Smarter Scheduling (Priorities, Preemptive Priority Scheduling, Lottery and Stride Scheduling)
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The document outlines various scheduling strategies in Linux, highlighting first come, first served, round-robin, and preemptive priority scheduling. It discusses the distinctions between planned and on-demand scheduling, emphasizing their relevance in real-time systems with specific task deadlines. Additionally, it touches on priority inversion and lottery scheduling as methods to enhance resource allocation and process execution efficiency.
Smarter Scheduling (Priorities, Preemptive Priority Scheduling, Lottery and Stride Scheduling)
- 2. Plan for Today Recap: Scheduling First Come, First Served / Round-Robin Priorities Lottery and Stride Scheduling Scheduling in Linux 1
- 4. Recap Main Goals of Scheduling Maximize Resource Use Fairness Switching is Expensive Fundamental tradeoff between maximizing usage and fairness 3
- 5. Planned vs. On-Demand Planned: schedule in advance Supervisor/developer plans schedule given a priori knowledge about all tasks and deadlines On-Demand: schedule on-the-fly Supervisor runs periodically and makes decision based on current information Where is planned used? 4
- 6. Real-Time Systems Hard Real-Time Missing a deadline is a total system failure. Soft Real-Time Undesirable to miss too many deadlines too badly. 5
- 7. Hard Real-Time Missing a deadline is a total system failure. Soft Real-Time Undesirable to miss too many deadlines too badly. 6
- 8. What must programs be able to do to have guaranteed schedules? 7
- 9. Planned vs. On-Demand Planned: schedule in advance Necessary (at least in part) for hard real time On-Demand: schedule on-the-fly Most normal systems have unpredictable tasks with unknown deadlines 8
- 10. What should your courses be? 9
- 11. What should your courses be? Exam 2 will be scheduled later (not next week) Problem Set 3 is due 11:59pm Wednesday, March 5 Demos on Wednesday, Thursday, Friday No demos after Spring Break barring snow-out (firm real-time deadline) But…if you planned a schedule around the previously posted deadlines, you can stick to them (see me after class) 10
- 12. Scheduling Strategies First Come, First Served (FIFO) P1 P2 P3 (effectively: non-preemptive multi-processing) 11
- 13. First Come, First Served (FIFO) P2 P1 P3 (effectively: non-preemptive multi-processing) Round-Robin P1 P2 Time Slice P3 P1 P2 P3 P1 P3 P1 P3 Blocked Each process gets to run for a set time slice, or until it finished or gets blocked. 12
- 14. Comparison First Come, First Served Round Robin 13
- 15. Which one is my laptop using? 14
- 16. Priorities More important processes: “higher priority” (except Linux inverts priority values!) Highest priority = 0 gets most preferential treatment Lowest priority = 99 highest number varies by Linux flavor 15
- 17. High Priority Processes ps -w -e -o pri,pid,pcpu,vsz,cputime,command | sort -n -r --key=5 | sort --stable -n --key=1 16
- 18. Pre-emptive Priority Scheduling Always run the highest priority process that is ready to run Round-robin schedule among equally high, ready to run, highest-priority processes Priority 0: P 629 Priority 1: P 44 Memory Read P 815 P 516 P 131 Network Data P 528 Priority 2: Waiting: P 124 P 221 Shared Bus P 1209 17
- 21. Pathfinder OS: Pre-emptive Priority Always run the highest priority process that is ready to run Round-robin schedule among equally high, ready to run, highest-priority processes Actuators Shared Bus CPU Radio Camera Flash Memory 20
- 22. Priority Inversion Task 1 (scheduler) – highest priority (Priority = 1) Task 2 (send data) – (Priority = 4) Task 3 (science analysis) – lowest priority (Priority = 97) Actuators Shared Bus CPU Radio Camera Flash Memory 21
- 23. How should we solve priority inversion? 22
- 24. Waiting: Priority 0: Priority 1: P 44 Holds Bus Lock P 815 P 516 P 131 Network Data P 528 Priority 2: Memory Read P 221 Shared Bus P 1209 PRI: 0 23
- 25. Should my MacBook use a priority pre-emptive scheduler with priority inheritance? 24
- 26. Should my MacBook use a priority pre-emptive scheduler with priority inheritance? 25
- 27. Kinds of Processes “Compute-Bound” P1 “I/O-Bound” P2 wait for disk… P2 wait for network… P2 wait for user… “Real Time” P3 need frame ^ P3 need frame ^ P3 need frame ^ P3 need frame ^ 26
- 30. Lottery Scheduling • Each user (process) gets a share of the “tickets” – e.g., 1000 total tickets, 20 processes each get 50 tickets (or more/less weighted by priority) • User/process can distribute tickets however it wants – Among its own threads, can “loan” to other processes’ threads • Scheduler: randomly picks a ticket – Associated thread gets to run for that time slice 29
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- 34. What is the running time? 33
- 35. > uname -a Linux power2 3.2.0-49-generic #75-Ubuntu SMP Tue Jun 18 17:39:32 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux > sysctl kernel.pid_max kernel.pid_max = 32768 What is the running time? 34
- 36. Charge Stride scheduling works (in real life also)! Much smarter than priority pre-emptive (never finish anything) or first-come first-served or earliest-deadlinefirst. (Unless you like to live serendipitously: then you should use lottery scheduling) Problem Set 3 should have high (but not quite hard real-time) priority! (and try to turn off interrupts when you work on it!) 35