Task-Aware Optimization of Dynamic Fractional Permissions

Abstract

Boyland’s original work on fractional permissions introduced a mechanism to statically reason about the correct use of shared memory in concurrent programs. Permissions are linear capabilities that can be passed from one task to another. By splitting a permission into fractions, a task can grant multiple other tasks concurrent read access. Because writing data requires the full permission–and by definition at most one task can have the full permission–, fractional permissions prevent read/write conflicts.

This paper presents an optimizing compiler for a dynamic variant of fractional permissions where memory accesses are checked at runtime. In this system, every object is associated with a list of tasks that have read and/or write permission for the object. Tasks can grant read permission to subtasks by splitting their own permission into fractions and later collect those fractions back to re-gain the original permission.

To reduce the time and space overhead associated with permission checks, the compiler uses task-ordering information to minimize the places where permission checks must be inserted. For three of the five benchmarks we have investigated, the fully optimized version is within 10 % of the original version without fractional permissions. The average performance overhead over all benchmarks is 48 % which can be attributed to the poor performance of one particular benchmark. (For the other benchmarks, the average overhead is 15 %.)