The CPU Performance Equation

40 The CPU Performance Equation41 The Performance Equation (PE) We would like to model how architecture impacts Performance (latency) This means we need to quantify Performance in terms of architectural parameters. Instruction Count --The number of instructions the CPU executes Cycles per instructions --The ratio of cycles for execution to the number of instructions executed. Cycle time --The length of a clock cycle in seconds The first fundamental theorem of computer architecture:Latency = Instruction Count * Cycles/Instruction * Seconds/CycleL = IC * CPI * CT42 The PE as Mathematical model Good models give insight into the systems they model Latency changes linearly with IC Latency changes linearly with CPI Latency changes linearly with CT It also suggests several ways to improve Performance Reduce CT (increase clock rate) Reduce IC Reduce CPI It also allows us to evaluate potential trade-offs Reducing cycle time by 50% and increasing CPI by is a net = Instructions * Cycles/Instruction * Seconds/Cycl

The PE as Mathematical Model • Good models give insight into the systems they model • Latency changes linearly with IC • Latency changes linearly with CPI • Latency changes linearly with CT • It also suggests several ways to improve performance • Reduce CT (increase clock rate) • Reduce IC • Reduce CPI • It also allows us to ...

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