Simply put, regardless of the voltage, as long as the CPU is running, it will “shrink the cylinder,” but generally, the lower the voltage, the slower the shrinking process.
The so-called “shrinking the cylinder” refers to the decline in the CPU’s performance, meaning that to run at the same frequency, a higher voltage is required, or the same voltage can only run at a lower frequency.
This is mainly caused by two factors: transistor aging and wire electromigration losses.
Transistor aging refers to the process in which electrons flowing through a transistor collide with the atoms that make up the transistor. If the electrons that collide happen to have particularly high energy, they can cause atoms to become displaced, forming lattice defects. The continuous accumulation of these defects causes the transistor’s performance to decline.
Electromigration in wires refers to the phenomenon where copper atoms in the wires are physically active and, under the influence of current, move, causing the copper wire to become thinner as it operates, increasing the resistance.
Whether due to aging or electromigration, the speed of these processes is related to two factors: the current flowing through and the operating temperature. The higher the temperature, the larger the current, the more high-energy charge carriers, and the more active the copper atoms, accelerating both aging and electromigration, thus leading to a faster shrinking of the CPU.
The relationship between voltage, current, and temperature is usually positively correlated.
First, the current and voltage are related in digital circuits, where the current is proportional to the frequency and the square of the voltage. Therefore, if the voltage is lowered while keeping the frequency constant, the current will be smaller. In the case of underclocking and undervolting, since current is related to the square of the voltage and frequency is linearly related, and desktop CPU frequency increases are limited, it’s essentially impossible for the voltage decrease to be outweighed by frequency gains, so underclocking will still lead to a reduction in current.
Next, temperature and current are also proportional, with operating temperature being roughly proportional to the square of the current. Since we’ve already deduced that the current decreases, the temperature naturally also decreases.
Since lowering the voltage reduces both current and temperature, it doesn’t make the shrinking process worse, it only slows it down.
As for why the phrase “lowering the voltage causes shrinking” exists, it is mainly because undervolting reduces the CPU’s redundancy, making the effects of shrinking more apparent. For example, when running at the same frequency, if the CPU has 10% redundancy before undervolting, it can still run stably even if performance shrinks by 5%; however, after undervolting, the redundancy might only be 1%. While the shrinking speed is slower, and after the same amount of time, only a 2% shrink occurs (much less than without undervolting), the reduced redundancy means it may no longer run stably.
There is also a situation where, regardless of whether the CPU is undervolted or not, it runs into a power wall rather than a frequency/temperature/current wall. Since power consumption equals current multiplied by voltage, undervolting in this case will cause the current to increase and the frequency to rise, which can indeed accelerate the shrinking process.
Even for such CPUs, undervolting will only accelerate the shrinking process in scenarios where it hits the power wall under full load. In light-load scenarios, undervolting can still slow down the shrinking process. Furthermore, these CPUs typically have a large design margin since they are already hitting the power wall first (otherwise they wouldn’t have hit the power wall). Therefore, even if they shrink, it’s difficult to notice.
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