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Thread: How Well Is AMD CnQ Working For You On Linux?

  1. #1
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    Default How Well Is AMD CnQ Working For You On Linux?

    Phoronix: How Well Is AMD CnQ Working For You On Linux?

    One of the latest possible Linux power-related regressions I've heard about is that AMD Cool 'n' Quiet may no longer be functioning too well on Linux-based systems...

    http://www.phoronix.com/vr.php?view=MTIxNjE

  2. #2
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    The only device with an AMD processor that I have right now is a Brazos-based netbook, and it doesn't seem to have any power management issues. If anything, it exceeds the specifications. I didn't check the power management when CnQ was disabled, though.

    That said, isn't CnQ/EIST losing is relevance lately, in light of ACPI processor (C) states? If nothing is using resources, it just goes to complete sleep by itself. No need to change the frequency, so there is no performance loss.

  3. #3
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    Default asus f1a75-M pro

    I have a f1a75-M pro in combination with AMD's A8-3870K and after enabling the powernow or any other power saving function in bios the system will locks up. No idea if this is due to the hardware or linux.
    Mard0

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    Quote Originally Posted by mard0 View Post
    I have a f1a75-M pro in combination with AMD's A8-3870K and after enabling the powernow or any other power saving function in bios the system will locks up. No idea if this is due to the hardware or linux.
    Mard0
    If PowerNow leads to lockups, this is almost certainly a hardware or BIOS issue. On K10 and later CPU generations, p-state transitioning is done in hardware, so there's little chance for any driver to screw up.

    PowerNow really did suck on K8 generation CPUs, but it's great on later CPUs.

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    I got the impression C'n'Q is and always has been a BIOS related thing, not OS, so i don't see why linux wouldn't use it. I guess depending on how you load the CPU, other OSes compared to windows might not use it as effectively but in the end, its how much power your system draws when idle that matters most.

  6. #6
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    Default For me it's perfect.

    Quote Originally Posted by schmidtbag View Post
    I got the impression C'n'Q is and always has been a BIOS related thing, not OS, so i don't see why linux wouldn't use it. I guess depending on how you load the CPU, other OSes compared to windows might not use it as effectively but in the end, its how much power your system draws when idle that matters most.
    For me C'n'Q works perfectly. It keeps my FX8120 CPU at almost room themperature most of time even at low cooler speed. Really remarkable efficiency I admit. Yet that CPU is able to offer quite impressive performance when it's actually needed. The only thing I wish: AMD GPUs should work the very same ways with Linux opensource drivers. Unfortunately it's not a case so far

  7. #7
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    Quote Originally Posted by 0xBADCODE View Post
    For me C'n'Q works perfectly. It keeps my FX8120 CPU at almost room themperature most of time even at low cooler speed. Really remarkable efficiency I admit. Yet that CPU is able to offer quite impressive performance when it's actually needed. The only thing I wish: AMD GPUs should work the very same ways with Linux opensource drivers. Unfortunately it's not a case so far
    I really do suspect that AMD will find a way to get proper powersaving out for their GPUs. Now that they are putting them onto the same die with the CPU, having them run on full blast all the time (and thus negating nearly all powersaving done by the CPU) is a big problem.

    Since these APUs were one of the driving forces behind releasing documentation and OSS drivers (the way I undestand it, at least), having them not work properly is not a good way to proceed. So I'm guessing that proper powersaving (already written) will be released once the technical review is done.

  8. #8
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    Default CnQ, FX-4170, GA-990FXA-UD3

    I run an FX-4170 under Linux (Ubuntu mostly) on a Gigabyte 990FXA-UD3 and CnQ works perfectly.

    This motherboard is dodgey though, it wont boot USB sticks that are created with Ubuntu's startup disk creator. Very annoying. It does work however with MultiSystem, and also sticks made with PenDriveLinux under Windows.

  9. #9
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    Quote Originally Posted by mard0 View Post
    I have a f1a75-M pro in combination with AMD's A8-3870K and after enabling the powernow or any other power saving function in bios the system will locks up. No idea if this is due to the hardware or linux.
    Mard0
    I own a f1a75-i that fails to adjust vcore with all the powernow features enabled.
    The same cpu/socket/chipset/software on an asrock A75M-ITX and a gigabyte GA-A75M-UD2H motherboard perform perfectly as expected.
    This leads me to believe that asus has not properly implemented their acpi tables in the bios. Or maybe their digi-vrm nonsense and special proprietary "features" are causing issues.
    Last edited by Soul_keeper; 10-28-2012 at 01:20 AM.

  10. #10
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    Quote Originally Posted by schmidtbag View Post
    I got the impression C'n'Q is and always has been a BIOS related thing, not OS, so i don't see why linux wouldn't use it.
    It's really not, at least not entirely. What the BIOS needs to do for PowerNow/CnQ to work is fill the p-state table in the CPU's model-specific registers, and that's it. This table contain values for operating frequency, voltage, etc. for all states. The OS requires a driver that reads the table and instructs the CPU to switch to a certain state, depending on load, according to some governor. (BTW, if the BIOS fails to set up the p-state table correctly, you can do that from userspace before loading PowerNow drivers.) The transition between states is handled by the CPU itself, so the driver is quite simple.

    Earlier on K8, the driver also needed to accurately do and time all the nitty-gritty low-level details of transitioning between states, and this was quite error-prone, depending on hardware.
    Last edited by brent; 10-28-2012 at 11:15 AM.

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