Slide 3.3: CPU performance and its factors

CPU Performance and Its Factors

Computer performance is related to execution time, which can be defined in different ways such as

Clock/response/elapsed time, which means the total time to complete a task, including disk accesses, memory accesses, input/output (I/O) activities, operating system overhead—everything.

CPU execution time, which is the actual time the CPU spends computing for a specific task, which can be further divided into:
- User CPU time, the CPU time spent in a program itself, and
- System CPU time, the CPU time spent in the operating system performing tasks on behalf of the program.

This book focuses on user CPU time. A simple formula relates the most basic metric (clock cycles and clock cycle times) to CPU time:

   CPU execution time for a program

 = CPU clock cycles for a program × Clock cycle time

Alternatively, because clock rate and clock cycle time are inverses,

   CPU execution time for a program

 = CPU clock cycles for a program / Clock rate

Clock cycle is the time between two adjacent pulses of the oscillator that sets the tempo of the computer processor, e.g.,
```
   250 picoseconds = 250 ps = 250 × 10^-12 seconds
```
Clock rate is the number of these pulses per second. It is generally measured in GHz (gigahertz, or billions of pulses per second), e.g.,
```
   4 GHz = 4 × 10⁹ pulses/second
 = [ 1 / ( 250 × 10^-12 ) ] pulses/second
```