Is it better to use an offset or manually set CPU voltage (with respect to CPU longevitity)?

Question

The Background:

I recently built a new computer, and I'm working with the ASUS P8Z68-V Pro motherboard and Intel i7 2600k processor. While this question does not pertain specifically to my hardware, I mention what I have to explain the voltages/temperatures I get. Do note that the information in your answers should not pertain to my specific case, but computer hardware in general. Furthermore, the information should apply regardless of if the system is under-clocked, stock-clocked, and over-clocked.

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The Details:

In my motherboard, there are two options that pertain to my question. The first is load-line calibration (LLC), and the second is setting the CPU voltage by manual/offset mode. After some experimenting with my manually set multiplier, I have come up with the following as a stable set of voltages in each voltage mode:

• Manual Voltage - 1.19V at idle , drops to 1.18V under load (LLC on high).
• Offset Voltage - 0.93V at idle, 1.19V under load, voltage spikes to 1.25V under load transitions (LLC is off).

Now I understand why the voltages result from each setting (like V_droop), and why I need to turn LLC on/off in each case, but there are two sides to the hypothetical coin here. While my load temperatures are about equal in each case, the CPU idles a few degrees cooler in offset voltage mode (due to the lower idle voltage).

That being said, in offset mode, I noticed an interesting side effect - load transitioning causes the voltage to spike up to 1.25V. I also noticed that the voltage stays at 1.25V when starting the computer (until Windows is fully loaded and SpeedStep begins to work... brownie points if you can also tell me why this happens). With LLC enabled on any setting in offset mode, the load and idle voltages remain the same, but the peak transition voltage gets a lot higher (over 1.3V).

Conversely, when I set the voltage to manual mode (with LLC enabled, since without it V_droop causes it to be unstable at idle), the CPU is constantly at ~1.17-1.18V, in both idle/load/startup. My point is that I don't see any voltage spikes between load transitioning - the voltage is almost constant, all the time.

Again, note that in both cases, my load temperatures are the same (a perfectly acceptable 65°C under a stress test, mid to high 50's under normal full load). Thus, I am not worried about temperatures (even at idle), but rather the longevity of the CPU with respect to these voltage settings.

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The Question:

For the long-term use and stability of a computer, with respect to CPU degradation and longevity, is it better to use an offset voltage (which results in a lower idle but higher transition voltage) or manual voltage (roughly constant voltage)? Will the offset voltage spikes (although within my manufacturer's specified voltages) harm the CPU or cause it to degrade faster over time?

Assume the system is under load 60% of the time it is on (which is why I want to use offset mode - cooler and less power at idle).

Reason for bounty: I would appreciate some hard evidence (datasheets, research papers, studies, or any proof really) in favour of one method or another, specifically pertaining to fluctuating versus constant voltage.

Answer 1

Both temperature and voltage kills a CPU. A high voltage spike can kill it quickly. In your case I wouldn't worry though. The voltage spikes you have are still low. The Intel spec sheets specify the max vCore VID as 1.52v for that processor. Now, I wouldn't actually run the processor at anything near that, but on the other hand I doubt a voltage spike up to that would kill it either.

Faced with the very same question you are posing I went with the offset. I figured the bigger killer would be the extra idle voltage and temperature. And I have a much higher overclock than you do. With the overclock you have I would still expect to get 10 years of life out of the processor either way.

Answer 2

In response to music2myear.

It's not just heat that kills a processor. Breakthrough is correct, the interconnects degrade with increased voltage:

V=IR

If you increase the voltage (while resistance remains constant), the current will increase proportionally. Increased current running through interconnects leads to electromigration which transports conductor material away from the interconnect due to momentum transfer between electrons and metal ions.

From a simplistic point of view you could think of it as a river eroding its path. Basically, if the increased current degrades the interconnects, they will eventually fail. All CPU's have a finite lifetime, but increasing voltage, and hence current will decrease its lifespan.

That said, I recommend offset voltage, it will result in far less current passing through your processor, and the small voltage spikes are of no consequence at 1.25V.

Answer 3

Just to add an extension to Mr Alpha's answer, I also found this interesting guide on [H]ard|Forum in regards to overclocking on the Sandy Bridge chipsets. In regards to SpeedStep settings:

It possible to overclock while keeping speedstep enabled and use the offset voltage method for Vcore increase this will provide the overall best in temperature and eatsink performance as well as generally efficiency and extension of the lifespan of the motherboards its components and the CPU. This is due to the Turbo Multiplier working and exucting ramping as Intel intended. Example leaving all CStates and Speedstep enabled will allow the CPU to idle down in Vcore and frequency (1600MHz) and ramp up when under load to 4.8GHz when needed.

Since I have a modest overclock, and have all power-saving settings enabled (there are no stability or performance issues in doing so), I believe that keeping the CPU voltage in offset mode would be the best choice in this case.

For anyone going down this route as well, do keep an eye on your temperatures and voltages using a temperature monitoring tool (e.g. HWMontior or HWiNFO). Pay special attention to the maximum CPU core voltage, and see if that fits within your needs. Also note the effects of V_droop (see my comments on Mr Alpha's answer for details).