3 ms·

My understanding is that this puts a fundamental limit on how fast CPU clocks can be. If the CPU die has a diameter of 3 centimeters, and the clock speed is a h

by p-e-w 3y ago

My understanding is that this puts a fundamental limit on how fast CPU clocks can be. If the CPU die has a diameter of 3 centimeters, and the clock speed is a hypothetical 20 GHz, light (and electrical signals) can traverse at most half the size of the die in the course of one clock cycle. It's hard to imagine how such a CPU could work, since components at opposite sides of the die cannot communicate within a cycle.

HALtheWise 3y ago
Definitely nitpicking here, but for a modern pipelined CPU there's already lots of components of the die that aren't designed to be able to exchange data within a single clock. I don't see what would be fundamentally different about laying out a CPU die such that operations that need to happen in the same clock are physically nearby, and it seems like it would (at worst) have similar properties to multi-socket and pipelined CPUs today where cache coherency operations and branch mispredictions have multi-cycle penalties associated with them. I'm pretty sure that multi-socket CPUs can't talk in less than one clock cycle anyway.
- laci37 3y ago
  I only had an intro course in CPU architecture, but AFAIK pipelining depends on parts of the pipeline running on the same clock. I'd say CPU manufacturers already sidestepped this limitation by using multicore dies.
  - foobiekr 3y ago
    No, it doesn’t. Putting aside that there is an entire field of clockless designs, asynchronous logic, free-running clocks, etc. You should think of this as “when the cpu is processing an instruction, what does it need to track to dispatch and retire it?” Designing to a click tick is one version and kind of where things were in the 1970s. It’s an educational model.
  - Dylan16807 3y ago
    That just means you can't have a single simple clock source fan out in a simple way across the entire chip, but that kind of design performs badly anyway. There are lots of ways to have multiple clock sources kept in sync or to alter the clock's phase to compensate for distance, so you can keep the entire CPU synchronized even if crossing it takes multiple clock cycles.
- Dylan16807 3y ago
  That's not nitpicking at all. The argument about light-nanoseconds crossing a chip is very common and completely wrong.

by p-e-w 3y ago

HALtheWise 3y ago

Definitely nitpicking here, but for a modern pipelined CPU there's already lots of components of the die that aren't designed to be able to exchange data within a single clock. I don't see what would be fundamentally different about laying out a CPU die such that operations that need to happen in the same clock are physically nearby, and it seems like it would (at worst) have similar properties to multi-socket and pipelined CPUs today where cache coherency operations and branch mispredictions have multi-cycle penalties associated with them. I'm pretty sure that multi-socket CPUs can't talk in less than one clock cycle anyway.

laci37 3y ago
I only had an intro course in CPU architecture, but AFAIK pipelining depends on parts of the pipeline running on the same clock. I'd say CPU manufacturers already sidestepped this limitation by using multicore dies.
- foobiekr 3y ago
  No, it doesn’t. Putting aside that there is an entire field of clockless designs, asynchronous logic, free-running clocks, etc. You should think of this as “when the cpu is processing an instruction, what does it need to track to dispatch and retire it?” Designing to a click tick is one version and kind of where things were in the 1970s. It’s an educational model.
- Dylan16807 3y ago
  That just means you can't have a single simple clock source fan out in a simple way across the entire chip, but that kind of design performs badly anyway. There are lots of ways to have multiple clock sources kept in sync or to alter the clock's phase to compensate for distance, so you can keep the entire CPU synchronized even if crossing it takes multiple clock cycles.
Dylan16807 3y ago
That's not nitpicking at all. The argument about light-nanoseconds crossing a chip is very common and completely wrong.