CS 213 Introduction to Computer Systems Randal E Bryant August 25 1998 Topics Theme Five great realities of computer systems How this fits within CS curriculum class01a ppt CS 213 F 98 Course Theme Abstraction is good but don t forget reality Courses to date emphasize abstraction Abstract data types Asymptotic analysis These abstractions have limits Especially in the presence of bugs Need to understand underlying implementations 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 class01a ppt 2 CS 213 F 98 Great Reality 1 Int s are not Integers Float s are not Reals Examples Is x2 0 Float s Yes Int s 65535 65535 131071 65535L 65535 4292836225 On Alpha Is x y z x y z Yes Int s Float s 1e10 1e10 3 14 3 14 1e10 1e10 3 14 0 0 class01a ppt 3 CS 213 F 98 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 Important issues for compiler writers and serious application programmers class01a ppt 4 CS 213 F 98 Great Reality 2 You ve got to know assembly Chances are you ll never write program in assembly Compilers are much better at this than you are Understanding assembly key to machine level execution model 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 5 CS 213 F 98 Great Reality 3 Memory Matters Memory is not unbounded It must be allocated and managed Many applications are memory dominated Especially those based on complex graph algorithms 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 class01a ppt 6 CS 213 F 98 Memory Referencing Bug Example main main long long int int a 2 a 2 double double dd 3 14 3 14 a 2 1073741824 a 2 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 Sun g 5 30498947741318e 315 3 1399998664856 3 14 O 3 14 3 14 class01a ppt 3 14 7 CS 213 F 98 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 system and compiler dependent Action at a distance Corrupted object logically unrelated to one being accessed Effect of bug may occur long after it occurs How can I with this 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 98 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 class01a ppt 9 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 CS 213 F 98 Matmult Performance Alpha 21164 Too big for L1 Cache Too big for L2 Cache 160 140 120 ijk 100 ikj jik 80 jki kij 60 kji 40 20 0 matrix size n class01a ppt 10 CS 213 F 98 Blocked matmult perf Alpha 21164 160 140 120 100 bijk bikj 80 ijk ikj 60 40 20 0 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500 matrix size n class01a ppt 11 CS 213 F 98 Great Reality 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 class01a ppt 12 CS 213 F 98 Tuning of BDD Packages Bwolen Yang in cooperation with researchers from Colorado Synopsys CMU and T U Eindhoven Application Symbolic model checking Analyze systems consisting of interacting state machines Shared memory parallel computer systems Air traffic collision avoidance systems Regularly deal with systems having 10 20 states or more Cannot possibly represent state space as explicit state graph Instead represent symbolically with Binary Decision Diagrams Procedure Generated set of benchmark traces Operation sequences that could be run using different packages and under varying conditions Identify strengths and weaknesses of 6 different packages and tune accordingly class01a ppt 13 CS 213 F 98 Effect of Optimization Compare pre vs post optimized results for 96 runs 6 different BDD packages 16 benchmark traces each Limit each run to maximum of 8 CPU hours and 900 MB Measure speedup Told Tnew or New Fail Bad Failed before but now succeeds Fail both times Succeeded before but now fails Results Overall speedup 4 3 Total time 6 4 days 1 5 days 6 cases achieve 100X speedup 13 New 6 Fail 1 Bad class01a ppt 14 CS 213 F 98 Optimization Results Summary Cumulative Speedup Histogram 80 75 70 76 76 speedup Told Tnew New Failed before but now succeeds Fail Fail both times Bad Succeeded before but now fails 61 50 40 33 30 22 20 6 class01a ppt bad new 0 failed speedups 0 95 1 2 1 5 0 6 10 10 13 100 of cases 60 15 CS 213 F 98 Optimization Results Summary 2 Time Comparison 1000000 n a 100x 10x speedup Told Tnew New Failed before but now succeeds Fail Fail both times Bad Succeeded before but now fails 1x initial results sec 100000 10000 new failed bad rest 1000 100 10 10 100 1000 n a 10000 100000 current results sec class01a ppt 16 CS 213 F 98 Great Reality 5 Computers do more than execute programs They need to get data in and out I O system critical to program reliability and
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