Performance

Performance Equation

Here:

• CPI - Cycles Per Instruction
  $\text{CPI} = \text{Total CPU cycles} / \text{Instruction count}$
• IPC Instructions Per Cycle
  $\text{IPC} = 1 / \text{CPI}$

Performance can be improved by:

• Reducing instruction count (better compiler)
• Lowering CPI (efficient microarchitecture)
• Reducing clock cycle time (faster hardware)

Performance Measurement and Benchmarking

Performance is domain-specific. Requirements evolve over time and depend on target devices.

SPEC

Standard Performance Evaluation Corporation. Provides standardized, reproducible benchmark suites.

• SPEC CPU: measures CPU-intensive workloads (integer and floating-point subsets)
• Results reported as a ratio relative to a reference machine
• Geometric mean used to combine scores across workloads

MIPS

Millions of Instructions Per Second.

Peak vs Sustained Performance

• Peak
  Maximum theoretical throughput under ideal conditions. Rarely achieved in practice.
• Sustained
  Actual throughput under real workloads. Reduced by memory bottlenecks, branch mispredictions, and pipeline stalls.