Bug Isolation via Remote Program Sampling Ben Liblit, Alex Aiken, Alice X.Zheng, Michael I.Jordan Presented by: Xia Cheng

Outline of My Talk Bug Isolation Using Predicate Elimination Statistical Debugging Related Work Privacy and Security Future Work Conclusions

Bug Isolation Using Predicate Elimination  Instrumentation strategy  Elimination strategies  Data collection and analysis  Refinement over time  Performance Impact

Bug Isolation Instrumentation strategy  Automatic isolation of deterministic bugs Release 1.2 of the ccrypt encryption tool  Randomly sampling function Return values may identify key operations that behave differently in successful versus crashed runs  Group return value into three classes Negative values, zero, and positive values

Bug Isolation Instrument ccrypt Syntactic call site Return scalar values Update one of three counters triple of counters Negative valuezeropositive value

Bug Isolation Elimination Strategies - Discard irrelevant predicates  Elimination by universal falsehood Disregard any counter that is zero on all runs Represent predicates that can never be true  Elimination by lack of failing coverage Disregard any triple of counters all three of which are zero on all failed runs Not even reached in failing executions

Bug Isolation Discard irrelevant predicates (cont…)  Elimination by lack of failing example Disregard any counter that is zero on all failed runs Not be true for a failure to occur  Elimination by successful counter example Disregard any counter that has a non-zero value on any successful run Can be true without a subsequent program failure

Bug Isolation universal falsehooddiscards 1569 counters- zero on all runs leaves 141 candidate predicates lack of failing coveragediscards 526 counter triples –all zero on all crashes leaves 132 candidate predicates lack of failing examplediscards 1665 counters- zero on all crashes Leaves 45 candidate predicates successful counterexample Discards 139 counters- non-zero on any successful run Leaves 1571 candidate predicates Data Collection and Analysis Successful counterexample distinct, the another three partially overlapped Falsehood and counterexample test disjoint properties, combined to good effect Falsehood and counterexample at successful runs, be analyzed together Failing example eliminates the most features, combine with counterexample Failing coverage is an inherently weaker strategy 2990 trial runs at sampling rate 1/1000; 88 of these end in a crash

Bug Isolation Refinement over time Elimination strategies benefit from increasing the number of runs On average, 1750 runs are enough to isolate twenty candidate features Greater diversity benefits the analysis

Bug Isolation Performance Impact  Sampling transformation a simpler but slower pattern of checking the next- sample countdown at each and every site  the performance impact minimal for sampled instrumentation  Overhead for 1/1000 sampling is less than 4%

Outline of My Talk Bug Isolation Using Predicate Elimination Statistical Debugging Related Work Privacy and Security Future Work Conclusions

Statistical Debugging Instrumentation strategy Crash prediction using logistic regression Data collection and analysis Performance Impact

Statistical Debugging Automatic isolation of non-deterministic bugs Instrumentation strategy  Instrument bc to guess and randomly check a large number of predicates  Goal: identify predicates capturing bad behavior- false on success, true on crashing Cast an extremely broad net An eye toward pointer and buffer error

Statistical Debugging Crash prediction using logistic regression  Goal: narrow down the set of features  Method: balance good classification performance with aggressive feature selection  Binary classifier Given by quantizing the logistic function output Takes feature values as input, and outputs a prediction of either 0 or 1  Feature selection Achieved by regularizing the function parameters to ignore most input feature, Forcing it to form a model that predicts success or failure using just a small selection of sampled features

Statistical Debugging to learn a good classifier, maximize the log likelihood of the training set distribution is modeled as logistic function penalized log likelihood function

Statistical Debugging Data collection and analysis  bc data set consists of 4390 runs with distinct random inputs and distinct randomized 1/1000 sampling

Statistical Debugging

Performance Impact

Outline of My Talk Bug Isolation Using Predicate Elimination Statistical Debugging Related Work Privacy and Security Future Work Conclusions

Related Work Performance profiling and optimization  Triggers - periodic hardware timers/interrupts, periodic software event counters or both [ M.Arnold, 2000]  Digital Continuous Profiling Infrastructure [Anderson 1997] choosing sampling intervals randomly Trace collection on program understanding  Difficulty Minimizing performance overhead and managing large quantities of captured data Directly adapt dynamic trace analysis techniques to domain

Related Work (cont…) Sharing the following techniques but with new approach  Daikon, fairly unstructured guesses and eliminate not holding ones [Ernst 2001] new approach: gathering data from production code  DIDUCE, identify bugs using analysis of executions [ Hangal 2002 ] new approach: more probabilistic, correlating predicate violations with increased likelihood of failure  Software tomography, through the GAMMA system, low- overhead distributed monitoring of deployed code [ Bowring 2002 ] new approach: bug isolation

Outline of My Talk Bug Isolation Using Predicate Elimination Statistical Debugging Related Work Privacy and Security Future Work Conclusions

Privacy and Security

Statistical model - an mechanism for protecting user anonymity Logistic regression - parameters updated with a new trace Statistical approach with noise - against malicious users Collaborative filtering system

Outline of My Talk Bug Isolation Using Predicate Elimination Statistical Debugging Related Work Privacy and Security Future Work Conclusions

Future Work Public Deployment of Cooperative Bug Isolation Scalable Statistical Bug Isolation Path Optimization in Programs and its Application to Debugging Statistical Debugging: Simultaneous Identification of Multiple Bugs The cooperative Bug Isolation Project, visit

Outline of My Talk Bug Isolation Using Predicate Elimination Statistical Debugging Related Work Privacy and Security Future Work Conclusions

sampling infrastructure - gathering information from the set of runs produced by the user community Bernoulli process to do the sampling several sample applications  Sharing the overhead of assertions  Predicating guessing and elimination to isolate a deterministic bug  Regularizing logistic regression to isolate a non-deterministic memory corruption error