1. Sogang University Advanced Computing System Chap 1. Computer Architecture Hyuk-Jun Lee, PhD Dept. of Computer Science and Engineering Sogang University Seoul, Korea Email: hyukjunl@sogang.ac.kr

2. Sogang University Contents Computer Architecture Overview Metrics – Performance – Power consumption What to improve? – Common case – Amdahl's law

3. Sogang University The Computer Revolution Progress in computer technology –Underpinned by Moore’s Law Makes novel applications feasible –Computers in automobiles –Cell phones –Human genome project –World Wide Web –Search Engines Computers are pervasive

4. Sogang University Classes of Computers Desktop computers –General purpose, variety of software –Subject to cost/performance tradeoff Server computers –Network based –High capacity, performance, reliability –Range from small servers to building sized Embedded computers –Hidden as components of systems –Stringent power/performance/cost constraints

5. Sogang University 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

6. Sogang University Below Your Program Application software –Written in high-level language System software –Compiler: translates HLL code to machine code –Operating System: service code Handling input/output Managing memory and storage Scheduling tasks & sharing resources Hardware –Processor, memory, I/O controllers

7. Sogang University Levels of Program Code High-level language –Level of abstraction closer to problem domain –Provides for productivity and portability Assembly language –Textual representation of instructions Hardware representation –Binary digits (bits) –Encoded instructions and data

8. Sogang University Components of a Computer Same components for all kinds of computer –Desktop, server, embedded Input/output includes –User-interface devices Display, keyboard, mouse –Storage devices Hard disk, CD/DVD, flash –Network adapters For communicating with other computers The BIG Picture

9. Sogang University Typical x86 PC System

10. Sogang University Inside the Processor AMD Barcelona: 4 processor cores

11. Sogang University ARM based smartphone structure IP block Embedded multimedia card

12. Sogang University 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…

13. Sogang University CPU Time Performance improved by –Reducing number of clock cycles –Increasing clock rate –Hardware designer must often trade off clock rate against cycle count

14. Sogang University 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?

15. Sogang University 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

16. Sogang University 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

17. Sogang University CPI in More Detail If different instruction classes take different numbers of cycles Weighted average CPI Relative frequency

18. Sogang University CPI Example Alternative compiled code sequences using instructions in classes A, B, C ClassABC CPI for class123 IC in sequence 1212 IC in sequence 2411 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

19. Sogang University 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, T c The BIG Picture

20. Sogang University Uniprocessor Performance Constrained by power, instruction-level parallelism, memory latency

21. Sogang University Power consumption For CMOS chips, traditional dominant energy consumption has been in switching transistors, called dynamic power For mobile devices, energy better metric For a fixed task, slowing clock rate (frequency switched) reduces power, but not energy Capacitive load a function of number of transistors connected to output and technology, which determines capacitance of wires and transistors Dropping voltage helps both, so went from 5V to 1V To save energy & dynamic power, most CPUs now turn off clock of inactive modules (e.g. Fl. Pt. Unit)

22. Sogang University Example of quantifying power Suppose 15% reduction in voltage results in a 15% reduction in frequency. What is impact on dynamic power?

23. Sogang University Power consumtion Because leakage current flows even when a transistor is off, now static power important too Leakage current increases in processors with smaller transistor sizes Increasing the number of transistors increases power even if they are turned off In 2006, goal for leakage is 25% of total power consumption; high performance designs at 40% Very low power systems even gate voltage to inactive modules to control loss due to leakage

24. Sogang University Amdahl’s Law Best you could ever hope to do:

25. Sogang University Amdahl’s Law example New CPU 10X faster I/O bound server, so 60% time waiting for I/O Apparently, its human nature to be attracted by 10X faster, vs. keeping in perspective its just 1.6X faster