1. CGS 3763 Operating Systems Concepts Spring 2013 Dan C. Marinescu Office: HEC 304 Office hours: M-Wd 11:30 - 12:30 AM

2. Last time:  Pthreads  Java threads  CPU scheduling Today:  Answers to student question from past week  CPU Scheduling Next time  Midterm Reading assignments  Chapter 5 of the textbook Lecture 20 – Friday, February 22, 2013 2Lecture 20

3. Student questions from week 6 lectures Feb 11 th Monday:  Which process is faster message passing or shared memory? What are the advantages and disadvantages between message passing and shared memory?  What is an example of a trusted intermediary?  Is a thin client like a workstation computer in an office with an active directory?  What is the relationship between the intermediary and the thin client?  What is an everyday example of the three-way handshake? Lecture 203

4. Student questions from week 6 lectures Feb 13 th Wednesday:  Little and Big endian are probably the most confusing concepts. Are there different situations that the little or big endian system would be used? What kind of system uses each?  What purpose does marshaling and unmarshaling serve?  In what situation would you use unstructured peer-to-peer networks? Give an example.  What are stubs? Where are they located and what do they do?  How do we manage so many threads with so little amount of memory? Lecture 204

5. Student questions from week 6 lectures Feb 15 th Friday:  Confusion on Physical, Data-link, network, transport, application layers.  Where does multithreading occur? Lecture 205

6. Metrics and objectives of scheduling policies Two types of systems  Batch processing of jobs  Interactive, e.g., transaction processing systems The metrics  Utilization  ratio of useful time versus total time  Throughput  number of transactions or jobs per unit of time  Turn around time  the time it takes to complete a job  Response time  the time to get a response at a request  Waiting time  the time a job or a transaction has to wait before being processed The objectives  Maximize: CPU utilization and Throughput  Minimize: Turn around time, Response time, Waiting time The objectives can be contradictory!! Lecture 206

7. Scheduling policies First-Come First-Serve (FCFS) Shortest Job First (SJF) Round Robin (RR) Priority scheduling Lecture 207

8. First-Come First-Served (FCFS) ThreadBurst Time P 1 24 P 2 3 P 3 3 Processes arrive in the order: P 1  P 2  P 3 Gantt Chart for the schedule: Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 Convoy effect  short process behind long process P1P1 P2P2 P3P3 2427300 8Lecture 20