CSC 320 Chapter #1, Spring 2015

Chapter 1 Computer Abstractions and Technology Chapter 1 — Computer Abstractions and Technology — 2 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 §1.1 Introduction Chapter 1 — Computer Abstractions and Technology — 3 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 Chapter 1 — Computer Abstractions and Technology — 4 The Processor Market Chapter 1 — Computer Abstractions and Technology — 6 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 Chapter 1 — Computer Abstractions and Technology — 7 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 §1.2 Below Your Program Chapter 1 — Computer Abstractions and Technology — 8 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 Chapter 1 — Computer Abstractions and Technology — 9 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 §1.3 Under the Covers The BIG Picture Chapter 1 — Computer Abstractions and Technology — 13 Opening the Box Chapter 1 — Computer Abstractions and Technology — 14 Inside the Processor (CPU) ! Datapath: performs operations on data ! Control: commands the datapath, memory, and the I/O devices according to the program instructions. ! Cache memory ! Small fast SRAM memory for immediate access to data ! Acts as buffer to the DRAM memory Chapter 1 — Computer Abstractions and Technology — 16 Abstractions ! Abstraction helps us deal with complexity ! Hide lower-level detail ! Instruction set architecture (ISA) ! The hardware/software interface ! Implementation ! The details underlying the interface The BIG Picture Chapter 1 — Computer Abstractions and Technology — 17 A Safe Place for Data ! Volatile main memory ! Loses instructions and data when power off ! Non-volatile secondary memory ! Magnetic disk ! Flash memory ! Optical disk (CDROM, DVD) Chapter 1 — Computer Abstractions and Technology — 18 Networks ! Communication and resource sharing ! Local area network (LAN): Ethernet ! Within a building ! Wide area network (WAN: the Internet ! Wireless network: WiFi, Bluetooth Chapter 1 — Computer Abstractions and Technology — 19 Technology Trends ! Electronics technology continues to evolve ! Increased capacity and performance ! Reduced cost Year Technology Relative performance/cost 1951 Vacuum tube 1 1965 Transistor 35 1975 Integrated circuit (IC) 900 1995 Very large scale IC (VLSI) 2,400,000 2005 Ultra large scale IC 6,200,000,000 DRAM capacity Chapter 1 — Computer Abstractions and Technology — 21 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? Chapter 1 — Computer Abstractions and Technology — 22 Relative Performance ! Define Performance = 1/Execution Time ! “X is n time faster than Y” = n = X Y Y X Execution time time Execution Performance e Performanc ! 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 Chapter 1 — Computer Abstractions and Technology — 23 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 — 24 CPU Clocking ! Operation of digital hardware governed by a constant-rate clock Clock (cycles) Data transfer and computation Clock period ! 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 Chapter 1 — Computer Abstractions and Technology — 26 CPU Time ! Performance improved by ! Reducing number of clock cycles ! Increasing clock rate ! Hardware designer must often trade off clock rate against cycle count Rate Clock Clock Cycles CPU CPU Clock Cycles Clock Cycle Time Time CPU = × = Example Our favorite program runs in 10 sec. on computer A, which has a 2 GHz clock. We are trying to help a computer designer build a computer B which will run this program in 6 seconds. The designer has determined that a substantial increase in the clock rate is possible but this increase will affect the rest of the CPU design, causing computer B to require 1.2 times as many clock cycles as computer A for this program. What clock rate should we tell the designer to target Chapter 1 — Computer Abstractions and Technology — 27 Chapter 1 — Computer Abstractions and Technology — 28 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 — 29 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? 4GHz 6s 10 24 6s 1.2 20 10 Rate Clock 20 10 2GHz 10s Clock Rate CPU Time Cycles Clock 6s Clock Cycles 1.2 Time CPU Clock Cycles Rate Clock 9 9 B 9 A A A A B B B = × = × × = × = × = × = × = = Chapter 1 — Computer Abstractions and Technology — 30 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 Rate Clock CPI n Count Instructio CPI Clock Cycle Time Instructio n Count Time CPU Cycles per Instruction Instructio n Count Cycles Clock × = × × = × = Chapter 1 — Computer Abstractions and Technology — 31 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 — 32 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? 1.2 500ps I 600ps I TimeA CPU TimeB CPU I 600ps 1.2 500ps I Cycle TimeB CPIB Instructio n Count TimeB CPU I 500ps 2.0 250ps I Cycle TimeA CPIA Instructio n Count TimeA CPU = × × = = × × × = × × = = × × × = × × = A is faster… …by this much Chapter 1 — Computer Abstractions and Technology — 33 CPI in More Detail ! If different instruction classes take different numbers of cycles ∑ = × = n 1 i i i Instructio n Count ) (CPI Cycles Clock ! Weighted average CPI ∑ =       × = = n 1 i i i n Count Instructio Instructio n Count CPI n Count Instructio Clock Cycles CPI Relative frequency Chapter 1 — Computer Abstractions and Technology — 34 CPI Example ! Alternative compiled code sequences using instructions in classes A, B, C Class A B C CPI for class 1 2 3 IC in sequence 1 2 1 2 IC in sequence 2 4 1 1 ! 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 Chapter 1 — Computer Abstractions and Technology — 35 Performance Summary ! Performance depends on ! Algorithm: affects IC, CPI ! Programming language: affects IC, CPI ! Compiler: affects IC, CPI ! Instruction set architecture: affects IC, CPI, Tc The BIG Picture cycle Clock Seconds n Instructio cycles Clock Program Instructions CPU Time × × = Chapter 1 — Computer Abstractions and Technology — 38 Uniprocessor Performance §1.6 The Sea Change: The Switch to Multiprocessors Constrained by power, instruction-level parallelism, memory latency Chapter 1 — Computer Abstractions and Technology — 39 Multiprocessors ! Multicore microprocessors ! More than one processor per chip ! Requires explicitly parallel programming ! Compare with instruction level parallelism ! Hardware executes multiple instructions at once ! Hidden from the programmer ! Hard to do ! Programming for performance ! Load balancing ! Optimizing communication and synchronization Chapter 1 — Computer Abstractions and Technology — 47 Amdahl’s Law ! Improving an aspect of a computer and expecting a proportional improvement in overall performance §1.8 Fallacies and Pitfalls unaffected affected improved T t factor improvemen T T + = Amdahl’s Law ! A program runs in 100 s on a computer, with multiply operations responsible for 80 s of this time. How much do I have to improve the speed of multiplication if I want my program to run five times faster? Chapter 1 — Computer Abstractions and Technology — 48 Amdahl’s Law ! A program runs in 100 s on a computer, with multiply operations responsible for 80 s of this time. How much do I have to improve the speed of multiplication if I want my program to run five times faster? Chapter 1 — Computer Abstractions and Technology — 49 20 80 20 + = n ! multiply accounts for 80s/100s ! How much improvement in multiply performance to get 5× overall? ! Corollary: make the common case fast ! Can’t be done! Chapter 1 — Computer Abstractions and Technology — 50 Fallacy: Low Power at Idle ! X4 power benchmark ! At 100% load: 295W ! At 50% load: 246W (83%) ! At 10% load: 180W (61%) ! Google data center ! Mostly operates at 10% – 50% load ! At 100% load less than 1% of the time ! Consider designing processors to make power proportional to load Chapter 1 — Computer Abstractions and Technology — 51 Pitfall: MIPS as a Performance Metric ! MIPS: Millions of Instructions Per Second ! Doesn’t account for ! Differences in ISAs between computers ! Differences in complexity between instructions 6 6 6 10 CPI rate Clock 10 rate Clock CPI n count Instructio n count Instructio time 10 Execution Instructio n count MIPS × = × × = × = ! CPI varies between programs on a given CPU Example ! Performance measurements for a program ! Which computer has the higher MIPS rating? ! Which computer is faster? Chapter 1 — Computer Abstractions and Technology — 52 Measurement Computer A Computer B Instruction count 10 billion 8 billion Clock rate 4 GHz 4 GHz CPI 1.0 1.1 Chapter 1 — Computer Abstractions and Technology — 53 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 §1.9 Concluding Remarks