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Undervolting AMD's Phenom II X4 955

Undervolting Your Phenom II And Core 2 Processors
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AMD’s Phenom II X4 955 has been the firm’s flagship desktop processor model since its introduction April. With its DDR3 support and 3.2 GHz core clock speed, AMD finally managed to threaten Intel’s Core 2 Quad in some benchmarks, but at somewhat lower cost both for the processor and an average platform. However, the Core i7’s reign remains untouched.

The Phenom II X4 is available at between 2.5 and 3.2 GHz (see an overview on the AMD Web site). The 800-series processors have 4 x 512 KB L2 cache per core and a shared 4 MB L2 cache, while the 900-series offers 50% more L3 cache. All Phenom II processors are manufactured by Globalfoundries using AMD’s 45 nm DSL SOI process, which allows for low power consumption and nice overclocking capabilities. We were curious to look at the undervolting results.

Default Voltage: 1.32 V

The automatic BIOS setting would run the Phenom II X4 955 at 1.32 V, as reported by CPU-Z. This resulted in a system peak power of up to 216 W at full CPU load. Clearly there is some room for improvement here.

Idle Voltage: Always 0.960 V (BIOS Configured)

All AMD processors with Cool’n’Quiet enabled will switch to a clock speed of 800 MHz when idle, and they will all switch to the default core voltage at this speed: 0.96 V. As you can see in the summary table below, the Phenom II switched to 0.96 V in Cool’n’Quiet mode when idle regardless of our BIOS CPU voltage settings. Consequently, the system idle power was the same at all times: 99 W. In this case there is nothing to improve here, unless the BIOS allowed modifying the idle voltage.

Optimized Peak Voltage: 1.152 V

We tried several voltage settings (see table below) and stressed all of them using Prime95 for at least 30 minutes. It turned out that the default voltage of 1.32 V could be lowered by as much as 12%, to 1.1175 V. This lowered system peak power from 216 W to only 179 W, which represents a 17.2% drop in system power consumption. Not bad at all.

Summary Table

AMD Phenom II X4 955
BIOS Voltage
Effective Idle Voltage
Effective Idle Power
Effective Load Voltage
Effective Load Power
Stable
Auto
0.96 V*
99 W
1.32 V
216 W
Yes
1.3125
0.96 V*
99 W
1.288 V
205 W
Yes
1.2875
0.96 V*
99 W
1.264 V
199 W
Yes
1.2625
0.96 V*
99 W
1.24 V
196 W
Yes
1.2375
0.96 V*
99 W
1.216 V
192 W
Yes
1.2125
0.96 V*
99 W
1.192 V
186 W
Yes
1.1875
0.96 V*
99 W
1.168 V
181 W
Yes
1.175
0.96 V*
99 W
1.152 V
179 W
Yes
1.1625
0.96 V*
99 W
1.136 V
177 W
No


*managed by Cool’n’Quiet

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Top Comments
  • 20 Hide
    johnbilicki , July 14, 2009 6:27 AM
    Glad to see this article considering I underclock a whole heck of a lot more then I overclock; working on the web doesn't require the same amount of power as playing a video game.

    My main concern is heat, I live in Florida right now and underclocking my computer and reduce the overall room temperature which is clearly higher then the average temperature of the rest of the house.

    What is nice about the Phenom II series is that I can drop my X3 720 BE from 2.8 to as low as 0.8 and adjust the cores individually. This let's me keep the first core at let's say 1.4 and drop the other two to 0.8 if I need my system running but won't be directly using it. I'm curious as how this effects the effective voltage if at all. My socket 939 Opteron 185 could only drop from 2.6 to about 1.8. Another thing to note is that CPU usage seems to have a much higher correlation to heat output; if my system is idle then it doesn't generate as much heat as if I was playing a video game.

    I was disappointed that the feature in the GeForce series that completely turned the video card off when not using 3D mode was removed. I prefer having one system to do my work and game and reducing heat output is my highest priority when I'm not spray-painting in Counter-Strike.
  • 11 Hide
    johnbilicki , July 14, 2009 6:41 AM
    @sohei Imagine being able to get 100mpg on a Ferrari...and when you want to kick it up a notch restore the normal power level. Less power means less heat...and it also means a smaller power bill. If entire data centers did/do this when demand is low they could/can save a ton of money.
Other Comments
  • 20 Hide
    johnbilicki , July 14, 2009 6:27 AM
    Glad to see this article considering I underclock a whole heck of a lot more then I overclock; working on the web doesn't require the same amount of power as playing a video game.

    My main concern is heat, I live in Florida right now and underclocking my computer and reduce the overall room temperature which is clearly higher then the average temperature of the rest of the house.

    What is nice about the Phenom II series is that I can drop my X3 720 BE from 2.8 to as low as 0.8 and adjust the cores individually. This let's me keep the first core at let's say 1.4 and drop the other two to 0.8 if I need my system running but won't be directly using it. I'm curious as how this effects the effective voltage if at all. My socket 939 Opteron 185 could only drop from 2.6 to about 1.8. Another thing to note is that CPU usage seems to have a much higher correlation to heat output; if my system is idle then it doesn't generate as much heat as if I was playing a video game.

    I was disappointed that the feature in the GeForce series that completely turned the video card off when not using 3D mode was removed. I prefer having one system to do my work and game and reducing heat output is my highest priority when I'm not spray-painting in Counter-Strike.
  • 11 Hide
    johnbilicki , July 14, 2009 6:41 AM
    @sohei Imagine being able to get 100mpg on a Ferrari...and when you want to kick it up a notch restore the normal power level. Less power means less heat...and it also means a smaller power bill. If entire data centers did/do this when demand is low they could/can save a ton of money.
  • 3 Hide
    stumpystumped , July 14, 2009 6:52 AM
    Even though the use of an top of the line Intel CPU is pointless, the article makes a valid point. Undervolting can save money and you can undervolt even more if you downclock as well. E5200 might be one of the best overclocking CPU but if you are using it in a HTPC like I do, 2.5G isn't really required to watch movies, listen to music or surf the web on the TV. I downclocked it to 2G and set voltage to 1.00V in BIOS. In Windows it sometimes goes as low as 0.82V. In referenece to johnbilicki's comment about GeForce CPUs, if you use nTune (I can't remember the new name for it) you can downclock you GPU to 25% of the factory setting and boost it to 100% or more when you are playing games.
  • 3 Hide
    jongwoonkim , July 14, 2009 7:06 AM
    excellent article. didn't have a clue about undervolting.
  • 1 Hide
    jongwoonkim , July 14, 2009 7:18 AM
    i don't like using i*t word so i won't use it. no company i know undervolt. and most computers are not oc'ed. and lastly but mostly undervolting doesn't reduce performance. get a job.
  • -1 Hide
    lapoki , July 14, 2009 7:37 AM
    But would you still love your ferrari if it sipped fuel even while sitting pretty in your driveway?
    And besides its not like you're crippling it for life, just resting while you sit on your a** doing nothing
  • 0 Hide
    jongwoonkim , July 14, 2009 7:46 AM
    on my Q6600,
    1.3v idle 186W, 100% 271W
    1.1v idle 170W, 100% 243W
  • 2 Hide
    rdawise , July 14, 2009 7:50 AM
    soheithe cpus use in the article are inappropriate if you want to show ,to bring in front economy, green computing etc ...you use components made for this undervoltage because if someone needs less heat and power consumption will newer buy components how from star use a lot of power because a high end cpu needs a high end motherboard high end gpu etc both amd and intel has this kind of products made for green computing


    Ture if you aim was simply "green computing". What if you simply want to save power for a short period of time, then wish to return to your "heavy load"? The problem with going simply "green" cpu is they lack horse power period. As you've pointed out they're not meant for heavy work. I guess an appropriate analogy would be having a car the could go 200 mph, but always doing the speed limit (let's assume 60). Would you rather have a car that could only do 60, or have a car that is capable of 200 if you need it to? That's my idea of it anyway. Good article.
  • 1 Hide
    andrej_valand , July 14, 2009 8:00 AM
    Nice article, i underclock my workstations notebooks and servers all the time for longer battery life and less power use. I use RMclock from RrightMark, it works excellent and it changes multiplier and voltage on the fly in windows depending on the load. So "sohei" no restarting is needed. Just Like "jongwoonkim" said you can have a efficient Ferrari which can go full when you need it to go fast... Regards
  • 0 Hide
    one-shot , July 14, 2009 8:16 AM
    I think there is a typo on page 4 it states "It turned out that the default voltage of 1.32 V could be lowered by as much as 12%, to 1.1175 V. This lowered system peak power from 216 W to only 179 W, which represents a 17.2% drop in system power consumption."

    However, in the graph, it shows 1.175Vcore. These are very different and I would like to know which one is correct. Thanks
  • 2 Hide
    andrej_valand , July 14, 2009 8:25 AM
    Yes that is true a high power cpu will consume more, but if you want and need to have an hi-end system with fast processor you buy it. True? But mostly you don't need full power all the time. Why not save something on the electric bill and do something for the planet and adjust the factory power management to even lower levels if it is possible? I see only positive points in that. Why would i buy a low power CPU if it is not enough for me? The article is just fine, because what I have found out that low power CPUS don’t underclock that well, they are already power optimized, but you can tweak the hi power CPUS for quite a high margin sometimes. But in my opinion it is better to use some dynamic underclock utility that as doing this. I agree with you doing this in BIOS makes less sense, and when the CPU goes to idle it destroys the settings, so that way the bad numbers for the AMD cpu at idle when its most important. If you want to do it right you have to control the multiplier and voltage all the time, depending on load.
  • 1 Hide
    drealar , July 14, 2009 9:49 AM
    LoL sohei, chill :) 

    @ PSchmid and ARoos
    "The power savings of 17.2% or 37 W..."
    "148 W instead of 185 W—that’s a 20% reduction." = 37W
    This is why I never rely solely on percentage%.. hehe

    As not so enthusiast & not so average user, I appreciate the fact that we're able to see what effect do SStep & C'n'Q have on undervolting.

    One thing that I'm wondering is how much is the CPU usage when running a process? Like if I see a 40% CPU usage normally, does that means I'll see 45~50% usage after undervolted? Or will I still see 40% but with a lil slower processing? Heh, I kinda got confused there, but would be useful to know since my CPU usage never goes beyond 70%.

    Another thing I wanna confirm is, I assume that undervolted cpu speed will be a tad slower; like 2.8GHz cpu's performance will be like a 2.6GHz cpu, except that now the 2.8GHz will use ~37W less than the 2.6GHz. Is it? Something like a strong muscular guy effortlessly lift a full tower PC compared to a thin muscle-less guy who give his all :p  :D 

    Would appreciate some clarification there Tom's guys :D 
    Please.
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