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Performance

Performance. Lecture notes from MKP, H. H. Lee and S. Yalamanchili. Reading. Section 1.4. Understanding Performance. Algorithm Determines number of operations executed Programming language, compiler, architecture Determine number of machine instructions executed per operation

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Performance

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  1. Performance Lecture notes from MKP, H. H. Lee and S. Yalamanchili

  2. Reading • Section 1.4

  3. Understanding Performance • Algorithm • Determines number of operations executed • Programming language, compiler, architecture • Determine number of machine instructions executed per operation • Processor and memory system • Determine how fast instructions are executed • I/O system (including OS) • Determines how fast I/O operations are executed Instruction Set Architecture

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

  5. Metrics • Response time (latency) • How long it takes to do a task • Throughput • Total work done per unit time • e.g., tasks/transactions/… per hour • Trading throughput vs. latency • Energy/Power • Measure of work being performed • Increases with clock frequency/voltage • Determines temperature • Is affected by temperature

  6. Relative Performance • “X is n time faster than Y” • Example: time taken to run a program • 10s on A, 15s on B • Execution TimeB / Execution TimeA= 15s / 10s = 1.5 • So A is 1.5 times faster than B

  7. CPU Clocking • Operation of digital hardware governed by a constant-rate clock Our cycle time 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

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

  9. CPU Time Example • Computer A: 2GHz clock, 10s CPU time • Designing Computer B • Aim for 6s CPU time • Can do faster clock, but causes 1.2 × clock cycles • How fast must Computer B clock be?

  10. 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

  11. Cycles and Instructions • Multiplication takes more time than addition • Floating point operations take longer than integer ones • Accessing memory takes (in general) more time than accessing registers • Important point: changing the cycle time often changes the number of cycles required for various instructions (more later) time

  12. Program Execution time Number of instruction classes ~= Instruction_count * CPIavg * clock_cycle_time technology architecture algorithms/compiler Relative frequency

  13. CPI Example • Computer A: Cycle Time = 250ps, CPI = 2.0 • Computer B: Cycle Time = 500ps, CPI = 1.2 • Same ISA • Which is faster, and by how much? A is faster… …by this much

  14. CPI Example • Alternative compiled code sequences using instructions in classes A, B, C • Sequence 1: IC = 5 • Clock Cycles= 2×1 + 1×2 + 2×3= 10 • Avg. CPI = 10/5 = 2.0 • Sequence 2: IC = 6 • Clock Cycles= 4×1 + 1×2 + 1×3= 9 • Avg. CPI = 9/6 = 1.5 Example:

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

  16. SPEC CPU Benchmark • 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)

  17. Other Benchmark Suites • Report performance metrics for execution on target platforms • Designed to assess how well the platforms function in specific domains • Examples • Media Bench - Multimedia • EEMBC – Embedded systems • Rodinia, Parboil: For GPU Systems • SPECWeb, SPECJbb – Enterprise systems • Many more……

  18. 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

  19. Told f (1 - f) Tnew (1 - f) f / P Amdahl’s Law • Speed-up = Exec_timeold/ Exec_timenew = • Performance improvement from using faster mode is limited by the fraction the faster mode can be applied. affected

  20. Concluding Remarks • Cost/performance is improving • Due to underlying technology development • Hierarchical layers of abstraction • In both hardware and software • Instruction set architecture • The hardware/software interface • Execution time: the best performance measure • Power is a limiting factor • Use parallelism to improve performance

  21. Study Guide • Practice problems provided on the class website

  22. Glossary • Amdahl’s Law • Benchmarks • CPI (cycles per instruction) • CPU Time • Latency • SPEC CPU • Throughput

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