ECE 1747H : Parallel Programming

1. ECE 1747H : Parallel Programming Lecture 1: Overview

2. ECE 1747H • Meeting time: Mon 4-6 PM • Meeting place: BA 4164 • Instructor: Cristiana Amza, http://www.eecg.toronto.edu/~amza amza@eecg.toronto.edu, office Pratt 484E

3. Material • Course notes • Web material (e.g., published papers) • No required textbook, some recommended

4. Prerequisites • Programming in C or C++ • Data structures • Basics of machine architecture • Basics of network programming • Please send e-mail to eugenia@eecg to get an eecg account !! (name, stuid, class, instructor)

5. Other than that • No written homeworks, no exams • 10% for each small programming assignments (expect 1-2) • 10% class participation • Rest comes from major course project

6. Programming Project • Parallelizing a sequential program, or improving the performance or the functionality of a parallel program • Project proposal and final report • In-class project proposal and final report presentation • “Sample” project presentation posted

7. Parallelism (1 of 2) • Ability to execute different parts of a single program concurrently on different machines • Goal: shorter running time • Grain of parallelism: how big are the parts? • Can be instruction, statement, procedure, … • Will mainly focus on relative coarse grain

8. Parallelism (2 of 2) • Coarse-grain parallelism mainly applicable to long-running, scientific programs • Examples: weather prediction, prime number factorization, simulations, …

9. Lecture material (1 of 4) • Parallelism • What is parallelism? • What can be parallelized? • Inhibitors of parallelism: dependences

10. Lecture material (2 of 4) • Standard models of parallelism • shared memory (Pthreads) • message passing (MPI) • shared memory + data parallelism (OpenMP) • Classes of applications • scientific • servers

11. Lecture material (3 of 4) • Transaction processing • classic programming model for databases • now being proposed for scientific programs

12. Lecture material (4 of 4) • Perf. of parallel & distributed programs • architecture-independent optimization • architecture-dependent optimization

13. Course Organization • First month of semester: • lectures on parallelism, patterns, models • small programming assignments, done individually • Rest of the semester • major programming project, done individually or in small group • Research paper discussions

14. Parallel vs. Distributed Programming Parallel programming has matured: • Few standard programming models • Few common machine architectures • Portability between models and architectures

15. Bottom Line • Programmer can now focus on program and use suitable programming model • Reasonable hope of portability • Problem: much performance optimization is still platform-dependent • Performance portability is a problem

16. ECE 1747H: Parallel Programming Lecture 1-2: Parallelism, Dependences

17. Parallelism • Ability to execute different parts of a program concurrently on different machines • Goal: shorten execution time

18. Measures of Performance • To computer scientists: speedup, execution time. • To applications people: size of problem, accuracy of solution, etc.

19. Speedup of Algorithm • Speedup of algorithm= sequential execution time / execution time on p processors (with the same data set). speedup p

20. Speedup on Problem • Speedup on problem= sequential execution time of best known sequential algorithm / execution time on p processors. • A more honest measure of performance. • Avoids picking an easily parallelizable algorithm with poor sequential execution time.

21. What Speedups Can You Get? • Linear speedup • Confusing term: implicitly means a 1-to-1 speedup per processor. • (almost always) as good as you can do. • Sub-linear speedup: more normal due to overhead of startup, synchronization, communication, etc.

22. Speedup speedup linear actual p

23. Scalability • No really precise decision. • Roughly speaking, a program is said to scale to a certain number of processors p, if going from p-1 to p processors results in some acceptable improvement in speedup (for instance, an increase of 0.5).

24. Super-linear Speedup? • Due to cache/memory effects: • Subparts fit into cache/memory of each node. • Whole problem does not fit in cache/memory of a single node. • Nondeterminism in search problems. • One thread finds near-optimal solution very quickly => leads to drastic pruning of search space.

25. Cardinal Performance Rule • Don’t leave (too) much of your code sequential!

26. Amdahl’s Law • If 1/s of the program is sequential, then you can never get a speedup better than s. • (Normalized) sequential execution time = 1/s + (1- 1/s) = 1 • Best parallel execution time on p processors = 1/s + (1 - 1/s) /p • When p goes to infinity, parallel execution = 1/s • Speedup = s.

27. Why keep something sequential? • Some parts of the program are not parallelizable (because of dependences) • Some parts may be parallelizable, but the overhead dwarfs the increased speedup.

28. When can two statements execute in parallel? • On one processor: statement 1; statement 2; • On two processors: processor1: processor2: statement1; statement2;

29. Fundamental Assumption • Processors execute independently: no control over order of execution between processors

30. When can 2 statements execute in parallel? • Possibility 1 Processor1: Processor2: statement1; statement2; • Possibility 2 Processor1: Processor2: statement2: statement1;

31. When can 2 statements execute in parallel? • Their order of execution must not matter! • In other words, statement1; statement2; must be equivalent to statement2; statement1;

32. Example 1 a = 1; b = 2; • Statements can be executed in parallel.

33. Example 2 a = 1; b = a; • Statements cannot be executed in parallel • Program modifications may make it possible.

34. Example 3 a = f(x); b = a; • May not be wise to change the program (sequential execution would take longer).

35. Example 5 a = 1; a = 2; • Statements cannot be executed in parallel.

36. True dependence Statements S1, S2 S2 has a true dependence on S1 iff S2 reads a value written by S1

37. Anti-dependence Statements S1, S2. S2 has an anti-dependence on S1 iff S2 writes a value read by S1.

38. Output Dependence Statements S1, S2. S2 has an output dependence on S1 iff S2 writes a variable written by S1.

39. When can 2 statements execute in parallel? S1 and S2 can execute in parallel iff there are no dependences between S1 and S2 • true dependences • anti-dependences • output dependences Some dependences can be removed.

40. Example 6 • Most parallelism occurs in loops. for(i=0; i<100; i++) a[i] = i; • No dependences. • Iterations can be executed in parallel.

41. Example 7 for(i=0; i<100; i++) { a[i] = i; b[i] = 2*i; } Iterations and statements can be executed in parallel.

42. Example 8 for(i=0;i<100;i++) a[i] = i; for(i=0;i<100;i++) b[i] = 2*i; Iterations and loops can be executed in parallel.

43. Example 9 for(i=0; i<100; i++) a[i] = a[i] + 100; • There is a dependence … on itself! • Loop is still parallelizable.

44. Example 10 for( i=0; i<100; i++ ) a[i] = f(a[i-1]); • Dependence between a[i] and a[i-1]. • Loop iterations are not parallelizable.

45. Loop-carried dependence • A loop carried dependence is a dependence that is present only if the statements are part of the execution of a loop. • Otherwise, we call it a loop-independent dependence. • Loop-carried dependences prevent loop iteration parallelization.

46. Example 11 for(i=0; i<100; i++ ) for(j=0; j<100; j++ ) a[i][j] = f(a[i][j-1]); • Loop-independent dependence on i. • Loop-carried dependence on j. • Outer loop can be parallelized, inner loop cannot.

47. Example 12 for( j=0; j<100; j++ ) for( i=0; i<100; i++ ) a[i][j] = f(a[i][j-1]); • Inner loop can be parallelized, outer loop cannot. • Less desirable situation. • Loop interchange is sometimes possible.

48. Level of loop-carried dependence • Is the nesting depth of the loop that carries the dependence. • Indicates which loops can be parallelized.

49. Be careful … Example 13 printf(“a”); printf(“b”); Statements have a hidden output dependence due to the output stream.

50. Be careful … Example 14 a = f(x); b = g(x); Statements could have a hidden dependence if f and g update the same variable. Also depends on what f and g can do to x.