TCSS 422 A – Winter 2019 1/22/2019 School of Engineering and Technology TCSS 422: OPERATING SYSTEMS Introduction to CPU Scheduling, Multi-Level Feedback Queue, Proportional Share Scheduling Wes J. Lloyd School of Engineering and Technology University of Washington - Tacoma TCSS422: Operating Systems [Winter 2019] January 23, 2019 School of Engineering and Technology, University of Washington - Tacoma FEEDBACK FROM 1/16 I am unclear about the Ch. 6 material What is limited direct execution? CPU modes: what is user mode? CPU modes: what is kernel mode? Three kinds of traps “switch” processing to kernel mode . System call (calls for I/O, kernel API calls, context switch) . Exception (error in code) . Interrupt: Maskable vs. Unmaskable What is Cooperative multi-tasking? What is Preemptive multi-tasking? What is a context switch? CPU scheduling: what is a time quantum (slice)? TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.2 School of Engineering and Technology, University of Washington - Tacoma Slides by Wes J. Lloyd L5.1 TCSS 422 A – Winter 2019 1/22/2019 School of Engineering and Technology FEEDBACK - 2 What criteria is used to decide whether a context switch is appropriate following the end of a job’s time slice? . After time quantum timer interrupt always fires, OS regains control (context switch) to evaluate next scheduling task Causes of context switches: . System call – process in user mode context switches to kernel mode to perform a kernel (API) call. Example: I/O . Multi-tasking – process uses full time quantum (10 ms default), timer interrupt fires, OS scheduler regains control of CPU to evaluate next task . Exception – CPU exception running current task (e.g. memory access fault) TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.3 School of Engineering and Technology, University of Washington - Tacoma FEEDBACK - 3 Explain the difference between unmaskable (interrupt) and maskable (interrupt): Define unmaskable Define maskable What happens if a non-maskable interrupt lock in interrupted by a higher priority (interrupt) ? . Only maskable interrupts can be interrupted . Non maskable interrupt handles are not pre-emptible TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.4 School of Engineering and Technology, University of Washington - Tacoma Slides by Wes J. Lloyd L5.2 TCSS 422 A – Winter 2019 1/22/2019 School of Engineering and Technology FEEDBACK – 4 What is “preemptibility” again? . OS: ability to be interrupted and replaced by a higher priority task What does “no preemption” mean for the Shortest Job First (SJF) CPU scheduler? Why is there no overhead / context switching for the FIFO scheduler? . Does the FIFO scheduler context switch? . Revisit example TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.5 School of Engineering and Technology, University of Washington - Tacoma FEEDBACK - 5 Where is the C tutorial? Link: http://faculty.washington.edu/wlloyd/courses/tcss422/a ssignments/TCSS422_w2019_C_Tutorial.pdf TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.6 School of Engineering and Technology, University of Washington - Tacoma Slides by Wes J. Lloyd L5.3 TCSS 422 A – Winter 2019 1/22/2019 School of Engineering and Technology OBJECTIVES Active Reading Quiz 1 (Tuesday) Assignment 0 / Linux Tutorial C Tutorial Assignment 1 – Soon… CPU Scheduling: Chapter 7 – Scheduling Introduction Chapter 8 – Multi-level Feedback Queue (MLFQ) Chapter 9 – Proportional Share Scheduler Linux Completely Fair Scheduler (CFS) TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.7 School of Engineering and Technology, University of Washington - Tacoma CHAPTER 7- SCHEDULING: INTRODUCTION TCSS422: Operating Systems [Winter 2019] January 23, 2019 School of Engineering and Technology, University of Washington - Tacoma L5.8 Slides by Wes J. Lloyd L5.4 TCSS 422 A – Winter 2019 1/22/2019 School of Engineering and Technology SCHEDULING METRICS Metrics: A standard measure to quantify to what degree a system possesses some property. Metrics provide repeatable techniques to quantify and compare systems. Measurements are the numbers derived from the application of metrics Scheduling Metric #1: Turnaround time The time at which the job completes minus the time at which the job arrived in the system 푻풕풖풓풏풂풓풐풖풏풅 = 푻풄풐풎풑풍풆풕풊풐풏 − 푻풂풓풓풊풗풂풍 How is turnaround time different than execution time? TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.9 School of Engineering and Technology, University of Washington - Tacoma SCHEDULING METRICS - 2 Scheduling Metric #2: Fairness . Jain’s fairness index . Quantifies if jobs receive a fair share of system resources n processes xi is time share of each process worst case = 1/n best case = 1 Consider n=3, worst case = .333, best case=1 With n=3 and x1=.2, x2=.7, x3=.1, fairness=.62 With n=3 and x1=.33, x2=.33, x3=.33, fairness=1 TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.10 School of Engineering and Technology, University of Washington - Tacoma Slides by Wes J. Lloyd L5.5 TCSS 422 A – Winter 2019 1/22/2019 School of Engineering and Technology SCHEDULERS FIFO: first in, first out . Very simple, easy to implement Consider . 3 x 10sec jobs, arrival: A B C, duration 10 sec each ퟏퟎ + ퟐퟎ + ퟑퟎ 푨풗풆풓풂품풆 풕풖풓풏풂풓풐풖풏풅 풕풊풎풆 = = ퟐퟎ 풔풆풄 ퟑ TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.11 School of Engineering and Technology, University of Washington - Tacoma SJF: SHORTEST JOB FIRST Given that we know execution times in advance: . Run in order of duration, shortest to longest . Non preemptive scheduler . This is not realistic . Arrival: A B C, duration a=100 sec, b/c=10sec ퟏퟎ + ퟐퟎ + ퟏퟐퟎ 푨풗풆풓풂품풆 풕풖풓풏풂풓풐풖풏풅 풕풊풎풆 = = ퟓퟎ 풔풆풄 ퟑ TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.12 School of Engineering and Technology, University of Washington - Tacoma Slides by Wes J. Lloyd L5.6 TCSS 422 A – Winter 2019 1/22/2019 School of Engineering and Technology SJF: WITH RANDOM ARRIVAL If jobs arrive at any time: duration a=100s, b/c=10s A @ t=0sec, B @ t=10sec, C @ t=10sec ퟏퟎퟎ + ퟏퟏퟎ − ퟏퟎ +(ퟏퟐퟎ − ퟏퟎ) 푨풗풆풓풂품풆 풕풖풓풏풂풓풐풖풏풅 풕풊풎풆 = = ퟏퟎퟑ. ퟑퟑ 풔풆풄 ퟑ TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.13 School of Engineering and Technology, University of Washington - Tacoma STCF - 2 Consider: duration a=100sec, b/c=10sec . Alen=100 Aarrival=0 . Blen=10, Barrival=10, Clen=10, Carrival=10 (ퟏퟐퟎ − ퟎ)+ ퟐퟎ − ퟏퟎ +(ퟑퟎ − ퟏퟎ) 푨풗풆풓풂품풆 풕풖풓풏풂풓풐풖풏풅 풕풊풎풆 = = ퟓퟎ 풔풆풄 ퟑ TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.14 School of Engineering and Technology, University of Washington - Tacoma Slides by Wes J. Lloyd L5.7 TCSS 422 A – Winter 2019 1/22/2019 School of Engineering and Technology SCHEDULING METRICS - 3 Scheduling Metric #3: Response Time Time from when job arrives until it starts execution 푻풓풆풔풑풐풏풔풆 = 푻풇풊풓풔풕풓풖풏 − 푻풂풓풓풊풗풂풍 STCF, SJF, FIFO . can perform poorly with respect to response time What scheduling algorithm(s) can help minimize response time? TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.15 School of Engineering and Technology, University of Washington - Tacoma RR: ROUND ROBIN Run each job awhile, then switch to another distributing the CPU evenly (fairly) Scheduling Quantum is called a time slice Time slice mustRR is befair, but performs poorly on metrics a multiple of the such as turnaround time timer interrupt period. Scheduling Quantum = 5 seconds TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.16 School of Engineering and Technology, University of Washington - Tacoma Slides by Wes J. Lloyd L5.8 TCSS 422 A – Winter 2019 1/22/2019 School of Engineering and Technology RR EXAMPLE ABC arrive at time=0, each run for 5 seconds OVERHEAD not considered ퟎ + ퟓ + ퟏퟎ 푻 = = ퟓ풔풆풄 풂풗풆풓풂품풆 풓풆풔풑풐풏풔풆 ퟑ ퟎ + ퟏ + ퟐ 푻 = = ퟏ풔풆풄 풂풗풆풓풂품풆 풓풆풔풑풐풏풔풆 ퟑ TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.17 School of Engineering and Technology, University of Washington - Tacoma ROUND ROBIN: TRADEOFFS Short Time Slice Long Time Slice Fast Response Time Slow Response Time High overhead from Low overhead from context switching context switching Time slice impact: .Turnaround time (for earlier example): ts(1,2,3,4,5)=14,14,13,14,10 .Fairness: round robin is always fair, J=1 TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.18 School of Engineering and Technology, University of Washington - Tacoma Slides by Wes J. Lloyd L5.9 TCSS 422 A – Winter 2019 1/22/2019 School of Engineering and Technology SCHEDULING WITH I/O STCF scheduler . A: CPU=50ms, I/O=40ms, 10ms intervals . B: CPU=50ms, I/O=0ms . Consider A as 10ms subjobs (CPU, then I/O) Without considering I/O: CPU utilization= 100/140=71% TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.19 School of Engineering and Technology, University of Washington - Tacoma SCHEDULING WITH I/O - 2 When a job initiates an I/O request . A is blocked, waits for I/O to compute, frees CPU . STCF scheduler assigns B to CPU When I/O completes raise interrupt . Unblock A, STCF goes back to executing A: (10ms sub-job) Cpu utilization = 100/100=100% TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.20 School of Engineering and Technology, University of Washington - Tacoma Slides by Wes J. Lloyd L5.10 TCSS 422 A – Winter 2019 1/22/2019 School of Engineering and Technology TCSS422: Operating Systems [Winter 2019] January 23, 2019 School of Engineering and Technology, University of Washington - Tacoma L5.21 CHAPTER 8 – MULTI-LEVEL FEEDBACK QUEUE (MLFQ) SCHEDULER TCSS422: Operating Systems [Winter 2019] January 23, 2019 School of Engineering and Technology, University of Washington - Tacoma L5.22 Slides by Wes J. Lloyd L5.11 TCSS 422 A – Winter 2019 1/22/2019 School of Engineering and Technology MULTI-LEVEL FEEDBACK QUEUE Objectives: .Improve turnaround time: Run shorter jobs first .Minimize response time: Important for interactive jobs (UI) Achieve without a priori knowledge of job length TCSS422: Operating Systems [Winter 2019] January 23, 2019 L5.23 School of Engineering and Technology, University of Washington - Tacoma MLFQ - 2 Round-Robin within a Queue Multiple job queues Adjust job priority based on observed behavior Interactive Jobs .