Performance

Defining Performance

• Response time (execution time): The time between start and completion of a task
• Throughput (Bandwidth): The number of tasks completed per unit time

Measuring Performance

• Clock cycle (s): The time for one clock period (usually the processor clock) - constant
• Clock rate (Hz): The inverse of clock period
• CPU execution time: The time CPU spends computing for a certain task (does not include time spent waiting for I/O or running other programs)

$$\begin{array}{rl}\text{CPU time}& =\text{CPU clock cycles}\times \text{Clock cycle time}\\ & =\frac{\text{CPU clock cycles}}{\text{Clock rate}}\end{array}$$

• CPI (Clock cycles per instruction): The average number of clock cycles per instruction for a program or program fragment.

$$\begin{array}{r}\text{CPU clock cycles}=\text{Total number of instructions}\times \text{CPI}\end{array}$$

Therefore, we have the following.

$$\begin{array}{rl}\text{CPU time}& =\frac{\text{Instruction count}\times \text{CPI}}{\text{Clock rate}}\end{array}$$