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Undervolting, Intel Turbo Boost, power usage, and you!

Nowadays, I use an Alienware 13R3 as my laptop. Before that, however, I was on the i7 model of the Surface Pro 4. It had a funny quirk, you see: it was powerful hardware... too powerful to be contained. The i7-6660U present in the device was rated for a TDP of 15 watts, but it turned out that merely using the CPU at a decently high load could blow past through that power allowance.

Thankfully, Intel processors come with two power limits. Beyond the TDP, you also have the “boosted” limit, which, in this case, was 25 watts. It still wasn’t enough to sustain both the CPU and GPU at full load at the same time, though. And consuming 25 watts generates a lot of heat, enough that the system will be like “hmm, things are toasting up in here, let’s throttle things down back to 15W.” While you could play games on the machine, that flaw made it annoying.

There were two things that you could do:

Pointing a small USB fan at the back of the device allowed it to significantly cool down (the insides and the chassis acted as heatsinks)

Using software to reduce the amount of power that goes through the chip; less power means less heat, which means less throttling, and so on.

A lot of people in the Surface subreddit back then recommended using something called Intel XTU (which stands for Extreme Tuning Utility), and while it’s probably a great tool for people who go overclocking and fine-tuning every low-level variable of their hardware, it’s very overkill if you just want to reduce the current that goes through your processor. (It also had a few very annoying quirks that needed extensive workarounds.)

We only need to reduce the current that goes through the chip: undervolting.

UNDERVOLTA-WHAT?

In a sense, it’s very similar to overclocking, while sorta being its opposite. When you try to overclock hardware, you try to push its clock speed as high as you can for the voltage that goes through it. Sometimes, overclockers go through very involved steps in order to be able to increase the voltage: custom heatsinks, liquid cooling, nitrogen, etc. because more current inevitably means more heat.

Undervolting is really just taking that same problem, but looking at it upside-down: “how much can I reduce the voltage for my current clock speed?” (mostly because 1) you can’t and shouldn’t overclock laptop processors, and 2) you’re trying to reduce heat, not keep it while increasing performance)

And unlike overclocking, this process does not carry, to the best of my knowledge, any risks to your hardware. There is a danger (albeit low) of damaging some of your components when you overclock, because you might stress them too hard, thermally or otherwise. Undervolting, however, can only cause crashes, BSODs, or freezes, when you might not be feeding the chip enough power.

HOW TO DO THAT THING THEN?

There is an excellent piece of software called ThrottleStop:

https://www.techpowerup.com/download/techpowerup-throttlestop/

It is much more lightweight and easy to use than Intel XTU! You can find all of the voltage controls under the [FIVR] button.

All that you need to change is the Offset Voltage for CPU Core, CPU Cache, and the Intel GPU. You will need to experiment by yourself to figure out how much voltage you can take away from the components. Core & Cache should be the same.

On my Surface Pro 4 i7, I was able to stay at -60 mV (I can’t remember how much for the GPU), but the i7-7700HQ in my Alienware laptop can go all the way down to -120 mV. However, my ThrottleStop preset that has Turbo Boost enabled only goes down to -115 mV, just in case. I’m only taking 10 mV away from the Intel GPU.

All hardware is different, however, even chips with the same model number. Your i7-7700HQ may be able to do more or less undervoltage. This is something called the Silicon Lottery. Here’s an explanation I copypasted off Reddit:

When chip manufacturers like Intel, TSMC, UMC, GF, etc. make wafers, there are slight variations in material quality across the wafer surface, there are local variations in how the lithography, metal vapor deposition, photoresist chemical deposition, etc. are done and this can yield a significant contrast between how good the best chip of a given batch will perform vs how bad the worst chip of the same batch will perform.

And of course, there is a difference across generations of hardware. I believe the “wear” on the processor (transistor degradation due to heat, etc.) may also have an influence. The i7-4770 (non-K) in my desktop computer will only let 40 mV be taken away before it becomes unstable under high loads. That said, even small undervolts are valuable and will save you power... and therefore heat... and therefore wear... in the long run!

I would recommend starting at -50 mV, running some stress tests (Prime95, maybe some gaming benchmarks) and then using your computer like that for a while, with usages that are heavy on resources. If it doesn’t freeze/crash/BSOD, you can keep undervolting down and down until it eventually happens. You might be able to get a decent idea of how much undervolting you can do by looking around for reports by other users on the same generation & class of processors.

I’ve got some test results which perfectly illustrate why this is a great idea, but first:

INTEL TURBO BOOST

Turbo Boost Technology (TBT) is a microprocessor technology developed by Intel that attempts to enable temporary higher performance by opportunistically and automatically increasing the processor's clock frequency. This feature automatically kicks in on TBT-enabled processors when when there is sufficient headroom - subject to power rating, temperature rating, and current limits.

https://en.wikichip.org/wiki/intel/turbo_boost_technology

It’s kind of like a built-in overclock that scales somewhat intelligently. The thing, though, is that it may not be very efficient in terms of how much power you’re spending vs. how much performance you’re getting out of that. An image is worth a thousand words:

This chart illustrates how much power the smartphone Exynos processors spend when they are at a certain frequency. It starts off being roughly linear, but at some point, starts bending very fast towards being exponential. There is a point where components have an ideal performance-per-watt ratio. For example, it’s this specific point of efficiency that “Max-Q” GPUs are attempting to exploit.

Turbo Boost arguably stays within this window of efficiency, of course... as far as desktops are concerned. For laptops, you may really want to minimize “waste of power”, both for battery and for heat. And toggling Turbo Boost off is actually a worthwhile tradeoff. Now let me show you the numbers. You can see the entire set of benchmark screenshots here in full-res in this Imgur gallery, or over here.

HERE COME THE BENCHMARKS

Let’s start with a synthetic benchmark: Prime95.

Without undervolting, and with Turbo Boost ON, the power consumption goes up to 52W. However, it doesn’t stay there for long, because the TDP of this processor is 45W! The boost limit is 60W, but the temperature is too much to keep that extended limit enabled: it’s going up to 87°C, which is TOASTY.

With undervolting, the power goes down to 43W, which is 82% of before. The temperature only went down by one degree, but the fans were spinning slower.

However, with undervolting and Turbo Boost OFF, the power went all the way down to a measly 25W! That’s 58% of before, and 52% from the first result, while only lowering frequency by ~20%. 3.4 GHz with all-cores Turbo Boost became 2.8 Ghz. The temperature is now only 65°C too!

Now, let’s check out a semi-synthetic usage: the BOINC client running scientific research tasks from the World Community Grid, on all eight cores.

Regular voltage, Turbo Boost ON = 38W, 79°C.

Undervolted, Turbo Boost ON = 29W, 70°C.

Something especially noteworthy: with Turbo Boost OFF and the undervolt in place, I can set BOINC to only use three cores, and the fans will (almost) never spin up, because the CPU isn’t using enough power to reach 60°C. It is effectively cooled passively! Isn’t that cool?

Here’s a benchmark more representative of real-world usage: I’m encoding a video in H.264/AVC at 4K resolution using Adobe Media Encoder.

Undervolted, Turbo Boost OFF = 21W, encoding time is 249s.

Undervolted, Turbo Boost ON = 34W, encoding time is 216s.

That’s a 15% speed gain for 61% more power. Not very efficient!

And last, let’s take a look at Dota 2. This was done at highest settings, but with vertical synchronization off to uncap framerate, as well as one-quarter screen resolution to make sure the test was bound by the CPU and not the GPU.

Undervolted, Turbo Boost OFF = 133 fps, 17W, 68°C.

Undervolted, Turbo Boost ON = 152 fps, 26W, 74°C.

This equals 52% more power for 14% more framerate and 6 more degrees.

While these benchmarks are fairly naive and don’t get into framerate percentiles (TBT may help with usage spikes and such), I think they paint a clear picture: on laptops, Turbo Boost is usually not worth it, and undervolting is a huge help.

ThrottleStop allows you to toggle Turbo Boost off with one simple box to tick.

You see the four little radio buttons in that box labeled “Performance”? Those are the presets. You have four, each pre-labeled. I use the first two as an easy way to disable/enable the boost. (you only need to click “Save” to make sure the presets get saved.)

As a side note: ThrottleStop allows you to enable SpeedShift, but I’d recommend against it, as it caused very odd issues for me: Dota 2 would only run at 55 to 57 fps while capped at 60, while uncapped behaviour was normal. To make that issue even more confusing, it only happened in DX11 mode.

Note that, if you wish to keep the frequency at maximum (which may be potentially useful for extreme usages such as virtual reality), you can do so by:

Windows 7/8: using the “High Performance” power plan

Windows 10: sliding this bar to the right

I hope this article was insightful and helps you get the most out of your laptop and the power you’re letting your computers draw. I’ve often contemplated making some sort of simple website to fight against all the uneducated myths that Gamers(tm) like to spread regarding performance that amount to “disable every power-saving feature ever and let your hardware run at full clock speeds all the time”, but I’m not much of a website designer or a writer.

That said, I thought the impact that undervolting can have on power consumption to be big enough to warrant a blog post. Even the modest -40 mV change on my desktop can measure itself in watts, and I’m sure that’s probably at least a few bucks off the power bill. And of course, we are currently living in dire times where every little thing we can do matters, from boycotting megacorps fucking up our environments, to little gestures like this.

Thanks for reading!

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