06.7 CPU Performance

Time is used to measure a computer’s performance

Wall clock time: elapsed time (including overheads)

CPU Cycles

CPU execution time (or CPU time): The time which the CPU spends on the program (without overhead)

To go from CPU time to CPU cycles, we have the formulas:

$${\text{CPU}}_{\text{time}}={\text{CPU}}_{\text{cycles}}\times \text{Clock cycle}$$ $${\text{CPU}}_{\text{time}}=\frac{{\text{CPU}}_{\text{cycles}}}{\text{Clock rate}}$$

Instruction Performance

CPU time depends on the number of instructions of a given program (CPI = Clock cycles per instruction)

$${\text{CPU}}_{\text{cycles}}=\text{Instructions}\times \text{CPI}$$

Using the previous CPU time equation, we can get that the CPU time for a program time

$${\text{CPU}}_{\text{time}}=\text{Instructions}\times \text{CPI}\times \text{Clock cycle}$$

Performance and Power

We have the dynamic power consumption ratio given by

$$P_D=fC{V}^2$$