Performance

Performance Measures

Improving performance:

• Quicker response time (one job faster)
• More jobs per unit time (pipelining)
• Better energy efficiency

Performance factors:

• Program execution time
• Throughput (overall time to finish)
• Energy per task

Iron Law of Processor Performance

$$\frac{\text{time}}{\text{program}}=\frac{\text{instructions}}{\text{program}}\cdot \frac{\text{cycles}}{\text{instructions}}\cdot \frac{\text{time}}{\text{cycle}}$$

• CPI = Cycles per Instruction, how many clock cycles for one average instruction to finish
  • ISA
  • Processor implementation
    • RISC-V, CPI = 1
    • CISC, CPI >> 1
    • Superscalar processors, CPI < 1
• Instructions per Program
  • Task
  • Algorithm
  • Programming language
  • Compiler
  • ISA
• Time per Cycle
  • Processor microarchitecture (critical path)
  • Technology
  • Power budget (lower voltage reduce transistor speed)