LeakChaser

Abstract

In large programs written in managed languages such as Java and C#, holding unnecessary references often results in memory leaks and bloat, degrading significantly their run-time performance and scalability. Despite the existence of many leak detectors for such languages, these detectors often target low-level objects; as a result, their reports contain many false warnings and lack sufficient semantic information to help diagnose problems. This paper introduces a specification-based technique called LeakChaser that can not only capture precisely the unnecessary references leading to leaks, but also explain, with high-level semantics, why these references become unnecessary. At the heart of LeakChaser is a three-tier approach that uses varying levels of abstraction to assist programmers with different skill levels and code familiarity to find leaks. At the highest tier of the approach, the programmer only needs to specify the boundaries of coarse-grained activities, referred to as transactions. The tool automatically infers liveness properties of these transactions, by monitoring the execution, in order to find unnecessary references. Diagnosis at this tier can be performed by any programmer after inspecting the APIs and basic modules of a program, without understanding of the detailed implementation of these APIs. At the middle tier, the programmer can introduce application-specific semantic information by specifying properties for the transactions. At the lowest tier of the approach is a liveness checker that does not rely on higher-level semantic information, but rather allows a programmer to assert lifetime relationships for pairs of objects. This task could only be performed by skillful programmers who have a clear understanding of data structures and algorithms in the program. We have implemented LeakChaser in Jikes RVM and used it to help us diagnose several real-world leaks. The implementation incurs a reasonable overhead for debugging and tuning. Our case studies indicate that the implementation is powerful in guiding programmers with varying code familiarity to find the root causes of several memory leaks---even someone who had not studied a leaking program can quickly find the cause after using LeakChaser's iterative process that infers and checks properties with different levels of semantic information.