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CS/ECE 3330 Computer Architecture

CS/ECE 3330 Computer Architecture. Chapter 1 Performance / Power. Last Time. Overview of computer architecture Low-level software High-level hardware The interface between the two (ISA) What’s new and cool Multicore and heterogeneity Power, temperature, reliability Logistics

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CS/ECE 3330 Computer Architecture

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  1. CS/ECE 3330Computer Architecture Chapter 1 Performance / Power

  2. Last Time • Overview of computer architecture • Low-level software • High-level hardware • The interface between the two (ISA) • What’s new and cool • Multicore and heterogeneity • Power, temperature, reliability • Logistics • Ben Kreuter brk7bx@virginia.edu Wed 7PM • Michelle McDaniel mm4ab@virginia.edu Thu 3PM

  3. Class Survey • What does it mean to have better performance?

  4. Defining Performance • Which airplane has the best performance?

  5. Response Time and Throughput • Response time (execution time) • How long it takes to do a task • Throughput (bandwidth) • Total work done per unit time • e.g., tasks/transactions/… per hour • How are response time and throughput affected • By replacing the processor with a faster version? • By adding more processors? • We’ll focus on response time for now…

  6. Relative Performance • Define Performance = 1/Execution Time • “X is n time faster than Y” • Example: time taken to run a program • 20s on A, 30s on B • So A is times faster than B • Execution TimeB / Execution TimeA= 30s / 20s = 1.5 • 1.5

  7. Measuring Execution Time • Elapsed time • Total response time, including all aspects • Processing, I/O, OS overhead, idle time • Determines system performance • CPU time • Time spent processing a given job • Discounts I/O time, other jobs’ shares • Comprises user CPU time and system CPU time • Different programs are affected differently by CPU and system performance

  8. CPU Clocking • Operation of digital hardware governed by a constant-rate clock Clock period Clock (cycles) Data transferand computation Update state • Clock period: duration of a clock cycle • e.g., 250ps = 0.25ns = 250×10–12s • Clock frequency (rate): cycles per second • e.g., 4.0GHz = 4000MHz = 4.0×109Hz

  9. CPU Time • Performance improved by • Reducing number of clock cycles • Increasing clock rate • Hardware designer must often trade off clock rate against cycle count

  10. CPU Time Example • Computer A: 2GHz clock, 20s CPU time • Designing Computer B • Aim for 12s CPU time • Can do faster clock, but causes 1.2 × clock cycles • How fast must Computer B’s clock be?

  11. Instruction Count and CPI • Instruction Count for a program • Determined by program, ISA and compiler • Average cycles per instruction • Determined by CPU hardware • If different instructions have different CPI • Average CPI affected by instruction mix

  12. CPI Example • Computer A: Cycle Time = 300ps, CPI = 2.0 • Computer B: Cycle Time = 600ps, CPI = 1.4 • Same ISA • Which is faster, and by how much? A is faster… …by this much

  13. CPI in More Detail • If different instruction classes take different numbers of cycles • Weighted average CPI Relative frequency

  14. CPI Example • Alternative compiled code sequences using instructions in classes A, B, C • Sequence 1: IC = 10 • Clock Cycles= 4×1 + 2×2 + 4×3= 20 • Avg CPI= 20/10 = 2.0 • Sequence 2: IC =12 • Clock Cycles= 8×1 + 2×2 + 2×3= 18 • Avg CPI= 18/12 = 1.5

  15. The Big Picture • Performance depends on • Algorithm: affects IC, possibly CPI • Programming language: affects IC, CPI • Compiler: affects IC, CPI • Instruction set architecture: affects IC, CPI, Tc

  16. Pitfall: Amdahl’s Law • Improving an aspect of a computer and expecting a proportional improvement in overall performance • Example: multiply accounts for 80s/100s • How much improvement in multiply performance to get 5× overall? • Can’t be done! • Corollary: make the common case fast

  17. Benchmarking • Real applications • Modified applications • Kernels (small, critical parts of real applications) • Toy benchmarks • Synthetic benchmarks + Accuracy -

  18. SPEC CPU Benchmarks • Programs used to measure performance • Supposedly typical of actual workload • Standard Performance Evaluation Corp (SPEC) • Develops benchmarks for CPU, I/O, Web, … • SPEC CPU2006 • Elapsed time to execute a selection of programs • Negligible I/O, so focuses on CPU performance • Normalize relative to reference machine • Summarize as geometric mean of performance ratios • CINT2006 (integer) and CFP2006 (floating-point)

  19. CINT2006 for Opteron X4 2356 High cache miss rates

  20. Summary • Performance can be measured a number of ways • Know the ways, and the potential misconceptions • Mostly boils down to the “standard performance equation” • Next Time… • Power has become a limiting factor • First problem set is due (2PM)

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