IPC vs Speed

[RAM555789]

Just wondering what the major difference is between IPC on a processor and speed (GHz) because I know that GHz is important but something can be clocked much slower but if it has a much higher IPC it will usually outperform the CPU with the higher speeds.

[porina]

IPC = instructions per clock. GHz is an indicator of the clock. They are two different measures. Multiply the two and you have an indicator of the throughput. IPC isn't usually given a value though, and it will vary with task and with a lot of other factors so is hard to nail down. You might see generalisations comparing architectures.

[Coaxialgamer]

IPC is directly related to the processor architecture itself . Generally , adding more hardware increases IPC . My disdain for the term aside , it's defined as the number of operations the processor can perform in one cycles , ie one hertz .

Clock speed refers to the number of cycles the processor will go through per second . Please note that clock speed does NOT refer to the speed the transistors  switch at .

One big problem is that power consumption and heat do not scale linearly with clocks . It grows exponentially. That's a meain reason why we see clock speeds topping out a 4-5ghz .

Generally , with high IPC , you cannot run at such high clocks because the extra hardware generates more heat .

It's a compromise the designers make when building the architecture.

It's a serial approach vs a parallel approach ( to some extent )

[porina]

2 minutes ago, Coaxialgamer said:

One big problem is that power consumption and heat do not scale linearly with clocks . It grows exponentially. That's a meain reason why we see clock speeds topping out a 4-5ghz .

Power consumption does scale with clock... if you keep the voltage constant. It is the increasing voltage required with clock that drives the power non linear.

[MilfShake]

Example: i5 vs Celeron D, both running at 3.0Ghz

 

They have the same clock speed, but the i5 has like 10 times IPC more than the celeron D.

 

Therefore, the i5 will give you more performance than the celeron D, while running at the same clock speed.

 

The difference is in the architecture

[Coaxialgamer]

42 minutes ago, porina said:

Power consumption does scale with clock... if you keep the voltage constant. It is the increasing voltage required with clock that drives the power non linear.

Yes. I know. But realistically that doesn't happen. 

[Godlygamer23]

IPC is Instructions Per Clock. For every cycle, the chip will execute a certain number of instructions. And bear in mind that a cycle is nothing more than an operation that a chip is performing over and over again. 

 

If you have a chip that operates at a speed of 4GHz(or 4 billion operations per second), and an IPC of 50, you multiply 4,000,000,000 by 50, and you have the total number of instructions the chip could execute per second. However, if the code is optimized for a particular architecture(for example, if you make it smaller), you could effectively increase the IPC, delivering better performance.