1. Process Scheduling in Windows XP, Linux, and UNIX By Sarah G. Levinson CSC 4320

2. Common Measures of Operating System Performance CPU Utilization Throughput Turnaround Time Response Time Wait Time

3. Other Important Factors “User-Perceived Performance”: This measure is concerned with how efficient the system seems to a human user. This deals less with overall efficiency and more with the time it takes for the user to see observable results of a response. Fairness: Processes that are similar are given similar access to system resources. The property of fairness is exemplified by the Round Robin scheduling algorithm.

4. The process scheduling algorithms used by an operating system reflect its primary performance objectives. Traditional UNIX – Overall Efficiency, Fairness and User-Perceived Performance Multi-level feedback queue implementation is used, so processes with higher priority have first access to CPU time. Priority of a process is based on the type of process, where the swapper has highest priority and user processes have lowest priority.

5. The process scheduling algorithms used by an operating system reflect its primary performance objectives. Traditional UNIX – Overall Efficiency, Fairness and User-Perceived Performance Round Robin algorithm is implemented within individual priority queues, and a process’s priority is decreased as it uses more CPU time. User programs that are processor-bound are penalized more than user programs that are I/O- bound.

6. The process scheduling algorithms used by an operating system reflect its primary performance objectives. Modern UNIX – Real-Time Processing and Overall Efficiency Processes can be assigned one of three priority levels: Real Time, Kernel, or Time Shared. Real Time processes have the highest priority and may pre-empt processes at any other level, including kernel processes.

7. The process scheduling algorithms used by an operating system reflect its primary performance objectives. Modern UNIX – Real-Time Processing and Overall Efficiency If multiple real-time processes are waiting to run, the highest-priority process is selected to run until it has exhausted its time quantum. If a higher-level process has not completed but is not currently in a running status, a process is selected from the highest-priority ready queue that is not empty.

8. The process scheduling algorithms used by an operating system reflect its primary performance objectives. Modern UNIX – Real-Time Processing and Overall Efficiency The priorities of time shared processes are dynamically set by the OS. A process’s priority is lowered when it uses up a time quantum and raised it if blocks on an event or resource.

9. The process scheduling algorithms used by an operating system reflect its primary performance objectives. Linux—Real-Time Processing and Fairness Linux uses two process scheduling algorithms: a real-time algorithm for processes that must be allowed to execute in a timely fashion, and a timesharing algorithm for all other processes. The Round Robin and FIFO scheduling algorithms are used to switch between real-time processes.

10. The process scheduling algorithms used by an operating system reflect its primary performance objectives. Linux—Real-Time Processing and Fairness Timesharing algorithm is prioritized and “credit- based”. Credit-Based: When a new process must be selected, the process with the most credits is selected. A process that is running loses a credit every time the timer interrupt occurs. When a process has no more credits, it may not run.

11. The process scheduling algorithms used by an operating system reflect its primary performance objectives. Linux—Real-Time Processing and Fairness Timesharing algorithm is prioritized and “credit- based”. Prioritized: When no processes have any remaining credits, the system re-assigns credits. Higher priority processes receive more credits.

12. The process scheduling algorithms used by an operating system reflect its primary performance objectives. Linux—Real-Time Processing and Fairness Timesharing algorithm is prioritized and “credit- based”. This crediting system automatically prioritizes interactive and I/O-bound processes, which increases fairness and user-perceived performance.

13. The process scheduling algorithms used by an operating system reflect its primary performance objectives. Windows XP – Real Time Processing and User- Perceived Performance Real time processes have the highest priority and can pre-empt any process with lower priority. System processes may be starved by real time processes because a process’s priority is never decreased.

14. The process scheduling algorithms used by an operating system reflect its primary performance objectives. Windows XP – Real Time Processing and User- Perceived Performance All processes receive a priority boost after a wait event, but processes that have experienced a keyboard I/O wait get a larger boost than those that have experienced a disk I/O wait. “Foreground” processes given higher priority.

15. Questions?