LeakChaser: Helping Programmers Narrow Down Causes of Memory Leaks Guoqing Xu, Michael D. Bond, Feng Qin, Atanas Rountev Ohio State University

Java Memory Leaks Objects are reachable, but not used – Kept by unnecessary references Existing memory leak detection techniques – Unaware of program semantics: track arbitrary objects – No focus: profiling the whole execution--- real causes buried in a sea of likely problems Developer insight is necessary in leak detection—a three-tier approach to exploit such insight Tier L Tier M Tier H 2 Manual LeakChaser Manual LeakChaser Manual LeakChaser

Exploiting Developer Insight 3 Heap assertions – GC Assertions: A reachability-based assertion framework [Aftandilian-PLDI’09] – E.g., assertOwns(a, b) – Works only for objects that have low-level structural reachability relationships Lifetime invariants exist in large-scale apps – Lifetimes for certain objects are strongly correlated Screen s = new Screen(…); Configuration c = new Configuration(); /*s and c should always be created together and eventually die together */

Tier L: Low-Level Liveness Assertions An assertion framework to specify lifetime relationships – assertDiesBefore(c, s) // s: Screen, c: Configuration – assertDiesBeforeAlloc(s, s) Our framework vs GC Assertions – Can be used to assert objects that have high-level semantic relationships (instead of low-level structural relationships) 4

High-Level Events: Transactions Frequently-executed code regions – Likely to contain memory leaks – Inspired by EJB transactions Allow programmers to specify transactions 5 ResultSet runQuery(String query){ Connection c = getConnection(…); Statement s = c.createStmt(); ResultSet r = s.executeQuery(query); return r; } … … … Heap

Transaction Specification Transaction – A user-specified spatial boundary – A transaction identifier object that is correlated with the livenss of this region: temporal boundary 6 ResultSet runQuery(String query){ transaction { Connection c = getConnection(…); Statement s = c.createStmt(); ResultSet r = s.executeQuery(query); } return r; } (query)

Tier M: Checking Transaction Properties Semantics for each object o created in this transaction do assertDiesBefore (o, query) Share region for each object o created in this transaction if o is not created in share do assertDiesBefore (o, query) 7 ResultSet runQuery(String query){ transaction (query) { Connection c = getConnection(…); share{ Statement s = c.createStmt(); globalMap.cache(s); } ResultSet r = s.executeQuery(query); } return r; } Programmers do not need to understand implementation details to use low-level assertions

Tier H: Inferring Transaction Properties Minimum requirement for user involvement – Specify a transaction – Tell LeakChaser it runs in the inference mode Semantics for each o created in this transaction if assertDiesBefore (o, query) = false startTrackStaleness(o) if(o.staleness >= S) reportLeak(); 8 ResultSet runQuery(String query){ transaction (query ) { Connection c = getConnection(…); Statement s = c.createStmt(); globalMap.cache(s); ResultSet r = s.executeQuery(query); } return r; }, INFER Programmers let LeakChaser do most of the work

Three-Tier Approach Tier H, Tier M, and Tier L – Decreasing levels of abstraction – More knowledge required for diagnosis – Increased precision LeakChaser: an iterative diagnosis process – Start Tier H with little knowledge – Gradually explore leaky behaviors to locate the root cause 9

Case Studies Six case studies on real-world applications – Eclipse diff (bug #115789) – SPECJbb 2000: LeakChaser found a memory problem never reported before – Eclipse editor (bug #139465): quickly concluded that this was not a bug – Eclipse WTP (bug #155898): found the cause for this bug that was reported three years ago and is still open – MySQL leak – Mckoi leak: first time found that a leaking thread is the root cause The ability of diagnosing problems for a large system at its client 10

Implementation Jikes RVM – Works for both baseline and optimizing compilers – Works for all non-generational tracing GCs Overhead on FastAdaptiveImmix (average) – Infrastructure: 10% on time, less than 10% on space – Transactions: 2.3X slowdown for transactions 11

Conclusions A three-tier approach for semantics-aware leak detection An implementation in Jikes RVM The transaction constructs can be easily incorporated into the Java language to help memory problem diagnosis LeakChaser is available for download – 12

Thank you 13