Which scenario saves more power?

Discussion in 'General Hardware' started by mikeymikec, Jan 3, 2013.

  1. mikeymikec

    mikeymikec Diamond Member

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    My PC spec:

    I'm not planning on changing the hardware, partly because I'm happy enough with it and because of a tight budget.

    The board has a bug that ASUS are being quite obtuse about, so I guess I'll have to accept the status quo. When this board resumes from standby/S3/sleep, the undervolting setting I've set in the BIOS is forgotten about until the PC is either restarted, cold-started or resumed from hibernation.

    I use the computer several times a day, and perhaps for hours at a time, or if I'm out on appointments I might use it several times a day for only say an hour on 2-3 occasions.

    The undervolting setting isn't much, but it is the maximum under-volt setting that the board's options will let me choose. When I had all six cores enabled, here's the power usage stats:

    -0.06v offset: 85-90W idle, 175W load
    standard: 89-95W idle, 186W load

    My feeling is that 5W saved on idle isn't much, but over a year if it saved a fiver it's better than no saving at all. However, the power usage penalty of resuming from hibernation three times a day might offset that. This is really the crux of my question. When a computer starts up (I imagine hibernation and a cold start are fairly similar in terms of their demands power-wise), the processor is cranked up quite a bit until the system finishes booting.

    Currently I've got the computer going into hibernation after an hour of idle time (I'm not sure why I changed it from 45 minutes to an hour now, as 45 minutes, but anyway).

    The advantage of going back to using S3 sleep is obviously that it resumes a heck of a lot quicker - (5 seconds versus 30 for hibernation), and probably uses less power to resume from sleep than hibernation.
     
  2. sm625

    sm625 Diamond Member

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    2 minutes of hibernation is all you need to make up the power lost by resuming, compared to just letting it sit idle. You should use hibernation as much as possible. It is great when you have an SSD.
     
  3. mikeymikec

    mikeymikec Diamond Member

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    My question was more along the lines of should I bother with undervolting when I have to use hibernate rather than use S3 sleep.
     
  4. Termie

    Termie Diamond Member

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    Your computer uses an extra 5 W-hours for every hour it's on and not undervolted. It probably uses about 50 W-seconds to pull itself out of hibernation. Absolutely use hibernation rather than sleep to save energy, unless the bootup time annoys you too much.

    But here's another solution - download Asus TurboVEvo right here: http://www.asus.com/Motherboards/AMD_AM3/M4A89GTD_PROUSB3/#download

    And then set an undervolt from within the app. That way you can reset your undervolt upon waking from sleep without rebooting.
     
    #4 Termie, Jan 3, 2013
    Last edited: Jan 3, 2013
  5. mfenn

    mfenn Elite Member <br> Currently on <BR> Moderator Sabb
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    OK, this is a pretty simple math problem.

    Let's say that you use the computer for three 2 hour sessions during the day. Let's assume that most usage is fairly light so that your average power usage is basically the idle measurement. That gives us 90W idle when undervolting and 95W idle when running at stock volts. Let's assume that you use full power (186W) for 30 seconds when powering the machine up. Let's also assume that the machine uses 5W while sleeping and 1W while hibernated.

    For the sleep case, we can model a day's power usage as:
    P_sleep = T_on * 95W + (24 - T_on) * 5W

    For the hibernate case, we can model a days power usage as:
    P_hiber = T_on * 90W + (24 - T_on - (N_sessions * 0.0083h)) * 1W + N_sessions * 0.0083h * 186W

    If we set the two equations equal to each other and solve, we get

    T_on = N_sessions * 0.542 - 96

    If you use your machine for an average of say 6 hours a day, then you will hibernation and sleep are equal if that usage is broken up into 188 difference sessions. If you have fewer sessions, then hibernation wins, more sessions and hibernation loses.

    So it's pretty obvious that for any sane usage pattern, hibernation is much more energy efficient.

    PS. I did this really quick, so anybody feel free to point out algebraic or arithmetic errors.