Lecture19.ppt
- 1. CS 501: Software Engineering Fall 2000 Lecture 19 Performance of Computer Systems
- 3. 3 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.
- 4. 4 Moore's Law and System Design Design system: 2000 Production use: 2003 Withdrawn from production: 2013 Processor speeds: 1 1.9 28 Memory sizes: 1 1.9 28 Disk capacity: 1 2.2 51 System cost: 1 0.4 0.01
- 5. 5 Moore's Law: Rules of Thumb Planning assumptions: Every year: cost/performance of silicon chips improves 25% cost/performance of magnetic media improves 30% 10 years = 100:1 20 years = 10,000:1
- 6. 6 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.
- 7. 7 False Assumptions Unix file system will never exceed 2 Gbytes (232 bytes). AppleTalk 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., etc., .....
- 8. 8 Moore's Law and the Long Term 1965 When? What level? 2000? Within your working life?
- 9. 9 Predicting System Performance • Mathematical models • Simulation • Direct measurement All require detailed understanding of the interaction between software and systems.
- 10. 10 Queues arrive wait in line service depart Single server queue
- 11. 11 Queues arrive wait in line service depart Multi-server queue
- 12. 12 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 families of distributions (e.g., negative exponential, gamma) Many of the results can be extended to multi-server queues.
- 13. 13 Utilization: Rule of Thumb utilization = mean service time mean inter-arrival time When the utilization of any system component exceeds 30%, be prepared for congestion.
- 14. 14 Behavior of Queues: Utilization mean delay utilization 1 0
- 15. 15 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)
- 16. 16 Timescale Operations per second CPU instruction: 400,000,000 Disk latency: 60 read: 25,000,000 bytes Network LAN: 10,000,000 bytes dial-up modem: 6,000 bytes
- 17. 17 Measurements on Operational Systems • Benchmarks: Run system on standard problem sets, sample inputs, or a simulated load on the system. • Instrumentation: Clock specific events.
- 18. 18 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
- 19. 19 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%).
- 20. 20 The Software Process Requirements Definition System and Software design Programming and Unit Testing Integration and System Testing Operation and Maintenance