Technology Monitor Overview - Intel's tool to monitor Turbo Boost
Intel Core i7 desktop processor Intel Turbo Boost Technology frequency table
Intel Processor Frequency ID Utility

Thanks for reading. If there's anything else it will be included in future updates.

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[URL="https://www.overclock.net/threads/1058894/"]Intel ACPI Guide - C/G/S/P states and OCs[/URL]

stren · Jul 5, 2011

Very useful, thanks!

H969 · Jul 6, 2011

Very nice,Thanks!!
I am very intrested in the "C" states!!
I do not have time to read but subbed for later

Intel ACPI Guide - C/G/S/P states and OCs

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