Section: New Results

Data Races in the Parallel Language X10

Participants : Tomofumi Yuki [Colorado State University and Inria/IRISA] , Paul Feautrier, Sanjay Rajopadhye [Colorado State University] , Vijay Saraswat [IBM Research] .

Parallel programmers are now required to efficiently utilize the massive amount of parallelism provided by multi-core and many-core systems. Parallel programming is difficult, and the existing tools are mostly low-level extensions to sequential languages or libraries. As an effort to improve this situation, several groups have initiated the design of parallel programming languages, mostly based on the partitioned global address space (PGAS) paradigm. One of these languages is X10, which is developed at IBM Research by a team led by Vijay Saraswat.

While such languages hide the low-level details of parallel programming, they cannot guarantee that the object code will be correct by construction. Parallelism introduces two new types of bugs: non-determinism and deadlocks, and experience shows that it is possible to guarantee the absence of one type but not both. X10 programs are guaranteed deadlock-free but may have non-determinism. Non-determinism can be detected at runtime, but this approach cannot give absolute guarantees. However, it is possible, at least for a restricted class of X10 programs, to check for non-determinism at compile time.

The first step in this direction is to define the polyhedral fragment of X10, in which the only control constructs are for loops with affine bounds, and the only data structures are arrays with affine subscripts. X10 has many parallel constructs: as a first effort, we focused on async , which creates an activity (lightweight thread) and finish , which waits for termination of all impending activities. The execution order (or happens-before relation) of such a program is an incomplete lexicographic order, in which terms relating operations in different activities are removed. The dataflow analysis method of  [23] has to be adapted to a partial execution order, which may have many extrema instead of a unique maximum. Multiple extrema denote data races, thus non-determinism. A detector along these lines has been implemented and presented at PPoPP'13 (Symposium on Principles and Practice of Parallel Programming) [10] .

X10 other parallel programming primitive directives are clocks and atomic . The at construct allows downloading a computation to another place. Clocks are a dynamic version of barriers. Their analysis involves counting their instances. For polyhedral programs, this can be done using the Ehrhart and Barvinok theories; the results are polynomials. Checking whether clocks remove non-determinism involves finding integer roots and hence is undecidable. However, modern SMT solvers are able to solve most of these problems. The resulting paper [13] has been submitted to the ECOOP conference.