4 ms·

How far an electron can travel in a ns is a crazy limiter in computer science.

• Dylan16807 6y ago
  It's not. The limit is how fast you can switch transistors. Electrons can go a pretty long distance inside a clock cycle, and waiting multiple clock cycles for signals to go from core to core is normal and easy.
  • a1369209993 6y ago
    Technically electrons barely move at all (on clock-cycle timescales); it's the electric field that moves at ~2/3 speed of light. Compare eg, wind speed (particles) versus the speed of sound (field).
    • Dylan16807 6y ago
      > Technically electrons barely move at all (on clock-cycle timescales); it's the electric field that moves at ~2/3 speed of light. Yes. > Compare eg, wind speed (particles) versus the speed of sound (field). Huh? The speed of particles in the air is even faster than the speed of sound.
      • ohazi 6y ago
        Aggregate wind speed, as in "5 knots southeasterly" vs. the speed of sound in that vicinity, which would still be ~340 m/s
• imtringued 6y ago
  It's actually very forgiving. For high frequency signal traces on a PCB there is this rule of thumb that your trace lengths must not differ by more 15cm per ns because that is how far electrons travel in one nanosecond. If you have a 10GHz signal that means you have a 0.01ns budget which translates to a 1.5mm max length difference between traces. Matching within 1.5mm is easy to do by hand. Now lets talk about actual processors instead of doing an analogy. 15cm per ns is more than enough to travel everywhere inside your CPU but the vast majority of logic is localized (usually signals stay in the same core). It's only awful if you go off package to DRAM or a second socket but then the budget is often higher than 1ns. Apple probably scores a lot of performance points here because the RAM is so close to the CPU.