Course Theme 15 213 The Class That Gives CMU Its Zip Abstraction is good but don t forget reality Courses to date emphasize abstraction Introduction to Computer Systems Abstract data types Asymptotic analysis These abstractions have limits Especially in the presence of bugs Need to understand underlying implementations Randal E Bryant August 29 2000 Useful outcomes Become more effective programmers Able to find and eliminate bugs efficiently Able to tune program performance Prepare for later systems classes Compilers Operating Systems Networks Computer Architecture Topics Theme Five great realities of computer systems How this fits within CS curriculum class01a ppt CS 213 F 00 Great Reality 1 class01a ppt 2 CS 213 F 00 Computer Arithmetic Int s are not Integers Float s are not Reals Does not generate random values Arithmetic operations have important mathematical properties Examples Cannot assume usual properties Is x2 0 Float s Yes Int s 65535 65535 131071 On most machines 65535L 65535 4292836225 On Alpha Is x y z x y z Unsigned Signed Int s Yes Float s 1e10 1e10 3 14 3 14 1e10 1e10 3 14 0 0 class01a ppt 3 CS 213 F 00 Due to finiteness of representations Integer operations satisfy ring properties usually Commutativity associativity distributivity Floating point operations satisfy ordering properties Monotonicity values of signs Observation Need to understand which abstractions apply in which contexts Important issues for compiler writers and serious application programmers class01a ppt 4 CS 213 F 00 Great Reality 2 Great Reality 3 You ve got to know assembly Memory Matters Chances are you ll never write program in assembly Compilers are much better patient at this than you are It must be allocated and managed Many applications are memory dominated Understanding assembly key to machine level execution model Memory referencing bugs especially pernicious Behavior of programs in presence of bugs High level language model breaks down Tuning program performance Understanding sources of program inefficiency Implementing system software Compiler has machine code as target Operating systems must manage process state class01a ppt 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 CS 213 F 00 5 Memory is not unbounded Memory Referencing Bug Example class01a ppt 6 CS 213 F 00 Memory Referencing Errors C and C do not provide any memory protection 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 15g n d printf d 15g n d exit 0 exit 0 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 Alpha MIPS Sun g 5 30498947741318e 315 3 1399998664856 3 14 O 3 14 3 14 class01a ppt 3 14 7 CS 213 F 00 Program in Java Lisp or ML Understand what possible interactions may occur Use or develop tools to detect referencing errors E g Purify class01a ppt 8 CS 213 F 00 Memory Performance Example Matmult Performance Alpha 21164 Implementations of Matrix Multiplication Too big for L1 Cache Too big for L2 Cache Multiple ways to nest loops 160 140 120 ijk ikj jik jki kij kji 100 ijk ijk for for i 0 i 0 i n i n i i for j 0 j n for j 0 j n j j sum sum 0 0 0 0 for for k 0 k 0 k n k n k k sum a i k sum a i k b k j b k j c i j c i j sum sum class01a ppt jik jik for for j 0 j 0 j n j n j j for i 0 i n for i 0 i n i i sum sum 0 0 0 0 for for k 0 k 0 k n k n k k sum a i k sum a i k b k j b k j c i j c i j sum sum 9 CS 213 F 00 80 60 40 20 0 matrix size n class01a ppt 10 CS 213 F 00 Great Reality 4 Blocked matmult perf Alpha 21164 There s more to performance than asymptotic complexity 160 140 Constant factors matter too 120 100 bijk bikj ijk ikj 80 60 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 40 20 0 50 7 5 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500 matrix size n class01a ppt 11 CS 213 F 00 class01a ppt 12 CS 213 F 00 Great Reality 5 Role within Curriculum Computers do more than execute programs CS 441 Networks They need to get data in and out CS 412 Operating Systems CS 411 Compilers Processes Mem Mgmt Machine Code Optimization 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 Network Protocols CS 212 Execution Models CS 213 Systems Data Structures Applications Programming CS 211 Fundamental Structures class01a ppt 13 CS 213 F 00 Course Perspective Exec Model Memory System Transition from Abstract to Concrete From high level language model To underlying implementation 14 CS 213 F 00 Our Course is Programmer 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 ECE 349 Embedded Systems Course Perspective Cont Most Systems Courses are Builder Centric class01a ppt class01a ppt ECE 347 Architecture 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 efficent Incorporate features that require hooks into OS E g concurrency signal handlers Not just a course for dedicated hackers We bring out the hidden hacker in everyone Cover material in this course that you won t see elsewhere CS 213 F 00 class01a ppt 16 CS 213 F 00
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