Undervolting and underclocking my Core i7 920

[RejZoR]

The last power bills are getting a bit high and i want to exclude my system from the equation for the time being. Since i don't have the watt-o-meter i'm trying to figure out how much i'd actually benefit from doing such thing on my Core i7 920.

I'll still use the 4,2GHz profile but when i'm just running my PC for P2P or music over night it's pointless to run it at full power.

So, this is how i'm hoping to lower power consumption:

- disabled HT
- enabled all the power saving states (not that they'll matter much)
- set VCORE to 0,8V
- locked multiplier to 12 (which is the lowest)

So now i basically have a 4 core CPU that is running at only 1,6GHz. Any idea what would be the power difference compared to stock clocked Core i7 920 ?

 

[Easy Rhino]

The last power bills are getting a bit high and i want to exclude my system from the equation for the time being. Since i don't have the watt-o-meter i'm trying to figure out how much i'd actually benefit from doing such thing on my Core i7 920.

I'll still use the 4,2GHz profile but when i'm just running my PC for P2P or music over night it's pointless to run it at full power.

So, this is how i'm hoping to lower power consumption:

- disabled HT
- enabled all the power saving states (not that they'll matter much)
- set VCORE to 0,8V
- locked multiplier to 12 (which is the lowest)

So now i basically have a 4 core CPU that is running at only 1,6GHz. Any idea what would be the power difference compared to stock clocked Core i7 920 ?

it is hard to know with out that watt o meter you don't have. the i7 920 is 130 watts so by crippling it like that you are probably down to like 90 or so??? taking the average price of KwH in the U.S, you will likely save $5 a month

 

[RejZoR]

And compared to a 4,2GHz? I know it wasn't clocked at that the entire time but half of the power saving features were disabled (C1E was still enabled though). But even then, it had 1,3V at disposal and even at the lowest multi it was running at 2,4GHz...

I'm from EU so i'm not sure how our electricity prices compare...

 

[Easy Rhino]

And compared to a 4,2GHz? I know it wasn't clocked at that the entire time but half of the power saving features were disabled (C1E was still enabled though). But even then, it had 1,3V at disposal and even at the lowest multi it was running at 2,4GHz...

I'm from EU so i'm not sure how our electricity prices compare...

honestly, downclocking and undervolting won't save you a lot of money every month. even if you got it down to 65 watts that is only ONE component of your pc using electricity. so even if you have expensive electricity costs where you live i would say the max you would save by undervolting/downclocking just for overnights would be $10-15

 

[unclewebb]

If you are trying to save power, running a modern CPU slow with the minimum multiplier is exactly what you don't want to do.

Power Optimization – a Reality Check
http://www.cs.berkeley.edu/~krioukov/realityCheck.pdf

Run your CPU at its normal speed or overclocked if you want. The way to save power is to enable C3 and C6 and that's all you need to do.

The new reality is called, "Race to Sleep". A CPU works most efficiently when it is allowed to work at full speed so it gets tasks done quickly and then as soon as the task is done, it gets to drop down into the C6 low power state where the voltage is reduced to nearly zero. Your setting of 0.8 volts sounds like a really low voltage but it is sky high compared to zero. A slow CPU will have to spend more time working in the C0 state at 0.8 volts and less time in the C6 state at 0.0 volts. Think about it. How is that going to save you power? The guys at Berkeley debunked this myth.

The latest version of RealTemp shows the percentage of time your Core i is spending in C6. Maximize your C6 residency time and you will minimize power consumption.

RealTemp Tech|Inferno Edition
http://www.overclock.net/t/1330144/realtemp-t-i-edition

 

[brandonwh64]

Turn on as many power savings as possible in the bios. Mine would do .9V on stock clocks stable so test that.

 

[unclewebb]

C6 at 0 volts is how you can save power.
The CPU-Z reported voltage is meaningless when your CPU is spending 99.5% of its idle time at 0 volts.

 

[RejZoR]

Well, when i have it in the 4,2GHz mode, i can't use any of the C states. All i can use is C1E and SpeedSteep. And thats it. If i use ANY kind of C state power saving the system just BSOD's on boot.

 

[RejZoR]

Ok, after fiddling with it for a while, the most i can use is C1, but that custom RealTem doesn't show C1 so i have no clue how much it's active or how much i even benefit from it.
But i'm guesing, if i can have at least C1, then i'll have it. Still need to make some stability tests but at least it boots unlike C3 and C6 which always BSOD on Windows logo while booting. Unless i make a very mild overclock in which case i might just as well leave it at stock speed.

So currently i'm at 4,2GHz with C1E and C1.

 

[unclewebb]

When you use C3 or C6 on your CPU, that engages full turbo boost and enables the 22 multiplier for single threaded tasks. Your CPU jumps up to 4400 MHz and that's why you crash. If you can reduce the multiplier in the bios you might be able to avoid this. You would have to disable Turbo Boost and you would be left running with the 20 multiplier. 4000 MHz wouldn't be so bad and it would be a lot more efficient if this allows you to enable C6.

 

[RejZoR]

Erm, as far as i know 21 is the max multi for 920... So multi 20 is normal max, 21 is when turbo is engaged. 22 is not possible.

4,2GHz (200x21) is full stable otherwise, but with C states enabled, it's not. Only with C1 as the highest option CPU will use.

 

[unclewebb]

Don't worry RejZoR, you're not the only one that doesn't realize that the Core i7-920 has a 22 multiplier hidden inside there. The 22 multiplier is only available when C3 or C6 is enabled and your CPU only uses the 22 multiplier when a single core is active. As soon as 2 or more cores are active, the multiplier drops down to a maximum of 21. Most enthusiasts prefer a stable multiplier so they disabled C3 and C6 and this disabled the 22 multiplier.

Reboot, set your BCLK at a nice safe 133 MHz, enable C3 and C6 and then run a single threaded benchmark. The RealTemp XS Bench is a nice light load. You won't see the full 22 multiplier because Windows background tasks will wake up additional cores but the average multiplier reported by RealTemp should be higher than 21.0 as long as you don't have a lot of background activities running on your computer while testing.

Page 18 of the Intel Specification Update mentions the 2 bins of Turbo Boost you get when a single core is active.

http://download.intel.com/design/processor/specupdt/320836.pdf

 

[jaggerwild]

@Unclewebb,
Nice little app realtemp is, just wanted to thank you as I've used it for a few years now.

 

[RejZoR]

Don't worry RejZoR, you're not the only one that doesn't realize that the Core i7-920 has a 22 multiplier hidden inside there. The 22 multiplier is only available when C3 or C6 is enabled and your CPU only uses the 22 multiplier when a single core is active. As soon as 2 or more cores are active, the multiplier drops down to a maximum of 21. Most enthusiasts prefer a stable multiplier so they disabled C3 and C6 and this disabled the 22 multiplier.

Reboot, set your BCLK at a nice safe 133 MHz, enable C3 and C6 and then run a single threaded benchmark. The RealTemp XS Bench is a nice light load. You won't see the full 22 multiplier because Windows background tasks will wake up additional cores but the average multiplier reported by RealTemp should be higher than 21.0 as long as you don't have a lot of background activities running on your computer while testing.

Page 18 of the Intel Specification Update mentions the 2 bins of Turbo Boost you get when a single core is active.

http://download.intel.com/design/processor/specupdt/320836.pdf

So, i'm suppose to give it even more volts then for it to run with C6 on max load and to still be able to use C6 ? That would result in rather insane 4,4GHz (at least for a i7 920). I do have a water cooling and i do have the D0 chip but i'm not sure it will work. Right now i'm at 1,35V on VCORE. I might try up to 1.4V but beyond that i don't think it's worth it.

I might also try setting max multi to 21 then, which should prevent it from using multi 22. Because right now i have it set to "Auto" and that means it can use full range of multipliers. If i set it to 21, it will only go up to 21 and no further.

Thx for the tip unclewebb, i'll report back in the afternoon...

 

[terrastrife]

As everyone has suggested, always let your CPU downnclock to idle. With your setting of a static speed/voltage, you are probably using MORE power than if everything was on auto/stock settings.

 

[unclewebb]

On the first generation Core i CPUs, I don't think there is any way to control the Turbo Boost feature. If you disable C3 / C6 you get a maximum of +1 bin of Turbo Boost whether 1, 2, 3 or 4 cores are active and if you enable C3 / C6 you get +2 bins of Turbo Boost when 1 core is active and +1 bin of Turbo Boost when 2, 3 or 4 cores are active.

In order to keep your maximum clock near 4200 MHz you would need to back off the BCLK to 191 MHz. With the core voltage you were originally using, you might be able to go up a couple of MHz on the BCLK while still maintaining full stability.

Your original goal was to reduce power consumption. This method will sacrifice a small amount of full load performance to accomplish that goal.

The other option would be to disable Turbo Boost and go with a fixed 20 multiplier and see if you can push your BCLK up towards 210 MHz. This will get you back up to 4200 MHz but will allow you to enable C6 for a big drop in light load / idle power consumption.

As everyone has suggested, always let your CPU downnclock to idle.

That's true when not using C6 but when using C6, there is no need to do this. Your CPU cores will shut themselves down automatically so it doesn't matter what speed or voltage they were at before entering C6.

 

[RejZoR]

Ok, the multi limiting didn't work but then i found a separate Turbo setting. After disabling it, system booted while i still managed to keep existing 4,2GHz @ 1,35V. I don't need some stupid multi bumping if i have 4,2GHz at disposal anyway.

Checked the C states with your custom RealTemp and C6 shows 80 pretty much right after the boot. So now i have all the power saving features enabled and still a very respectable 4,2GHz clock when heavy processing is required. I think this should do the trick

It's always nice to have someone around who knows in-depth stuff like this. Thx again unclewebb!

 

[RejZoR]

Oh for crying out loud. Now it refuses to use 21x multi and is continuously stuck at 20x. Wtf!? Encoding a video and compressing a 7z with 8 threads and its 200x20. All i did was disable turbo mode. Seems like 21x multi is bind to the Turbo function or something?

 

[unclewebb]

The 21 multiplier is only possible when using Turbo Boost. Some bios versions pretend that you can select a 21 multiplier and disable turbo at the same time but in the background, turbo boost gets turned back on to access the 21 multiplier.

With turbo boost disabled, you get your default multiplier which on a Core i7-920 is the 20 multiplier. Hopefully you can figure out a new combo with better power savings. The 2nd and 3rd Generation Core i CPUs allow you to select how many bins of Turbo Boost you would like so it is not an all or nothing feature any more.

 

[RejZoR]

Or i'll drop the volts as i don't need 1,35V for only 4GHz. It's not a bad clock anyway so i might just go with that.