James successfully defended his dissertation today. Below is a taste. Congrats, James!
Robby Many modern high-level or scripting languages are implemented around an interpretive run-time system, often with a JIT compiler. Examples include the Racket runtime system, the Parrot virtual machine, and the virtual machines underlying Perl, Python, Ruby, and other productivity-oriented languages. These runtime systems are often the result of many man-years of effort, and they have been carefully tuned for capability, functionality, correctness, and performance. For the most part, such runtime systems have not been designed to support parallelism on multiple processors. Even when a language supports constructs for concurrency, they are typically implemented through co-routines or OS-level threads that are constrained to execute one at a time. This limitation has become a serious issue, as it is clear that exploiting parallelism is essential to harnessing performance in future processor generations. Whether computer architects envision the future as involving homogeneous or heterogeneous multicores, and with whatever form of memory coherence or consistency model, the common theme is that the future is parallel and that language implementations must adapt. The essential problem is making the language implementation safe for low-level parallelism, i.e., ensuring that even when two threads are modifying internal data structures at the same time, the runtime system behaves correctly. One approach to enabling parallelism would be to allow existing concurrency constructs to run in parallel, and to rewrite or revise the runtime system to carefully employ locking or explicit communication. Experience with that approach, as well as the persistence of the global interpreter lock in implementations for Python and Ruby, suggests that such a conversion is extremely difficult to perform correctly. Based on the even longer history of experience in parallel systems, one would also expect the result to scale poorly as more and more processors become available. The alternative of simply throwing out the current runtime and re-designing and implementing it around a carefully designed concurrency model is no better, as it would require discarding years or decades of effort in building an effective system, and this approach also risks losing much of the language’s momentum as the developers are engaged in tasks with little visible improvement for a long period. This dissertation investigates a new technique for parallelizing runtime systems, called slow-path barricading. The technique is based on the observation that the core of many programs – and particularly the part that runs fast sequentially and could benefit most from parallelism – involves relatively few side effects with respect to the language implementation’s internal state. Thus, instead of wholesale conversion of the runtime system to support arbitrary concurrency, we add language constructs that focus and restrict concurrency where the implementation can easily support it. Specifically, the set of primitives in a language implementation is partitioned into safe (for parallelism) and unsafe categories. The programmer is then given a mechanism to start a parallel task; as long as the task sticks to safe operations, it stays in the so-called fast path of the implementation and thus is safe for parallelism. As soon as the computation hits a barricade, the runtime system suspends the computation until the operation can be handled in the more general, purely sequential part of the runtime system. Although the programming model allows only a subset of language operations to be executed in parallel, this subset roughly corresponds to the set of operations that the programmer already knows (or should know) to be fast in sequential code. Thus, a programmer who is reasonably capable of writing fast programs in the language already possesses the knowledge to write a program that avoids unsafe operations—and one that therefore exhibits good scaling for parallelism. Furthermore, this approach enables clear feedback to the programmer about when and how a program uses unsafe operations. Thesis: We can incrementally add effective parallel programming primitives and tool support to legacy sequential runtime systems with a modest investment of effort. ____________________ Racket Users list: http://lists.racket-lang.org/users
