1 Testing Effectiveness and Reliability Modeling for Diverse Software Systems CAI Xia Ph.D Term 4 April 28, 2005

2 Outline Introduction Background study Reliability modeling Testing effectiveness Future work Conclusion

3 Introduction Software reliability engineering techniques  Fault avoidance structure programming, software reuse, and formal methods  Fault removal testing, verification, and validation  Fault tolerance single-version technique multi-version technique (design diversity)  Fault prediction reliability modeling

4 Software Fault Tolerance Layers of Software fault tolerance

5 SFT techniques Single-version techniques  Checkpointing and recovery  Exception handling  Data diversity Multi-version techniques (Design diversity)  Recovery block  N-version programming  N self-checking programming

6 Design diversity To deploy multiple-version programs to tolerate software faults during operation Principle: redundancy Applications  Airplane control systems, e.g., Boeing 777 & AIRBUS A320/A330/A340  aerospace applications  nuclear reactors  telecommunications products

7 Design diversity (cont’) Controversial issues:  Failures of diverse versions may correlate with each other Reliability modeling on the basis of failure data collected in testing  Testing is a critical issue to ensure the reliability Testing completeness and effectiveness  Test case selection and evaluation  code coverage?  Real-world empirical data are needed to perform the above analysis

8 Research questions 1.How to predict the reliability of design diversity on the basis of the failure data of each individual version? 2.How to evaluate the effectiveness of a test set? Is code coverage a good indicator?

9 Experimental description Motivated by the lack of empirical data, we conducted the Redundant Strapped-Down Inertial Measurement Unit (RSDIMU) project It took more than 100 students 12 weeks to develop 34 program versions 1200 test cases were executed on these program versions 426 mutants were generated by injecting a single fault identified in the testing phase A number of analyses and evaluations were conducted in our previous work

10 Outline Introduction Background study Reliability modeling Testing effectiveness Future work Conclusion

11 Eckhardt and Lee (1985)  Variation of difficulty on demand space  Positive correlations between version failures Littlewood and Miller (1989)  Forced design diversity  Possibility of negative correlations Dugan and Lyu (1995)  Markov reward model Tomek and Trivedi (1995)  Stochastic reward net Popov, Strigini et al (2003)  Subdomains on demand space  Upper/lower bounds for failure probability Reliability models for design diversity Conceptual models Structural models In between

12 PS Model Alternative estimates for probability of failures on demand (pfd) of a 1-out-of-2 system

13 PS Model (cont’) Upper bound of system pfd “Likely” lower bound of system pfd - under the assumption of conditional independence

14 DL Model Example: Reliability model of DRB

15 DL Model (cont’) Fault tree models for 2-, 3-, and 4-version systems

16 Comparison of PS & DL Model PS Model DL Model Assumptions The whole demand space can be partitioned into disjoint subdomains; knowledge on subdomains should be given The faults among program versions can be classified into unrelated faults and related faults Prerequisite 1.Probability of subdomains 2.Failure probabilities of programs on subdomains 1.Number of faults unrelated and related among versions 2. Probability of hardware and decider failure Target systemSpecific 1-out-of-2 system configurations All multi-version system combinations Measurement objective Upper and lower bounds for failure probability Average failure probability Experimental results Give tighter bounds under most circumstances, yet whether tighter enough needs further investigation The prediction results agree well with observation, yet may have deviations to a specific system

17 Outline Introduction Background study Reliability modeling Testing effectiveness Future work Conclusion

18 Testing effectiveness The key issue in software testing is test case selection and evaluation What is a good test case?  testing effectiveness and completeness  fault coverage To allocate testing resources, how to predict the effectiveness of a given test case in advance?

19 Code coverage: an indicator of fault detection capability?  Positive evidence high code coverage brings high software reliability and low fault rate both code coverage and fault detected in programs grow over time, as testing progresses.  Negative evidence Can this be attributed to causal dependency between code coverage and defect coverage? Testing effectiveness

20 Is code coverage a good indicator for fault detection capability? ( That is, what is the effectiveness of code coverage in testing? ) 1.Does such effect vary under different testing profiles? 2.Do different code coverage metrics have various effects? Testing effectiveness (cont’)

21 Basic concepts: code coverage Code coverage - measured as the fraction of program codes that are executed at least once during the test. Block coverage - the portion of basic blocks executed. Decision coverage - the portion of decisions executed C-Use - computational uses of a variable. P-Use - predicate uses of a variable

22 Basic concepts: testing profiles Functional testing – based on specified functional requirements Random testing - the structure of input domain based on a predefined distribution function Normal operational testing – based on normal operational system status Exceptional testing - based on exceptional system status

23 Experimental requirement Complicated and real-world application Large population of program versions Controlled development process Bug history recorded Real faults studied Our RSDIMU project satisfies above requirements

24 Test cases description I II III IV V VI

25 The correlation between code coverage and fault detection Is code coverage a good indicator of fault detection capability? In different test case regions Functional testing vs. random testing Normal operational testing vs. exceptional testing In different combinations of coverage metrics

26 The correlation: various test regions Test case coverage contribution on block coverage Test case coverage contribution on mutant coverage

27 The correlation: various test regions Linear modeling fitness in test case regions Linear regression relationship between block coverage and defect coverage in whole test set

28 The correlation: various test regions Linear regression relationship between block coverage and defect coverage in region VI Linear regression relationship between block coverage and defect coverage in region IV

29 The correlation: various test regions Observations: Code coverage: a moderate indicator Reasons behind the big variance between region IV and VI Region IVRegion VI Design principleFunctional testingRandom testing Coverage range 32% ~ 50% 48% ~ 52% Number of exceptional test cases 277 (Total: 373) 0

30 The correlation: functional testing vs. random testing Code coverage: - a moderate indicator Random testing – a necessary complement to functional testing  Similar code coverage  High fault detection capability Testing profile (size)R-square Whole test set (1200)0.781 Functional test cases (800)0.837 Random test cases (400)0.558

31 The correlation: functional testing vs. random testing Failure details of mutants failed at less than 20 test cases: detected by 169 functional test cases (800 in total) & 94 random test cases (400 in total)

32 The correlation: functional testing vs. random testing Failure number of mutants that detected only by functional testing or random testing Test case typeMutants detected exclusively (total mutants killed) Average number of test cases that detect these mutants Std. deviation Functional testing 20 (382) Random testing 9 (371)

33 The correlation: normal operational testing vs. exceptional testing The definition of operational status and exceptional status  Defined by specification  application-dependent For RSDIMU application  Operational status: at most two sensors failed as the input and at most one more sensor failed during the test  Exceptional status: all other situations The 1200 test cases are classified to operational and exceptional test cases according to their inputs and outputs

34 The correlation: normal operational testing vs. exceptional testing Normal operational testing  very weak correlation Exceptional testing  strong correlation Testing profile (size)R-square Whole test case (1200)0.781 Normal testing (827)0.045 Exceptional testing (373)0.944

35 The correlation: normal operational testing vs. exceptional testing Normal testing: small coverage range (48%-52%) Exceptional testing: two main clusters

36 The correlation: normal operational testing vs. exceptional testing Failure number of mutants that detected only by normal operational testing or exceptional testing Test case type Mutants detected exclusively (total mutants detected) Average number of test cases that detect these mutants Std. deviation Normal testing 36/ Exceptional testing 20/

37 The difference between two pairs of testing profiles The whole testing demand space can be classified into seven subsets according to system status S i,j :  S 0,0 S 0,1 S 1,0 S 1,1 S 2,0 S 2,1 S others  i: number of sensors failed in the input  j: number of sensors failed during the test Functional testing vs. random testing  big overlap on seven system status Normal testing vs. exceptional testing  no overlap on seven system status This may explain the different performance of code coverage on testing effectiveness under two pairs of testing profiles

38 The correlation: under different combinations Combinations of testing profiles Observations:  Combinations containing exceptional testing indicate strong correlations  Combinations containing normal testing inherit weak correlations

39 The correlation: under different coverage metrics Similar patterns as block coverage Insignificant difference under normal testing  Decision/P-use: control flow change related  Larger variation in code coverage brings more faults detected

40 Discussions 1.Does the effect of code coverage on fault detection vary under different testing profiles?  A significant correlation exists in exceptional test cases, while no correlation in normal operational test cases.  Higher correlation is revealed in functional testing than in random testing, but the difference is insignificant. 2.Do different coverage metrics have various effects on such relationship?  Not obvious with our experimental data

41 Discussions (cont’) This is the first time that the effect of code coverage on fault detection are examined under different testing profiles Overall, code coverage is a moderate indicator for testing effectiveness The correlation in small code coverage range is insignificant Our findings of the positive correlation can give guidelines for the selection and evaluation of exceptional test cases

42 Future work Generate 1 million test cases and exercise them on current 34 versions to collect statistical failure data Conduct cross-comparison with previous project to investigate the “variant” and “invariant” features in design diversity Quantify the relationship between code coverage and testing effectiveness

43 Conclusion Survey on software fault tolerance evolution, techniques, applications and modeling Evaluate the performance of current reliability models on design diversity Investigate the effect of code coverage under different testing profiles and find it is a clear indicator for fault detection capability, especially for exceptional test cases

44 Q & A Thank you!