Undervolting & Underclocking Celeron G1820

Collider

Senior member
Jan 20, 2008
522
7
81
Need some advice on undervaluing/under-clocking my Celeron G1820.

Looking to run at lowest speed/voltage possible. The machine runs Win8 but mostly sitting around w an idle processor, so looking to reduce power consumption as much as possible.

What settings should I use ?

thanks.
 

BSim500

Golden Member
Jun 5, 2013
1,480
216
106
Need some advice on undervaluing/under-clocking my Celeron G1820.

Looking to run at lowest speed/voltage possible. The machine runs Win8 but mostly sitting around w an idle processor, so looking to reduce power consumption as much as possible.
There's no point underclocking it as SpeedStep auto-underclocks anyway when idle. As for undervolting, basically just keep nudging the vcore down slightly maybe -0.01v steps each time, then test stability, then another nudge down, then test stability again, etc, until it becomes unstable, then back off 1-2 steps for headroom. You might knock off anywhere between -0.02v to -0.15v which should reduce your load power consumption by up to 10w or so. Idle power consumption probably only by 1-3w as SpeedStep does most of that anyway.
 

crashtech

Lifer
Jan 4, 2013
10,635
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The only reason to do this is if you want to run the CPU at 100% sometimes within a more limited thermal envelope, for example a passively cooled application. Otherwise, buying the right PSU is going to save more energy.
 

Collider

Senior member
Jan 20, 2008
522
7
81
There's no point underclocking it as SpeedStep auto-underclocks anyway when idle. As for undervolting, basically just keep nudging the vcore down slightly maybe -0.01v steps each time, then test stability, then another nudge down, then test stability again, etc, until it becomes unstable, then back off 1-2 steps for headroom. You might knock off anywhere between -0.02v to -0.15v which should reduce your load power consumption by up to 10w or so. Idle power consumption probably only by 1-3w as SpeedStep does most of that anyway.

I see your point.

The CPU is indeed idle most of the time, 1-3W savings doesnt mean much..

Would there be any way to reduce idle consumption? I understand how SpeedStep functions, but it also has a lower bound to witch it will downclock if I'm not mistaken - any way to push that further ?
 

coercitiv

Diamond Member
Jan 24, 2014
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Would there be any way to reduce idle consumption? I understand how SpeedStep functions, but it also has a lower bound to witch it will downclock if I'm not mistaken - any way to push that further ?
You will see little or no gains from that. One of the main features of the new Haswell architecture was an improved sleep state which brought idle power consumption to around 1W. This is the reason not all PSUs are compatible with Haswell, in the sense that enabling this improved sleep states requires a modern PSU.
 

Collider

Senior member
Jan 20, 2008
522
7
81
You will see little or no gains from that. One of the main features of the new Haswell architecture was an improved sleep state which brought idle power consumption to around 1W. This is the reason not all PSUs are compatible with Haswell, in the sense that enabling this improved sleep states requires a modern PSU.

thanks. I ended up lowering max CPU % in power options in windows and limitted my cpu to 800 Mhz at all times, fast enough to browse the web. Idle usage dropped by 2-3w.
 

Yuriman

Diamond Member
Jun 25, 2004
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thanks. I ended up lowering max CPU % in power options in windows and limitted my cpu to 800 Mhz at all times, fast enough to browse the web. Idle usage dropped by 2-3w.

Even that might not be good for overall power consumption, though I'm not sure.

The concept of "race to idle" is as follows:

Given that a CPU tends to become less efficient at higher clocks because it needs more voltage, one would think that running it at the highest clock that can be achieved at the lowest stable voltage for any clock, would be best. That ignores the overhead though - things like chipset, peripherals, etc., and keeping the CPU awake longer, even if it's drawing less power, can keep these other things awake longer too, which might more than offset the power savings.

I.E. 50w for 2 seconds and then 1w for 8 seconds is still less power than 25w for 5 seconds and 1w for 5 seconds.
 

BSim500

Golden Member
Jun 5, 2013
1,480
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thanks. I ended up lowering max CPU % in power options in windows and limitted my cpu to 800 Mhz at all times, fast enough to browse the web. Idle usage dropped by 2-3w.
Decreasing voltage makes far more of a difference than crippling clock speed. The latter doesn't save anything if it now takes 3.5x longer in the load state to do the task (800MHz vs 2700MHz). If you really want the lowest power consumption, and your motherboard supports, it, then it's worth doing a little manual undervolting, put max CPU% back as it was and let SpeedStep do its job of keeping at 800Mhz 95% of the time anyway but allowing more speed when you need it (which as Yuriman says - it will "race to idle" faster). It's not just instantaneous power consumption that counts but power used per task over time. Underclocking doesn't improve that (because it increases the % of load time), but undervolting does.

Likewise, it's not just "cores" but "uncore" (eg, L3 cache, iGPU, etc) plus all other peripherals (SSD or HDD, USB devices, monitor, printer, etc) that draw power anyway regardless of CPU load. So 800MHz will use more than just 1/3rd of the power of 2.4GHz - leaving you worse off if it's now loaded +250% longer just to save 10-20% power.

If you really want a true "Green PC", you basically go through the whole system:-

- Gold / Platinum rated 80+ plus PSU at the lowest wattage you can get, eg, Seasonic G360 is good for a "full ATX", but Celeron's with no dGPU you could even go for a 60-160w Pico-PSU depending on your setup.

- 2.5" HDD's use less power than 3.5". SSD's use the lowest overall (especially Samsung's) and are incomparable for performance.

- LED TFT monitors use less power than the old CCFL LCD's (you could knock 10-15w off with that upgrade alone), ie, an old CCFL LCD TFT that draws 35-40w would be reduced down to 25-30w for LED screen at same screen brightness & size (and do so 100% of the time whether the CPU is loaded or not).

And general common sense, eg, power off active speakers when not using them, aggressive standby timers if you have a habit of leaving the PC idling for more than 30mins or so, etc.
 
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Rakehellion

Lifer
Jan 15, 2013
12,181
35
91
Need some advice on undervaluing/under-clocking my Celeron G1820.

Looking to run at lowest speed/voltage possible. The machine runs Win8 but mostly sitting around w an idle processor, so looking to reduce power consumption as much as possible.

What settings should I use ?

thanks.

If you're running idle on Win 8, you're already using very little power. Windows 8 is designed for mobile and has numerous power saving features.

But if you're using a motherboard that even allows you to change the clock speed, chances are it's power hungry.
 

tential

Diamond Member
May 13, 2008
7,348
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What about Power Saving features and powersupplies like the PicoPSU? Haswell requires a powersupply that is compliant, what about a PSU like the Pico?
 

Jovec

Senior member
Feb 24, 2008
579
2
81
It's generally best if power efficiency (for cost savings) is your goal, to do so in the context of planned upgrades. You could buy a 80 platinum PSU or new CPU, but a 10-20 year payback cycle is not unrealistic. Same goes for all of your other components.

A 3 watt savings works out to be roughly $4/yr at 24/7/365 with electricity at $0.15/kwh. At 33% idle, 33% sleep/off, %33 usage scenario, since only the idle is effected without undervolting, that drops down to $1.33/yr

There is also stress testing to consider. 24 hours of P95 or similar at even just +50w would eat into 16/54 days of that 3w reduction at 100% idle/33% idle.

Some might also want to consider the secondary effects, such as the production, packaging, and shipping of the new component along with the secondary use or disposal of the part being replaced.
 
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