1Reliability-Sept2001

Software Reliability Through

Hardware Reliability

Dolores R. WallaceSRS Information Services

Software Assurance Technology Centerhttp://satc.gsfc.nasa.gov/

NASA OSMA

SAS '01

2Reliability-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

3Reliability-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

4Reliability-Sept2001

The Mathematics

Typical Statistical Distributions•Exponential

•Gamma

•Weibull

•Binomial

•Poisson

•Normal

•Lognormal

•Bayes

•Markov

Sample software reliability models•Musa Basic:

m(t) = ß0(1-exp(-ß1t))

•Schneidewind:

D(T) = (α/β) [1-exp(-β ((T-s+1)))] + Xs-1

5Reliability-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

6Reliability-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

7Reliability-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

8Reliability-Sept2001

Example of SMERFS^3 Ouput

9Reliability-Sept2001

Loglet – same data

10Reliability-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

11Reliability-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