U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science The Implementation of the Cilk-5 Multithreaded Language (Frigo, Leiserson, and Randall) Alistair Dundas Department of Computer Science University of Massachusetts

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 2 Outline What is Cilk? Cilk example: the Fibonacci algorithm. The work-first principle. Work Stealing. The T.H.E. Protocol. Empirical results. Summary and questions.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 3 What Is Cilk? Extension of C for parallel programming. Designed for SMP machines with support for shared memory. Benefits: Provably efficient work stealing scheduler. Clean programming model. Benefits over normal thread programming: discussion topic! Specifically: Source to source compiler generating C.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 4 Example: Fibonacci Algorithm int main (int argc, char *argv[]) { int n, result; n = atoi(argv[1]); result = fib(n); printf(“Result:%d\n”, result); return 0; } int fib (int n) { if (n<2) return n; else { int x, y; x = fib (n-1); y = fib (n-2); return (x+y); } }

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 5 Example: Fibonacci In Parallel cilk int main (int argc, char *argv[]) { int n, result; n = atoi(argv[1]); result = spawn fib(n); sync; printf(“Result:%d\n”, result); return 0; } cilk int fib (int n) { if (n<2) return n; else { int x, y; x = spawn fib (n-1); y = spawn fib (n-2); sync; return (x+y); } }

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 6 Source to Source Compiler

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 7 The Work First Principle Work is the amount of time needed to execute the computation serially. Critical path length is the execution time on an infinite number of processors. The Work-First Principle: Minimize scheduling overhead borne by work at the expense of increasing the critical path.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 8 Theory: The Work First Principle Where T P is the time on P processors: T P = T 1 /P + O(T  ) (1) Making critical path overhead explicit: T P <= T 1 /P + c  T  (2) Define average parallelism (max speedup): P AVERAGE = T 1 /T  Define parallel slackness: P AVERAGE /P

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 9 The Work First Principle (cont) Assumption of parallel slackness: P AVERAGE /P ≫ c  Combining these with the inequality, we get: T P ≈ T 1 /P Define work overhead: c 1 = T 1 /T S T P ≈ c 1 T S /P Conclusion: Minimize work overhead.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 10 Work Stealing Algorithm Each worker keeps a ready deque (double ended queue) of procedure instances waiting to run. Workers treat the deque as a stack, pushing and popping procedure calls on to the end. When workers have no more work, they steal from the front of another workers’ deque. Parents are stolen before children. This is implemented using two versions of each procedure: a fast clone, and a slow clone.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 11 Fast Clone Run fast clone when a procedure is spawned. Little support for parallelism. Whenever a call is made, save complete state, and push on to end of deque. When call returns, check to see if procedure was stolen. If stolen, return immediately. If not stolen, carry on execution. Since children are never stolen, sync is a no op.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 12 Fast Clone Example cilk int fib (int n) { if (n<2) return n; else { int x, y; x = spawn fib (n-1); y = spawn fib (n-2); sync; return (x+y); } }

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 13 1 int fib (int n) 2 { 3 fib.frame *f; frame pointer 4 f = alloc(sizeof(*f)); allocate frame 5 f->sig = fib.sig; initialize frame 6 if (n!2) { 7 free(f, sizeof(*f)); free frame 8 return n; 9 } 10 else { … } Fast Clone Example

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 14 Fast Clone Example 11 int x, y; 12 f->entry = 1; save PC 13 f->n = n; save live vars 14 *T = f; store frame pointer 15 push(); push frame 16 x = fib (n-1); do C call 17 if (pop(x) == FAILURE) pop frame 18 return 0; procedure stolen 19 second spawn 20 ; sync is free! 21 free(f, sizeof(*f)); free frame 22 return (x+y); 23 } }

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 15 Slow Clone Slow clone used when a procedure is stolen. Similar to fast clone, but also supports concurrent execution. It restores program counter and procedure state using copy stored on deque. Calling sync makes call to runtime system for check on children’s status.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 16 The T.H.E. Protocol Deques held in shared memory. Workers operate at the end, thiefs at the front. We must prevent race conditions where a thief and victim try to access the same procedure frame. Locking deques would be expensive for workers. The T.H.E Protocol removes overhead of the common case, where there is no conflict.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 17 The T.H.E. Protocol Assumes only reads and writes are atomic. Head of the deque is H, tail is T, and (T ≥ H) Only thief can change H. Only worker can change T. To steal thiefs must get the lock L. At most two processors operating on deque. Three cases of interaction: Two or more items on deque – each gets one. One item on deque – either worker or thief gets frame, but not both. No items on deque – both worker and thief fail.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 18 One item on deque case Both thief and worker assume they can get a procedure frame and change H or T. If both thief and worker try to steal frame, one or both of them will discover (H > T), depending on instruction order. If thief discovers (H > T) it backs off and restores H. If worker discovers (H > T) it restores T, and then tries for the lock. Inside lock, procedure can be safely popped if still there.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 19 Empirical Results On an eight processor Sun SMP, achieved average speed up of 6.2 from elison (serial C non-threaded versions). Assumptions of work-first seem sound: Applications tested all showed high amounts of “average parallelism”. Work overhead small for most programs. Least speed up is where overhead is greatest.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 20 Summary Extension of C for parallel programming. Aims to simplify parallelization. Main idea is to prevent overhead for workers rather than focus on critical path. Empirical results show speed up average of 6.2 on an 8 processor machine.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 21 My Questions A cilk spawn is always just a C call. Who starts the threads, and how many are there? Why use Cilk rather than use threads directly? What about using Cilk on a bewoulf cluster? Are their test programs representative of SMP applications?

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 22 Other Extentions Inlets – a wrapper around spawned procedure returns. Abort – terminates work no longer needed (e.g. in parallel search). Locking facilities for access to shared data.

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 23 T.H.E. Protocol: The Worker/Victim push() { T++; } pop() { T--; if (H > T) { T++; lock(L); T--; if (H > T) { T++; unlock(L); return FAILURE; } unlock(L); } return SUCCESS; } steal() { lock(L); H++; if (H > T) { H--; unlock(L); return FAILURE; } unlock(L); return SUCCESS; }

U NIVERSITY OF M ASSACHUSETTS, A MHERST – Department of Computer Science 24 Fibonacci Illustration