– Why does clock speed matter on a CPU, and why do some top-tier CPU’s have lower clock speeds than some from nearly 10 generations ago?

Question

I have a good understanding of what clock speed is, but why does it matter?

For the second question, I was wondering since for example, the new i9-14900K has a base clock speed of 3.2 GHz, whereas my previous desktop CPU, the i7-4790K, had a base clock speed of 4.0 GHz. Why hasn’t this number steadily gone up thought the years?

In: Technology

Answer 1

The clock is the pulse of the CPU. It triggers new instructions, and then allows the results of those instructions to be synchronized when they are finished, ready for the next instruction. Generally, instructions are a single clock cycle, but some complex instructions do take more.

The reason you need a clock is that the individual binary gates (AND, OR, NOT, etc) that make up the CPU have a “propagation time). So if you send a binary “1” into a sequence of five gates, it will have a result faster than if you send it through a sequence of twenty gates. But you need all those results at the same time. At the end of all those gate sequences is a latch that holds the result, and then the clock signal transition releases all those latches at the same time. So your minimum clock speed depends on the longest gate propagation time of a single-clock instruction.

But wait, there’s more. The clock has to be distributed to all the parts of the CPU. It actually takes some time for that signal to propagate, as well. The larger and more complex the physical CPU, the longer it takes for the clock to propagate to all parts. This is why CPUs using smaller features can run faster, but more complex CPUs that are larger run slower. Using several CPUs on a die allows management of feature size, complexity, clock propagation, and performance.

There is a cost to keeping everything in step like this – heat. Heat is the result of gates changing state. So clock speed also manages heat in the CPU. This is why modern CPUs use boost clocks and throttling to manage the balance between performance and heat.

I know that ARM and others were looking at clock-less “asynchronous” CPUs at one stage. Here, everything just cascades and once all the latches are in a final state, the next instruction proceeds. Saves on power and reduces heat, but much harder to design and ensure that everything actually has completed before moving on. I suspect it has turned out to be a computational dead end.