Linux Developers Discussing Possible Kernel Driver for Intel CPU Undervolting

hmm maybe less thermal throttling?

Might avoid hitting a thermal threshold and downclocking.

Yes, definitely! Your laptop is generally constrained by how much heat it can push through the heat sink. If you get "lucky" you have a CPU that can remain stable at lower voltages. So it's effectively the same as an "overclock" because your laptop can turbo the core speed up for longer (or forever). Your laptop rarely runs at maximum turbo all day. It can only do that for a few hundred ms usually. It'll basically just hold the clock speed at whatever it can before hitting some specific temp (85-95c). When you undervolt, the frequency that it can "hold" at that temp threshold is higher.

Of course it does. Undervolting is simply finding the lowest voltage your unit can stably work at.

The only result of undervolting on modern CPUs is lower temperature; performance actually goes up as the processor still aims to hit its set TDP/TPL limits.

Lower voltage = higher clocks for longer on all Intel CPUs that can be undervolted. I believe it's the same for new AMDs, but I haven't had the chance to test one.

You're basically fine tuning your pet rock - some silicon is higher quality and can do better than the stock voltages that were set for thousands/millions of units out of the factory.

Many computers, especially those fan less laptop/2-in-1 tablet, doesn't have adequate cooling system.

If CPU was used too much, it instantly reach the temperature threshold, then forced down clock and stall kicks in to save the CPU, repeat the cycle.

Which way do you prefer? endless cycle of 60fps for 10 seconds followed by shuttering for 10 seconds, OR stable 20fps.

IME on most portable computers it's heat dissipation that's giving you the upper bound on clock speed not the CPU's actual upper bound.

of course it does - less voltage => more power headroom and less overall power, less heat dissipated => more thermal headroom.

If the silicon doesn't need the voltage, more voltage is only harmful. I can run my skylake alike laptop at -170mV offset (which is borderline golden chip), stable at prime 95 and whatnot. Prime95 runs below 70C.

Voltage is an energy gap between on and off state of the transistor. There is also random noise on top of that gap. And that noise can sometimes cause the transistor to flip randomly, which results in an error. More voltage means system is more stable and has less random errors. But in gaming people often don't care if there is an error or two now and then, so they can tradeoff stability for lower power usage. Companies don't like that, because that means more technical support when programs randomly crash.

It helps a bit to reduce power/thermal throttling, from my experience the effect is not very strong though (maybe 5-10 % at most I'd say). If your CPU gets thermally throttled a lot it's more likely that you have a faulty or underdimensioned heat sink, in that case I'd try to apply better thermal paste and/or ensure the heat sink properly connects to the CPU. My Alienware throttled the CPU a lot due to overheating because they didn't apply the thermal compound right during fabrication (and seemed to use cheap compound with low conductivity), applying better thermal compound fixed that problem reliably, while undervolting the CPU did almost nothing in that case.

A lot of laptops are built with pretty lack luster thermal design.

Less voltage means less waste heat. If you can lower voltage without lowering clock speed, you'll be able to maintain turbo boost longer instead of constantly thermal throttling.

Not my first recommendation but it is a valid strategy.

The same issues occur with both.

Undervolting goes hand in hand with overclocking. The goal is higher clocks. The enemies are stability and heat. Undervolting reduces heat but the impact on stability depends on the silicon lottery. For mobile chipsets, undervolting at the same clocks is a nice performance uplift because of the reduced heat (on a system that undoubtedly does not have the best heat dissipation.)

Overvolting is also useful for overclocking; at a point it can help increase stability. But generally I think you want to hit a point at which you are getting the highest clocks with the lowest voltage that is still stable.

Roughly, the transistors in a chip can have more or less internal resistance (Rds) due to impurities and variations during production. There's no way to know from one chip to the next until you try them.

If it has less internal resistance it will generate less heat for the same amount of work. Semiconductors are constrained by the maximum junction temperature[1], and CMOS logic chips like modern processors primarily consume power when switching.

So there's a thermal limit to how fast they can switch. Thus having lower internal resistance can allow you to clock higher (switch more often) before reaching the thermal limit.

However, when switching they act as varying resistors, and the amount of power dissipated in any resistor depends on the square of the applied voltage (P = V^2/R).

So if you can run your chip on a lower voltage, it will dissipate a lot less energy when switching, thus allowing you to switch faster before being thermally limited.

When overclocking, you ask the transistors to switch faster. To turn a MOSFET[3] transistor on or off, you need to transfer a certain amount of charge (aka electrons) into or out of the gate. The faster you can transfer the charge (more electrons per second ie the higher the current) the quicker the transistor turns on. And the way to do that is to increase the voltage[2]. So overclockers tend to bump up the voltage when going for the super-high clocks. But as you see from above that causes a massive increase in heat, thus requiring significantly more cooling.

I'm no semiconductor expert though, so I might got some of this wrong, but I hope I got the essence right. Also I'm not sure if the impurities and such that lead to the variability of the internal resistance is related to the ones affecting the ability to undervolt. Is it mainly the threshold voltage or?

There are other aspects as well though, I assume the parasitic elements[4] play a role as well, especially when overclocking.

[1]: https://en.wikipedia.org/wiki/Junction_temperature

[2]: https://en.wikipedia.org/wiki/Ohm%27s_law

[3]: https://en.wikipedia.org/wiki/MOSFET

[4]: https://techweb.rohm.com/knowledge/si/s-si/03-s-si/4873

[4]: https://techweb.rohm.com/knowledge/si/s-si/03-s-si/4873

For anyone struggling with this, look at the TLP project which is a package in most distros that makes (I think pretty anodyne) changes to improve power consumption/battery life. Not uncommon to get 50% improvements straightaway after installing it.

https://linrunner.de/tlp/

> Undervolting lowers voltage, but the processor will now stay at higher clocks for a longer time, hitting the same TDP/power limits.

No,at the same power, but for shorter amount of time, because now it is able do a lot more work per unit of time. So no, you'll definitely use less energy, although dissipate same amount of power. Keeping in mind, that your will be idling 90% of time, if not more, your savings will be even bigger.

Power scales with voltage squared and linearly with freq. Turboboost allows for higher clocks but you'd be getting that with or without undervolting.

Forget XTU, use Throttlestop. It's amazing what functionality the developer baked into it by mostly trial and error with Intel's hidden MSRs.

Linux also has utilities for this, using the same MSRs, although support for a lot of processors is lacking.

If you're running a compute task that is slow and takes an hour, then 5% additional speed saves you 3 minutes. If the computer gets 2x faster, then an additional 5% improvement only saves you 1.5 minutes. So if utility is proportional to time (and hence inversely proportional to speed), then this makes sense.

At a large enough N 5% to 10% is massive, at a small enough N 5% to 10% gets you closer to competitive, somewhere between though is where those extra percentage points are "meh".

For instance I run my computing on a TR 1950X and I could probably undervolt or overclock the chip but, to be honest, it's not worth the perceived "risk". Undervolting could break specific instructions in undetectable ways and the only way to be sure things haven't broken would be to run something like sandsifter [0] multiple times. Because I, and many others who would have the technical know how, write software on these systems it's not very much worth the headache. If I had a seperate system I built just for gaming or doing a specific task and I wanted to reduce how much I spent, or the power usage, etc undervolting would then shift out of headache -> super worth it.

The last thing I want to have to do is run every piece of code I need through godbolt and pray there's no AVX instructions that "may" be fishy.

I think this is largely why cloud providers don't do things like this as aggressively. They do it to some extent with custom off-roadmap chips but not controlled dynamically in userspace. If we can get something like this original post into the kernel that would be a massive win for everyone.

[0] - https://github.com/xoreaxeaxeax/sandsifter

> Or maybe it’s the law of diminishing returns, utility approaching some kind of asymptote.

It's exactly this. Many things have sublinear utility (Money is often assumed to have a log or square-root utility).

speed often has this effect magnified because what people actually care about is time and time is the inverse of speed. Lets say that doing task Z takes X + Y time. At some point increasing the performance of Y has no measurable gain in utility because X dominates.

An analogy with airplanes: you pay cubic dollars to fly faster at the high end.

That is, it's easy to build an airplane that flies at 90 knots. You can then clean up the aerodynamics a bit, burn a bit more fuel to get a bit more power to fly 110 knots. If you want to fly 130 knots, you probably need to add fairings and wheel pants and put in a bigger engine that burns even more fuel. It's very hard to get to 150 knots without retractable landing gear, a massive engine, or removing seats. To get to 200 knots you need 50% more cylinders or a turbocharger. To go past that you usually start looking at a turboprop at 10x the cost and 4x the fuel burn, then jets (...etc.)

Back to computers... doubling your power consumption and heat output for a 100% speed improvement is a perfect, linear improvement. If you need more performance, it's a fair trade. Even if you care about battery life, you should still make the trade, because drawing half as much power for twice as long still burns the same amount of total energy, so you don't gain anything by slowing down.

But would you double your power consumption and heat output for a 50% speed improvement? What about a 20% improvement?

It's the same idea. If the chip makers are setting voltages near the high-end, exponential cost, diminishing returns side of the curve, then you'll get more total "value" back by going back to the linear part of the curve, assuming you care about all of the variables, and not just "more speed at any cost."

well that's the subject of the Linux kernel thread as the original patch was just to remove a warning from the use of the Intel MSR https://lore.kernel.org/lkml/20200907094843.1949-1-Jason@zx2...

I guess it's a question of whether you want drivers to be in the kernel source tree with supported interfaces, where interactions with other drivers can be mediated, or in userspace, where they can iterate without the kernel release process, which takes a long time to get to distros and end-users.

As this Intel MSR is not well documented, I would not argue strongly either way.

Maybe to handle things like suspend/resume, ...

This must be the reason with used Haswell mobos are cheaper than used Ivy Bridge ones.