6 ms·

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

by Geee 6y ago

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.

SomeoneFromCA 6y ago
Lower voltage slows down recharging of the gates. Bugs in the integrated graphics can easily lock up the whole thing.
kalium-xyz 6y ago
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.
avianes 6y ago
> 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.
geogriffin 6y ago
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/ https://plundervolt.com/
- ngneer 6y ago
  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.
toast0 6y ago
> 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.

by Geee 6y ago

SomeoneFromCA 6y ago

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

kalium-xyz 6y ago

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.

avianes 6y ago

> 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.

geogriffin 6y ago

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/ https://plundervolt.com/

ngneer 6y ago
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.

toast0 6y ago

> 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.