1. 1 Reliability-Sept2001 Software Reliability Through Hardware Reliability Dolores R. Wallace SRS Information Services Software Assurance Technology Center http://satc.gsfc.nasa.gov/ http://satc.gsfc.nasa.gov/ NASA OSMA SAS '01

2. 2 Reliability-Sept2001 The Problem Critical NASA systems must execute successfully for a specified time under specified conditions -- Reliability Most systems rely on software Hence, a means to measure software reliability is essential to determining readiness for operation Software reliability modeling provides one data point for reliability measurement

3. 3 Reliability-Sept2001 The Issues Identify mathematics of hardware reliability not used in software Identify differences between hardware, software affecting reliability measurement Develop improvements to software reliability modeling – Dr. Norman Schneidewind, Naval Postgraduate School Develop and implement scenario for typical application on GSFC project data Identify how software reliability modeling can be used at GSFC

4. 4 Reliability-Sept2001 The Mathematics Typical Statistical Distributions Exponential Gamma Weibull Binomial Poisson Normal Lognormal Bayes Markov Sample software reliability models Musa Basic: m(t) = ß 0 (1-exp(-ß 1 t)) Schneidewind: D(T) = (α/β) [1-exp(-β ((T- s+1)))] + X s-1

5. 5 Reliability-Sept2001 Differences between Hardware and Software Hardware Deterioration over time Design faults removed before manufacture No faults entered during life Initial use, end of life failures No need to allow time to correct faults Models based on continuous use Software No deterioration over time Faults removed after build Faults possibly entered during fault correction Initial failures during test period, early use; then stable Models do not account for time to correct faults Software test may not be continuous time

6. 6 Reliability-Sept2001 Fault Correction Adjustments Reliability growth occurs from fault correction Failure correction proportional to rate of failure detection Adjusted model with delay dT (based on queuing service) but same general form as faults detected at time T Process: use Schneidewind model to get parameters; apply to revised model via spreadsheet Results –Show reliability growth due to fault correction –Predict stopping rules for testing Optimal Selection of Failure Data Next: GSFC data; Fault insertion

7. 7 Reliability-Sept2001 Applying Software Reliability Modeling The modeling process –Apply AIAA Recommended Practice –Learn about the system Data collection requirements –Dates of failure, fix –Activities/ phase when failures occur –Preparation of the data (interval; time between failure) Available software tools –Public domain SMERFS^3 –Loglet: commercial, but for our validation Interpretation of results

8. 8 Reliability-Sept2001 Example of SMERFS^3 Ouput

9. 9 Reliability-Sept2001 Loglet – same data

10. 10 Reliability-Sept2001 Options for SRM at NASA SATC Service –Projects submit failure data and project information –SATC executes models and prepares analysis Deployment to Project staff –SATC provides training and public domain tool –Project staff utilize the models Partnering –SATC provides tutorials and tools –Project staff may prepare the data and SATC the analysis –Project staff may exercise the models but discuss results with SATC

11. 11 Reliability-Sept2001 Proposed Next Steps Continue research on improvements to existing software reliability models Explore non-parametric methods Complete experiments with NASA data Provide technology transfer of knowledge