CS 501 Software Engineering Fall 1999 Lecture 9

CS 501: Software Engineering Fall 1999 Lecture 9 (a) Software Engineering as Engineering? (b) Computer Systems

Administration Talk: "Changes in Computer Science: Real and Imagined" The reorganization of computer science at Cornell and its impact from the undergraduate point-of-view. Charles Van Loan, chair CS department Olin 155 Tuesday September 28, 1999

What is Engineering?

What is Engineering? The profession of: . . . creating cost-effective solutions. . . to practical problems. . . by applying scientific knowledge. . . and established practices. . . building things. . .

Crafts, Science, Engineering Science Production Commercial Craft From: Shaw and Garlan Professional Engineering

Crafts, Science, Engineering algorithms data structures Science Production Craft From: Shaw and Garlan software development methodologies Commercial compiler construction Professional Engineering

Software Engineering as Engineering? Part craft -- part engineering Embryonic scientific basis Evolving body of knowledge Too much flux for the apparatus of a profession (e. g. , accreditation) Example: Texas

Moore's Law Original version: The density of transistors in an integrated circuit will double every year. (Gordon Moore, Intel, 1965) Current version: Cost/performance of silicon chips doubles every 18 months. Planning assumptions: Every year: cost/performance of silicon chips improves 25% cost/performance of magnetic media improves 30%

Moore's Law and System Design system: Production use: Withdrawn from production: 1999 2002 2012 Processor speeds: Memory sizes: Disk capacity: 1 1. 9 2. 2 28 28 51 System cost: 1 0. 4 0. 01

Moore's Law and Parkinson's Law Original: Work expands to fill the time available. (C. Northcote Parkinson) Planning assumptions: (a) Demand will expand to use all the hardware available. (b) Low prices will create new demands. (c) Your software will be used on equipment that you have not envisioned.

False Assumptions Unix file system will never exceed 2 Gbytes (232 bytes). Apple. Talk networks will never have more than 256 hosts (28 bits). GPS software will not last 1024 weeks. Nobody at Dartmouth will ever earn more than $10, 000 per month. etc. , . . .

Moore's Law and the Long Term What level? 1965 1999? When?

Predicting System Performance Mathematical models Simulation Direct measurement All require detailed understanding of the interaction between software and systems.

Mathematical Models Queueing theory Good estimates of congestion can be made for single-server queues with: arrivals that are independent, random events (Poisson process) service times that follow certain families of distributions (e. g. , negative exponential, gamma) Many of the results can be extended to multi-server queues.

Behavior of Queues: Utilization mean delay 0 1 utilization

Utilization: Rule of Thumb mean service time utilization = mean inter-arrival time When the utilization of any system component exceeds 30%, be prepared for congestion.

Simulation Model the system as set of states and events advance simulated time determine which events occurred update state and event list repeat Discrete time simulation: Time is advanced in fixed steps (e. g. , 1 millisecond) Next event simulation: Time is advanced to next event Events can be simulated by random variables (e. g. , arrival of next customer, completion of disk latency)

Timescale Operations per second CPU instruction: 400, 000 Disk latency: read: 60 25, 000 bytes Network LAN: dial-up modem: 10, 000 bytes 6, 000 bytes

Measurements on Operational Systems Benchmarks: Run system on standard problem sets, sample inputs, or a simulated load on the system. Instrumentation: Clock specific events.

Serial and Parallel Processing Single thread v. multi-thread e. g. , Unix fork Granularity of locks on data e. g. , record locking Network congestion e. g. , back-off algorithms

Example: Performance of Disk Array Each transaction must: wait for specific disk platter wait for I/O channel signal to move heads on disk platter wait for I/O channel pause for disk rotation read data Close agreement between: results from queueing theory, simulation, and direct measurement (within 15%).

Bouncing Ball Diagrams Example: http: //www. cs. cornell. edu/ domain name TCP connection HTTP get Client Servers

Reading Before next class, read and be ready to discuss: Sommerville: Chapters 12 and 13 pages 209 to 224.