Computer Architecture Chapter 4 Assessing and Understanding Performance

1. Computer ArchitectureChapter 4Assessing and Understanding Performance Yu-Lun Kuo 郭育倫 Department of Computer Science and Information Engineering Tunghai University, Taichung, Taiwan R.O.C. sscc6991@gmail.com http://www.csie.ntu.edu.tw/~d95037/

2. Opening Indeed, the cost-performance ratio of the product will depend most heavily on the implementer, just as ease of use depends most heavily on the architect. The Mythical Man-Month, Brooks

3. Introduction • Measure, Report, and Summarize • Make intelligent choices • See through the marketing hype • Key to understanding underlying organizational motivation • Why is some hardware better than others for different programs? • What factors of system performance are hardware related? (e.g., Do we need a new machine, or a new operating system?) • How does the machine’s instruction set affect performance?

4. Best Performance • How much faster is the Concorde compared to the 747? • How much bigger is the 747 than the Douglas DC-8? Response Time Throughput

5. Performance Metrics • Purchasing perspective • given a collection of machines, which has the • best performance ? • least cost ? • best cost/performance? • Design perspective • faced with design options, which has the • best performance improvement ? • least cost ? • best cost/performance?

6. Performance Metrics • Both require • Basis for comparison • Metric for evaluation • Our goal is to understand what factors in the architecture • Contribute to overall system performance • The relative importance (and cost) of these factors

7. Computer Performance • Response Time (latency)(execution time) • The total time required for the computer to compute a task, including disk access, memory access, I/O activities, OS overhead, CPU execution time, and so on. • How long does it take for my job to run? • How long does it take to execute a job? • How long must I wait for the database query? • Throughput (生產量) • How many jobs can the machine run at once? • What is the average execution rate? • How much work is getting done?

8. Computer Performance • If we upgrade a machine with a new processor what do we increase? • If we add a new machine to the lab what do we increase?

9. Measuring Performance • Elapsed Time • Counts everything (disk and memory accesses, I/O, etc.) • A useful number, but often not good for comparison purposes • CPU time • Doesn’t count I/O or time spent running other programs • Can be broken up into system time, and user time • Our focus: user CPU time • Time spent executing the lines of code that are “in” our program

10. Example (Linux instruction) • Time instruction: “time” • > 90.7u 12.9s 2:39 65% • CPU time ratio: (90.7+12.9) / 159 = 65% • I/O time  more than 1/3

11. performanceX execution_timeY -------------------- = --------------------- = n performanceY execution_timeX Book’s Definition of Performance • Normally interested in reducing • Response time (aka execution time) – the time between the start and the completion of a task • Important to individual users • Thus, to maximize performance, need to minimize execution time performanceX = 1 / execution_timeX If X is n times faster than Y, then

12. Book’s Definition of Performance • Throughput • The total amount of work done in a given time • Important to data center managers • Decreasing response time almost always improves throughput

13. Book’s Definition of Performance • Problem: • Machine X runs a program in 10 seconds • Machine Y runs the same program in 15 seconds

14. Performance Factors • Want to distinguish elapsed time and the time spent on our task • CPU execution time (CPU time) – time the CPU spends working on a task • Does not include time waiting for I/O or running other programs

15. CPU execution time # CPU clock cycles = x clock cycle time for a program for a program CPU execution time # CPU clock cycles for a program = ------------------------------------------- for a program clock rate Performance Factors or • Can improve performance by reducing either the length of the clock cycle or the number of clock cycles required for a program

16. Clock Cycles • Instead of reporting execution time in seconds, we often use cycles • Clock “ticks” indicate when to start activities • Cycle time = time between ticks = seconds per cycle • Clock rate (frequency) = cycles per second (1Hz = 1 cycle/sec) • A 4GHz clock has a cycle time

17. Review: Machine Clock Rate • Clock rate (MHz, GHz) is inverse of clock cycle time (clock period) CC = 1 / CR one clock period 10 nsec clock cycle => 100 MHz clock rate 5 nsec clock cycle => 200 MHz clock rate 2 nsec clock cycle => 500 MHz clock rate 1 nsec clock cycle => 1 GHz clock rate 500 psec clock cycle => 2 GHz clock rate 250 psec clock cycle => 4 GHz clock rate 200 psec clock cycle => 5 GHz clock rate

18. How to Improve Performance • So, to improve performance (everything else being equal) you can either (increase or decrease?)_decrease_ the # of required cycles for a program, or_decrease_ the clock cycle time or, said another way,_increase_ the clock rate.

19. 1st instruction 2nd instruction 3rd instruction ... 4th 5th 6th How many cycles are required for a program? • Could assume that number of cycles equals number of instructions • This assumption is incorrect, different instructions take different amounts of time on different machines.Why? hint: remember that these are machine instructions, not lines of C code time

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

21. Improving Performance • Our favorite program runs in 10 seconds on computer A, which has a 4GHz clock. • We are trying to help a computer designer build a new machine B, that will run this program in 6 seconds. • The designer can use new (or perhaps more expensive) technology to substantially increase the clock rate, but has informed us that this increase will affect the rest of the CPU design, causing machine B to require 1.2 times as many clock cycles as machineA for the same program. What clock rate should we tell the designer to target?

22. Improving Performance

23. Improving Performance

24. # CPU clock cycles # Instructions Average clock cycles = x for a program for a program per instruction Clock Cycles per Instruction (CPI) • Not all instructions take the same amount of time to execute • One way to think about execution time is that it equals the number of instructions executed multiplied by the average time per instruction

25. Clock Cycles per Instruction (CPI) • Clock cycles per instruction (CPI) • The average number of clock cycles each instruction takes to execute • A way to compare two different implementations of the same ISA

26. Effective CPI • Computing the overall effective CPI is done by looking at the different types of instructions and their individual cycle counts and averaging • Where ICi is the count (percentage) of the number of instructions of class i executed • CPIi is the (average) number of clock cycles per instruction for that instruction class • n is the number of instruction classes n Overall effective CPI =  (CPIi x ICi) i = 1

27. Instruction_count x CPI CPU time = ----------------------------------------------- clock_rate Performance Equation • Our basic performance equation is then CPU time = Instruction_count x CPI x clock_cycle or

28. Determinates of CPU Performance • CPU time = Instruction_count x CPI x clock_cycle

29. Determinates of CPU Performance • CPU time = Instruction_count x CPI x clock_cycle X X X X X X X X X X X X

30. CPI Example • Suppose we have two implementations of the same instruction set architecture (ISA). • For some program, Machine A has a clock cycle time of 250 ps and a CPI of 2.0 • Machine B has a clock cycle time of 500 ps and a CPI of 1.2 • What machine is faster for this program, and by how much? • If two machines have the same ISA which of our quantities (e.g., clock rate, CPI, execution time, # of instructions, MIPS) will always be identical?

31. CPI Example (2) • 假設相同指令集架構(ISA)下的兩種機器設計 • 對某一個程式的執行 • Machine A has a clock cycle time of 10 ns. and a CPI of 2.0 • Machine B has a clock cycle time of 20 ns. and a CPI of 1.2 • 那一台機器比較快, 而且快多少? • 假使有兩台機器有相同的 ISA, 下面的那個計算效能值不變(e.g., clock rate, CPI, execution time, # of instructions, MIPS:million instructions per second)?

32. Improving Performance • 因為是同樣的程式在不同的機器上執行, 所以執行的指令數相同, 但執行時間不同 • 假設這個程式有I個指令數 • CPU clock cyclesA=I*2.0 • CPU clock cyclesB=I*1.2 • CPU time = CPU clocks cycles * clock cycle time • CPU performanceA/CPU performanceB = ExecutionB / ExecutionA = (1.2*20)/(2*10)=1.2

33. Improving Performance

34. Now that We Understand Cycles • A given program will require • some number of instructions (machine instructions) • some number of cycles • some number of seconds • We have a vocabulary that relates these quantities: • cycle time (seconds per cycle) • clock rate (cycles per second) • CPI (cycles per instruction) a floating point intensive application might have a higher CPI • MIPS (millions of instructions per second)this would be higher for a program using simple instructions

35. Performance • Performance is determined by execution time • Do any of the other variables equal performance? • # of cycles to execute program? • # of instructions in program? • # of cycles per second? • average # of cycles per instruction? • average # of instructions per second?

36. Performance - Time

37. # of Instructions Example

38. # of Instructions Example

39. # of Instructions Example

40. Performance: What to measure • Usually rely on benchmarks vs. real workloads • To increase predictability, collections of benchmark applications-- benchmark suites -- are popular • SPECCPU: popular desktop benchmark suite • CPU only, split between integer and floating point programs • SPECint2000 has 12 integer, SPECfp2000 has 14 integer pgms • SPECCPU2006 to be announced Spring 2006 • SPECSFS (NFS file server) and SPECWeb (WebServer) added as server benchmarks

41. Performance: What to measure • Transaction Processing Council measures server performance and cost-performance for databases • TPC-C Complex query for Online Transaction Processing • TPC-H models ad hoc decision support • TPC-W a transactional web benchmark • TPC-App application server and web services benchmark

42. Benchmarks • Performance best determined by running a real application • Use programs typical of expected workload • Or, typical of expected class of applications e.g., compilers/editors, scientific applications, graphics, etc. • Small benchmarks • nice for architects and designers • easy to standardize • can be abused

43. Benchmarks • SPEC (System Performance Evaluation Cooperative) • companies have agreed on a set of real program and inputs • valuable indicator of performance (and compiler technology) • can still be abused

44. SPEC Benchmarks www.spec.org

47. MIPS example

49. MIPS Example

50. Amdahl’s Law • Execution Time After Improvement = • Execution Time Unaffected +( Execution Time Affected / Amount of Improvement ) • Example:"Suppose a program runs in 100 seconds on a machine, with multiply responsible for 80 seconds of this time. How much do we have to improve the speed of multiplication if we want the program to run 4 times faster?" How about making it 5 times faster???!! • Principle: Make the common case fast