Software Reliability Through Hardware Reliability by jhs20192

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									                          NASA OSMA SAS '01



                 Software Reliability Through
                     Hardware Reliability



                          Dolores R. Wallace
                       SRS Information Services
                 Software Assurance Technology Center
                        http://satc.gsfc.nasa.gov/
                                                        1
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


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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



                                                                3
Reliability-Sept2001
                       The Mathematics

Typical Statistical         Sample software
Distributions               reliability models
•Exponential
                            •Musa Basic:
•Gamma
•Weibull
•Binomial                    m(t) = ß0(1-exp(-ß1t))
•Poisson
•Normal                     •Schneidewind:
•Lognormal
•Bayes
•Markov                      D(T) = (α/β) [1-exp(-β ((T-
                            s+1)))] + Xs-1

                                                           4
Reliability-Sept2001
                       Differences between Hardware
                               and Software
Hardware                             Software
• Deterioration over time            • No deterioration over time
•Design faults removed before        •Faults removed after build
manufacture                          • Faults possibly entered
                                     during fault correction
•No faults entered during life
                                     •Initial failures during test
•Initial use, end of life failures   period, early use; then
•No need to allow time to            stable
correct faults                       •Models do not account for
• Models based on continuous         time to correct faults
use                                  •Software test may not be
                                     continuous time

                                                                     5
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
                                                                 6
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
                                                                7
Reliability-Sept2001
                   Example of SMERFS^3 Ouput




                                               8
Reliability-Sept2001
                       Loglet – same data




                                            9
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
                                                                 10
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



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Reliability-Sept2001

								
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