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Comment by freeqaz | Hacker News Reader

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.

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It's not just temperature: many laptops also hit their power limits under full load.

I had a Surface Book 2 that was particularly bad for this, I think because it uses the same connector as older versions, and it could not supply enough power. Undervolting seemed to help quite a bit.

MacBook Pros have this problem. You cannot achieve maximum performance without using both the external adapter and the internal battery. Not sure about the most recent models but true of mine (2013).

Annoyingly, there are 3 major related limits for Intel mobile CPUs- thermal, power (both an instantaneous, absolute max limit and a set of "okay you can use 25W for up to 5 seconds but then have to stop" limits), and current.

Within that envelope, the CPU can boost up to its max turbo frequency when needed.

I wonder what exactly causes "instability" when overclocking/undervolting in a processor. Transistors don't have enough time to switch, but it matters where it happens.

If there's an error in e.g. floating-point unit, it would just cause random errors in the output, but the game would probably keep running. That might be completely fine, if it's rare and you could undervolt just that unit a lot. Especially in somewhere in the graphics pipeline, where it would just generate visual noise. Also, I think energy consumed / heat is proportional to voltage squared, so it has a big effect.

Lower voltage slows down recharging of the gates. Bugs in the integrated graphics can easily lock up the whole thing.

Afaik impurities, small imperfections, and micro fractures increase resistance which increases power consumption and generate heat. Im not an expert on this but I assume even how well the chip was soldered to the board might make a serious difference here.

> If there's an error in e.g. floating-point unit, it would just cause random errors in the output, but the game would probably keep running

The processor circuitry does not only consist of transistors dedicated to the datapath, a significant amount of transistors are used to control the datapath. If control transistors are faulty there is no guarantee about the processor behavior.

Also, I don't know about Intel cpu protections, but in modern secure hardware we include dedicated circuits for power/clock glitch detection. The glitch detection circuit is designed with a higher sensibility to power/clock fault than the rest of the circuit, this way its avoids that clock/power faults affect the integrity of the circuit output.

The Plundervolt [1] paper is one study in what exactly goes haywire when a processor is undervolted. They found that the computation errors induced are quite consistent and reproducible. MUL and DIV are vulnerable, and AES-NI happened to be particularly badly broken in the face of undervolting.

[1] https://plundervolt.com/

Indeed. CLKSCREW is the progenitor of that work. Note that in the context of security, it only matters if undervolting can cross security boundaries. This was possible on Intel systems at the time of Plundervolt but has since been fixed.

> I wonder what exactly causes "instability" when overclocking/undervolting in a processor. Transistors don't have enough time to switch, but it matters where it happens.

Not x86, but in college a series of courses used a battery powered embedded system with a Motorolla 68HC11. When the batteries ran low, conditional branches would never branch, which was very confusing. Everything else seemed to work fine though.

Yeah, even with my NUC that had a pretty decent fan running at top speed I literally put a block of metal I kept in the freezer on top of it to keep it from downthrottling due to overheating. It was ridiculous that it could let itself get that hot instead of doing something more graceful.

Please share the name of the NUC so others can avoid it.

It's a 5th generation core i7 in the Intel NUC. It's been off the market for four years and the thermal issues were known and addressed in the intervening years to the best of my understanding.

I'm just saying that linux does not do a great job of gracefully limiting CPU power to avoid a lower BIOS level limiter kicking in and performing worse than if it just throttled down a bit earlier.

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