Memory models
Sarita V. Adve · 2009
The era of parallel computing for the masses is here, but writing correct parallel programs remains a challenge--popular parallel environments offer no analogs for the concepts of structured and safe sequential programming. The memory model forms the heart of the concurrency semantics of any (shared-memory) parallel language or hardware. Unfortunately, it has involved a tradeoff between programmability and performance, and has arguably been one of the most challenging and contentious areas in shared-memory specification. Recent broad community-scale efforts have finally led to a convergence in this debate, with popular languages such as Java and C++ and most hardware vendors publishing compatible memory model specifications. Although this convergence is a dramatic improvement, it has exposed fundamental shortcomings in current popular languages and systems that prevent achieving the vision of structured and safe parallel programming.I will discuss the path to the above convergence, the hard lessons learned, and their implications. A cornerstone of this convergence has been the view that the memory model should be a contract between the programmer and the system--if the programmer writes well-structured (data-race-free) programs, the system will provide high programmability (sequential consistency) and performance. I will discuss why this view is the best we can do with current popular languages, and why it should be unacceptable moving forward. I will then discuss research directions that eliminate worrying about the memory model, but require rethinking popular parallel languages and hardware. In particular, I will argue for languages that eliminate data races by design and provide determinism by default, while retaining the advantages of modern object-oriented programming. I will also argue for hardware that takes advantage of such disciplined programming models to enable energy-efficient performance scalability. I will use the Deterministic Parallel Java language and DeNovo hardware projects at Illinois as examples of such directions.