SE-3910 Real-time Systems

SE-3910Real-time Systems • Week 9, Classes 1 and 2 • Announcement* (regexp style) • Significance Testing • Failure statistics • Data flow diagrams SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling, Some from Dr. Hornick, etc.

3-Question Review Quiz • http://www.polleverywhere.com/survey/Beo1ZC-L5 • (See following slides for source material) SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

Normal Distribution SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

Normal Distribution (alternate) http://education-portal.com/academy/lesson/normal-distribution-of-data-examples-definition-characteristics.html#lesson SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

http://www.stat.ufl.edu/~athienit/Tables/Ztable.pdf SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

Determining if two processes are “significantly” different SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

In-Class Example • Suppose two processes have measured means of 12s and 13s runtime (measured from 8 samples each), and we know the standard deviation ACTUALLY IS 1 s. • Construct a statistic for determining if this is significant • Provide a confidence interval on said statistic with p=0.01 • What is the p-value for this result? SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

Zoomed-in t table SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

Modified process (1) • Suppose we don’t know what the standard deviation is… • What do we do? SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

Modified process (2) • What do we do? (if we don’t know the true std.) • Estimate it! • But there is a side-effect. Our estimate has some error as well. • This increases the size of our confidence interval. • By how much? Enter the t-test!

Modified process (3) • Use the same confidence approach as before, except: • Simply replace the “true” with the standard • And use one of the “df” (degrees of freedom) rows from the table • For difference of means, df = 2(N – 1) = 2N – 2 • Where N is the number of samples • Use exactly the same formula for z: • z = SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

Zoomed-in t table SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

In-Class Activity • Suppose two processes have a mean of 12.5 ms and 15ms runtime (with 5 samples for each), and we know the standard deviation ACTUALLY IS 2 ms Is this significant at the p = 0.01 level? SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

Example • An airplane software system has a failure probability of 10-6 failures per hour. What is the probability of reliable operation for a 10 hour flight? SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

Example • An airplane software system has a failure probability of 10-9per hour. Supposing this failure will cause the plane to crash. What is the chance that the plane will crash within the year if it flies 2400 hours a year? • Compute assuming both independent and dependent. SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling

Data flow diagrams SE3910 Real Time Systems • Entity • An entity is the source or destination of data. • The source in a DFD represents these entities that are outside the context of the system. • Entities either provide data to the system (referred to as a source) or receive data from it (referred to as a sink). • Process • The process is the manipulation or work that transforms data, performing computations, making decisions (logic flow), or directing data flows based on business rules. Data Store • Data Store • A data store is where a process stores data between processes for later usage by the same process or another process. • Data Flow • Data flow is the movement of data between the entity, the process, and the data store. Data flow portrays the interface between the components of the DFD.

Data Flow Diagram Symbols SE3910 Real Time Systems

Case study: Traffic Control SE3910 Real Time Systems • We are going to walk through the design of a Traffic Control System • Starting with the needs of the system. • We want to talk about how data flows through the system.

An Intersection System SE3910 Real Time Systems

Intersection Control Diagram SE3910 Real Time Systems

Dataflow diagram SE3910 Real Time Systems

Data Dictionary SE3910 Real Time Systems • An essential aspect of a structured design • Includes entries for data flows, control flows, data stores, buffers, etc.

Data Dictionary Example SE3910 Real Time Systems

Discussion: Which software failure we have talked about should have been caught using this approach? SE3910 Real Time Systems

DFD – Practical Example Launched Dec. 11, 1998, the Climate Orbiter plunged too steeply into the Martian atmosphere Sept. 23, 1999, and either burned up or crashed. In an initial failure report released Oct. 15, 2000 the review board blamed the navigation error on a communications foul-up between NASA's Jet Propulsion Laboratory and prime contractor Lockheed Martin.

SE-3910 Real-time Systems