Disable C-State, Why That?

C-State what is this?

  • C0 – Active: CPU is on. C0 is the operating state.
  • C1 – Auto Halt: core clock is off. C1 is a state where the processor is not executing instructions, but can return to an executing state essentially instantaneously. Some processors also support an Enhanced C1 state (C1E) for lower power consumption.
  • C2 – Stop Clock: core and bus clocks are off. C2 is a state where the processor maintains all software-visible state, but may take longer to wake up.
  • C3 – Deep Sleep: clock generator is off. C3 is a state where the processor does not need to keep its cache coherent, but maintains other states. Some processors have variations on the C3 state (Deep Sleep, Deeper Sleep, etc.) that differ in how long it takes to wake the processor.
  • C4 – Deeper Sleep: reduced VCC
  • DC4 – Deeper C4 Sleep: further reduced VCC


What is the issue?

Servers running Windows Server 2008 R2 with Hyper-V may encounter a blue screen aka BSOD after enabling Hyper-V (KB974598). The installation of Hyper-V and the loading of C-states during initialization of Hyper-V causes a conflict, resulting in a blue screen. The system uses a C-state that is not supported by Hyper-V.


Good practise but…

If you have C-State already turned on, reboot your server, enter the BIOS and just turn off any C-State setting… You don’t want to clock down or stop your processors’ clock because the server is idle, do you?

Now if you use VMware DPM, that’s something you definitely need to keep turned on because DPM uses all those ACPI states to reduce power consumption of your ESX hosts, more info at VMware.com.  Jason Boche posted a good article on how he could save a few Dollars by turning on those features in his home lab.

And do you know that since ESX3.0/VC2.0, VMware Tools provides support for guest operating systems that enable ACPI S1 sleep. 


And what about G-State, S-State and even D-State?

This is known as APM (Advanced Power Management). ACPI allows for control of power management to be turned over to the OS. With ACPI the system will have different states of “sleep.” States range from suspend mode to soft off state.

Global States Sleep States CPU states Device States
Device States S0System is on C0Working D0Working State
G1Sleeping State S1CPU stopped C1Stop Grant D1, D2, D3Device Spec specific
  S2Power on suspend
C2Clock Stopped D2,D3Device Spec specific
S3Suspend To RAMContext saved to RAM No Power D3No power except for wake up logic
S4Suspend to DiskContext saved to DiskAlso known as ‘hibernate’. No Power D3No power except for wake up logic
G2/G5 S5Soft offContext is not savedCold boot is required. No Power D3No power except for wake up logic
G3Mechanical off The power supply is unplugged from thepower source No power to the system No Power No power for wake up logic, except when provided by battery or ext. source

Sources: Intel.com, Microsoft.com, HP.com

About PiroNet

Didier Pironet is an independent blogger and freelancer with +15 years of IT industry experience. Didier is also a former VMware inc. employee where he specialised in Datacenter and Cloud Infrastructure products as well as Infrastructure, Operations and IT Business Management products. Didier is passionate about technologies and he is found to be a creative and a visionary thinker, expressing with passion and excitement, hopefully inspiring and enrolling people to innovation and change.
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6 Responses to Disable C-State, Why That?

  1. Pingback: A Year Blogging In Summary And Season’s Greetings « DeinosCloud

  2. Pingback: Ingmar Verheij – The dutch IT guy » Damn you C-states! (Unexpected XenServer reboot) » Ingmar Verheij - The dutch IT guy

  3. /b/ro says:

    thank you, I don’t own a server but this was useful for me as I want to get rid of any kind of stuttering in games and while doing other things on my computer

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  5. Wonderful! Keep it up. 😀

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