15 213 Introduction to Computer Systems Frank Pfenning January 16 2007 Topics Theme and objectives Five lessons in computer systems How this fits within CS curriculum Course mechanics and overview 01 overview ppt 15 213 S 07 Course Theme Abstraction is good but programs run on real hardware Courses to date emphasize abstraction Abstract data types Asymptotic analysis These abstractions have limits Need to understand underlying implementations Performance time and space Useful outcomes Become more effective programmers Able to find and eliminate bugs efficiently Able to tune program performance Prepare for later systems classes in CS ECE Compilers Operating Systems Networks Computer Architecture Embedded Systems 2 15 213 S 07 Lesson 1 Int s are not Integers Float s are not Reals Examples Is x2 0 Float s Yes Int s 40000 40000 1600000000 50000 50000 Is x y z x y z Unsigned Signed Int s Yes Float s 1e20 1e20 3 14 3 14 1e20 1e20 3 14 3 15 213 S 07 Computer Arithmetic Does not generate random values Arithmetic operations have important mathematical properties Cannot assume usual properties Due to finiteness of representations Integer operations satisfy ring properties Commutativity associativity distributivity Floating point operations satisfy ordering properties Monotonicity values of signs Observation Need to understand which abstractions apply in which contexts 4 Important issues for compiler writers and serious application programmers 15 213 S 07 Lesson 2 You ve got to know assembly Chances are you ll never write a program in assembly Compilers are much better and more patient than you are Understanding assembly key to machine level execution model Behavior of programs in presence of bugs High level language model is inadequate Tuning program performance Understanding sources of program inefficiency Implementing system software Compiler has machine code as target Operating systems must manage process state 5 15 213 S 07 Lesson 3 Memory Matters Random Access Memory is an un physical abstraction Memory is not unbounded It must be allocated and managed Many applications are memory dominated Memory referencing bugs especially pernicious Effects are distant in both time and space Memory performance is not uniform Cache and virtual memory effects can greatly affect program performance Adapting program to characteristics of memory system can lead to major speed improvements 6 15 213 S 07 Memory Referencing Bug Example main main long long int int a 2 a 2 double double dd 3 14 3 14 a 2 a 2 1073741824 1073741824 Out Out of of bounds bounds reference reference printf d printf d 15g n 15g n d d exit 0 exit 0 Alpha MIPS Linux g 5 30498947741318e 315 3 1399998664856 3 14 O 3 14 3 14 3 14 Linux version gives correct result but implementing as separate function gives segmentation fault 7 15 213 S 07 Memory Referencing Errors C and C do not provide any memory protection Out of bounds array references Invalid pointer values Abuses of malloc free Can lead to nasty bugs Whether or not bug has any effect depends on system and compiler Action at a distance Corrupted object logically unrelated to one being accessed Effect of bug may be first observed long after it is generated How can I deal with this Program in Java Lisp ML or Cyclone Understand what possible interactions may occur 8 Use or develop tools to detect referencing errors 15 213 S 07 Memory System Performance Example void copyij int int int i j for i 0 i for j 0 dst i j src 2048 2048 dst 2048 2048 2048 i j 2048 j src i j 59 393 288 clock cycles void copyji int int int i j for j 0 j for i 0 dst i j src 2048 2048 dst 2048 2048 2048 j i 2048 i src i j 1 277 877 876 clock cycles 21 5 times slower Measured on 2GHz Intel Pentium 4 Hierarchical memory organization Performance depends on access patterns Including how step through multi dimensional array 9 15 213 S 07 The Memory Mountain Pentium III Xeon 550 MHz 16 KB on chip L1 d cache 16 KB on chip L1 i cache 512 KB off chip unified L2 cache 1200 Read throughput MB s copyij 1000 L1 800 copyji 600 400 xe L2 200 10 2k 8k 32k 128k 512k 2m 8m s15 s13 s9 s11 Stride words Mem s7 s5 s3 s1 0 Working set size bytes 15 213 S 07 Memory Performance Example Implementations of Matrix Multiplication Multiple ways to nest loops ijk ijk for for i 0 i 0 i n i n i i for for j 0 j 0 j n j n j j sum sum 0 0 0 0 for for k 0 k 0 k n k n k k sum sum a i k a i k b k j b k j c i j c i j sum sum 11 jik jik for for j 0 j 0 j n j n j j for for i 0 i 0 i n i n i i sum sum 0 0 0 0 for for k 0 k 0 k n k n k k sum sum a i k a i k b k j b k j c i j c i j sum sum 15 213 S 07 Lesson 4 There s more to performance than asymptotic complexity Constant factors matter too Easily see 10 1 performance range depending on how code written Must optimize at multiple levels algorithm data representations procedures and loops Must understand system to optimize performance How programs compiled and executed How to measure program performance and identify bottlenecks How to improve performance without destroying code modularity and generality 12 15 213 S 07 Lesson 5 Computers do more than execute programs They need to get data in and out I O system critical to program reliability and performance They communicate with each other over networks Many system level issues arise in presence of network Concurrent operations by autonomous processes Coping with unreliable media Cross platform compatibility Complex performance issues 13 15 213 S 07 Role within Curriculum CS 412 Operating Systems CS 441 Networks Network Protocols CS 212 Execution Models Processes Mem Mgmt CS 411 Compilers Machine Code Optimization Data Structures Applications Programming Transition from Abstract to Concrete Warning About to Change 14 ECE 349 Embedded Systems Exec Model Memory System CS 213 Systems CS 211 Fundamental Structures ECE 447 Architecture CS 113 C Programming From high level language model To underlying implementation 15 213 S 07 Course Perspective Most Systems Courses are Builder Centric Computer Architecture Design pipelined processor in Verilog Operating Systems Implement large portions of operating system Compilers Write compiler for simple language Networking Implement and simulate network protocols 15 15 213 S 07 Course Perspective Cont Our Course is Programmer Centric Purpose is to show how by knowing more about the underlying system one can be more effective as a programmer Enable you to Write programs that are more reliable and efficient Incorporate features
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