CPU Performance. Lecture 8 CAP

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1 CPU Performance Lecture 8 CAP

2 Defining Performance Which airplane has the best performance? 1.6 Performance Boeing 777 Boeing 777 Boeing 747 BAC/Sud Concorde Douglas DC-8-50 Boeing 747 BAC/Sud Concorde Douglas DC Passenger Capacity Cruising Range (miles) Boeing 777 Boeing 777 Boeing 747 BAC/Sud Concorde Douglas DC-8-50 Boeing 747 BAC/Sud Concorde Douglas DC Cruising Speed (mph) Passengers x mph Chapter 1 Computer Abstractions and Technology 2

3 Response Time and Throughput Response time How long it takes to do a task Throughput 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? Adding more processors? We ll focus on response time for now Chapter 1 Computer Abstractions and Technology 3

4 Relative Performance Define Performance = 1/Execution Time X is n time faster than Y Performance X Execution time Performance Y Y Execution time X n Example: time taken to run a program 10s on A, 15s on B Execution Time B / Execution Time A = 15s / 10s = 1.5 So A is 1.5 times faster than B Chapter 1 Computer Abstractions and Technology 4

5 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 Chapter 1 Computer Abstractions and Technology 5

6 CPU Clocking Operation of digital hardware governed by a constant-rate clock Clock (cycles) Data transfer and computation Update state Clock period Clock period: duration of a clock cycle e.g., 250ps = 0.25ns = s Clock frequency (rate): cycles per second e.g., 4.0GHz = 4000MHz = Hz Chapter 1 Computer Abstractions and Technology 6

7 CPU Time CPU Time CPU Clock Cycles Clock Cycle Time CPU Clock Cycles Clock Rate Performance improved by Reducing number of clock cycles Increasing clock rate Hardware designer must often trade off clock rate against cycle count Chapter 1 Computer Abstractions and Technology 7

8 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? Chapter 1 Computer Abstractions and Technology 8

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? Clock Rate B Clock Cycles CPU Time B B 1.2 Clock Cycles 6s A Clock Cycles A CPU Time A Clock Rate A 10s2GHz Clock Rate B s s 9 4GHz Chapter 1 Computer Abstractions and Technology 9

10 Instruction Count and CPI Clock Cycles Instruction Count Cycles per Instruction CPU Time Instruction Count CPI Clock Cycle Time Instruction Count CPI Clock Rate 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 Chapter 1 Computer Abstractions and Technology 10

11 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? Chapter 1 Computer Abstractions and Technology 11

12 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? CPU Time A CPU Time B CPU Time B CPU Time A Instruction Count CPI A I ps I500ps Instruction Count CPI B I ps I 600ps I 600ps I500ps 1.2 Cycle Time A Cycle Time B A is faster by this much Chapter 1 Computer Abstractions and Technology 12

13 CPI in More Detail If different instruction classes take different numbers of cycles Clock Cycles n i1 (CPI Instructio n Count i i) Weighted average CPI CPI Clock Cycles Instructio n Count n i1 CPI i Instructio n Count Instructio n Count i Relative frequency Chapter 1 Computer Abstractions and Technology 13

14 CPI Example Alternative compiled code sequences using instructions in classes A, B, C Class A B C CPI for class IC in sequence IC in sequence Which code sequence executes the most instructions? Which one will be faster? What is the CPI for each sequence? Chapter 1 Computer Abstractions and Technology 14

15 CPI Example Alternative compiled code sequences using instructions in classes A, B, C Class A B C CPI for class IC in sequence IC in sequence Sequence 1: IC = 5 Clock Cycles = = 10 Avg. CPI = 10/5 = 2.0 Sequence 2: IC = 6 Clock Cycles = = 9 Avg. CPI = 9/6 = 1.5 Chapter 1 Computer Abstractions and Technology 15

16 Performance Summary The BIG Picture CPU Time Instructio ns Program Clock cycles Instructio n Seconds Clock cycle Performance depends on Algorithm: affects IC, possibly CPI Programming language: affects IC, CPI Compiler: affects IC, CPI Instruction set architecture: affects IC, CPI, T c Chapter 1 Computer Abstractions and Technology 16

17 Power Trends 1.7 The Power Wall In CMOS IC technology Power Capacitive load Voltage 2 Frequency 30 5V 1V 1000 Chapter 1 Computer Abstractions and Technology 17

18 Reducing Power Suppose we developed a new, simpler processor that has 85% of the capacitive load of the more complex older processor. Further, assume that it has adjustable voltage so that it can reduce voltage 15% compared to processor B, which results in a 15% shrink in frequency. What is the impact on dynamic power? Chapter 1 Computer Abstractions and Technology 18

19 Reducing Power Suppose a new CPU has 85% of capacitive load of old CPU 15% voltage and 15% frequency reduction 2 Pnew Cold 0.85(Vold 0.85) Fold P C V F old The power wall old We can t reduce voltage further We can t remove more heat old old 0.52 How else can we improve performance? Chapter 1 Computer Abstractions and Technology 19

Chapter 2. Why is some hardware better than others for different programs?

Chapter 2. Why is some hardware better than others for different programs? Chapter 2 1 Performance Measure, Report, and Summarize Make intelligent choices See through the marketing hype Key to understanding underlying organizational motivation Why is some hardware better than