The Cilk system for parallel multithreaded computing

Although cost-effective parallel machines are now commercially available, the widespread use of parallel processing is still being held back, due mainly to the difficulty of parallel programming. In particular, it is still difficult to build efficient implementations of parallel applications whose communication patterns are either highly irregular or dependent upon dynamic information. Multithreading has become an increasingly popular way to implement these dynamic, asynchronous, concurrent programs. Cilk (pronounced "silk") is our C-based multithreaded computing system that provides provably good performance guarantees. This thesis describes the evolution of the Cilk language and runtime system, as well as applications which affected the evolution of the system. Using Cilk, programmers are able to express their applications either by writing multithreaded code written in a continuation-passing style, or by writing code using normal call/return semantics and specifying which calls can be performed in parallel. The Cilk runtime system takes complete control of the scheduling, load-balancing, and communication needed to execute the program, thereby insulating the programmer from these details. The programmer can rest assured that his program will be executed efficiently since the Cilk scheduler provably achieves time, space, and communication bounds all within a constant factor of optimal. For distributed memory environments, we have implemented a software shared memory system for Cilk. We have defined a "dag-consistent" memory model which is a lock-free consistency model well suited to the needs of a multithreaded program. Because dag consistency is a weak consistency model, we have been able to implement coherence efficiently in software. The $\star$Socrates computer chess program is the most complex application written in Cilk. $\star$Socrates is a large, nondeterministic, challenging application whose complex control dependencies make it inexpressible in many other parallel programming systems. Running on an 1824-node Paragon, $\star$Socrates finished second in the 1995 World Computer Chess Championship. Currently, versions of Cilk run on the Thinking Machines CM-5, the Intel Paragon, various SMPs, and on networks of workstations. The same Cilk program will run on all of these platforms with a little, if any, modification. Applications written in Cilk include protein folding, graphic rendering, backtrack search, and computer chess. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)