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QEMU/Linux Assignments, Scheduling Continued Operating Systems 9/20/2018 Programming Assignment #2 About QEMU • DUE DATE: Monday, October 1 2018 • A processor emulator • Emulates code for target CPU on host CPU MINI-HOMEWORK DEADLINE: Tuesday Sep 25 • Download assignment files • Build the Linux kernel • Boot the QEMU image with the Linux kernel Guest System • Report output of the procedure Host System – uname -a 1 2 Why QEMU? Download the Code • Allow you to change the kernel • Get the files you need: – OS requires privilege to overwrite kernel file – Use git clone • Isolate kernel changes from the real machine – Approximate space requirements • Make programming and debugging easier – Kernel source and object code: 4 GB – Everything: About 4 GB of disk space 3 4 CSC 256/456 1 Operating Systems 9/20/2018 Get It Started! Start Up a New Kernel • Start QEMU and default Debian installation • Specify path to your kernel bzImage to – cd install runqemu.sh – sh runqemu.sh – sh runqemu.sh linux-3.18- • No password 77/arch/x86_64/boot/bzImage • Use poweroff command to shutdown • When QEMU boots, use uname -a to check if you booted the correct kernel image • Can kill qemu from different window if kernel panics • README.md file contains useful information 5 6 CPU Scheduling • Selects from among the processes/threads that are ready to execute, and allocates the CPU to it • CPU scheduling may take place at: 1. Hardware interrupt/software exception CPU Scheduling 2. System calls • Nonpreemptive: – Scheduling only when the current process terminates CS 256/456 or not able to run further Department of Computer Science • Preemptive: University of Rochester – Scheduling can occur at any opportunity possible 9/20/2018 CSC 2/456 7 9/20/2018 CSC 2/456 8 CSC 256/456 2 Operating Systems 9/20/2018 First-Come, First-Served (FCFS) Scheduling Criteria Scheduling • Minimize turnaround time – amount of time to execute a Process CPU Time particular process (includes I/O, CPU, memory time, waiting time in the ready queue) P1 24 P 3 • Maximize throughput – # of processes that complete their 2 execution per time unit P3 3 • Suppose that the processes arrive in the order: P1 , P2 , P3 • Maximize CPU utilization – the proportion of the CPU that The schedule is: is not idle • Minimize response time – amount of time it takes from when a request was submitted until the first response is P1 P2 P3 produced (interactivity) 0 24 27 30 • Waiting time: time spent in the ready queue • Turnaround time for P1 = 24; P2 = 27; P3 = 30 • Fairness: avoid starvation • Average turnaround time: (24 + 27 + 30)/3 = 27 9/20/2018 CSC 2/456 9 9/20/2018 CSC 2/456 10 Shortest-Job-First (SJF) FCFS Scheduling (Cont.) Scheduling Suppose that the processes arrive in the order • Associate with each process the length of its CPU time. P , P , P . Use these lengths to schedule the process with the 2 3 1 shortest CPU time • The schedule is: • Two variations: – Non-preemptive – once CPU given to the process it P2 P3 P1 cannot be taken away until it completes – preemptive – if a new process arrives with CPU time 0 3 6 30 less than remaining time of current executing process, • Turnaround time for P1 = 30; P2 = 3; P3 = 6 preempt • Average turnaround time: (30 + 3 + 6)/3 = 13 • Preemptive SJF is optimal – gives minimum average • Much better than previous case. turnaround time for a given set of processes • Short process delayed by long process: Convoy effect • Problem: – don’t know the process CPU time ahead of time 9/20/2018 CSC 2/456 11 9/20/2018 CSC 2/456 12 CSC 256/456 3 Operating Systems 9/20/2018 Example of Preemptive SJF Priority Scheduling • A priority number (integer) is associated with each process Process Arrival Time CPU Time • The CPU is allocated to the process with the highest priority P1 0.0 7 – preemptive P2 2.0 4 P3 4.0 1 – nonpreemptive P4 5.0 4 • SJF is a priority scheduling where priority is the predicted • SJF (preemptive) CPU time • Problem: Starvation – low priority processes may never P P P P P P 1 2 3 2 4 1 execute • Solution: Aging – as time progresses, increase the priority of 0 11 16 2 4 5 7 the process • Average turnaround time = (16 + 5 + 1 +6)/4 = 7 9/20/2018 CSC 2/456 13 9/20/2018 CSC 2/456 14 What Happened on the Mars Round Robin (RR) Pathfinder (1997)? • Each process gets a fixed unit of CPU time (time quantum), https://cs.unc.edu/~anderson/teach/comp790/papers/mars_pathfinder_long_version.html usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready Problem: Priority Inversion queue • If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units • Performance – q small fair, starvation-free, better interactivity – q large FIFO https://www.rapitasystems.com/blog/what-really-happened-to-the-software-on-the-mars-pathfinder-spacecraft – q must be large with respect to context switch cost, Solution: Priority Inheritance [L. Sha, R. Rajkumar, and J. P. Lehoczky. Priority otherwise overhead is too high Inheritance Protocols: An Approach to Real-Time Synchronization. In IEEE Transactions on Computers, vol. 39, pp. 1175-1185, Sep. 1990.] 9/20/2018 CSC 2/456 15 9/20/2018 CSC 2/456 16 CSC 256/456 4 Operating Systems 9/20/2018 Example of RR with Quantum = 20 Multilevel Scheduling • Ready tasks are partitioned into separate classes: Process CPU Time foreground (interactive) P1 53 background (batch) P 17 2 • Each class has its own scheduling algorithm, P 68 3 foreground – RR P4 24 background – FCFS • The schedule is: • Scheduling must be done between the classes. – Fixed priority scheduling; (i.e., serve all from P P P P P P P P P P 1 2 3 4 1 3 4 1 3 3 foreground then from background). Possibility of 0 20 37 57 77 97 117 121 134 154 162 starvation • Typically, higher average turnaround than SJF, but – Time slice – each class gets a certain amount of CPU better response time which it can schedule amongst its processes; e.g., • 80% to foreground in RR • 20% to background in FCFS 9/20/2018 CSC 2/456 17 9/20/2018 CSC 2/456 18 Example of Multilevel Feedback Multilevel Feedback Queue Queue • A process can move between the various queues; aging can • Three queues: be implemented this way – Q – RR with time quantum 8 milliseconds • Multilevel-feedback-queue scheduler defined by the 0 following parameters: – Q1 – RR time quantum 16 milliseconds – number of queues – Q2 – FCFS – scheduling algorithms for each queue • Scheduling – method used to determine when to upgrade a process – A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 – method used to determine when to demote a process milliseconds, job is moved to queue Q1. – method used to determine which queue a process will – At Q job is again served FCFS and receives 16 additional enter when that process needs service 1 milliseconds. If it still does not complete, it is preempted and moved to queue Q2. 9/20/2018 CSC 2/456 19 9/20/2018 CSC 2/456 20 CSC 256/456 5 Operating Systems 9/20/2018 Multilevel Feedback Queues Lottery Scheduling • Give processes lottery tickets for various system resources • Choose ticket at random and allow process holding the ticket to get the resource • Hold a lottery at periodic intervals • Properties – Chance of winning proportional to number of tickets held (highly responsive) – Cooperating processes may exchange tickets – Fair-share scheduling easily implemented by allocating tickets to users and dividing tickets among child processes 9/20/2018 CSC 2/456 21 9/20/2018 CSC 2/456 22 Real-Time Scheduling Cost of Context Switch • Hard real-time systems – required to complete a critical task within a guaranteed amount of time • Direct overhead of context switch • Soft real-time computing – requires that critical processes – saving old contexts, restoring new contexts, … … receive priority over less fortunate ones • EDF – Earliest Deadline First Scheduling • Indirect overhead of context switch – caching, memory management overhead 9/20/2018 CSC 2/456 23 9/20/2018 CSC 2/456 24 CSC 256/456 6 Operating Systems 9/20/2018 Solaris Dispatch Table Solaris Scheduling 9/20/2018 CSC 2/456 25 9/20/2018 CSC 2/456 26 Priorities and Time-slice length Linux Task Scheduling • Linux 2.5 and up uses a preemptive, priority-based algorithm with two separate priority ranges: – A time-sharing class/range for fair preemptive scheduling (nice value ranging from 100-140) – A real-time class that conforms to POSIX real-time standard (0- 99) • Numerically lower values indicate higher priority • Higher-priority tasks get longer time quanta (200-10 ms) • One runqueue per processor (logical or physical); load balancing phase to equally distribute tasks among runqueues • Runqueue indexed by priority and contains two priority arrays – active and expired • Choose task with highest priority on active array; switch active and expired arrays when active is empty • Time-sharing tasks are assigned the nice value +/- 5 9/20/2018 CSC 2/456 27 9/20/2018 CSC 2/456 28 CSC 256/456 7 Operating Systems 9/20/2018 List of Tasks Indexed According to Multiprocessor Context Switch Priorities • Disabling signals not sufficient • Acquire scheduler lock when accessing any scheduler data structure, e.g., yield: disable_signals acquire(scheduler_lock) // spin lock enqueue(ready_list, current) reschedule release(scheduler_lock) re-enable_signals 9/20/2018 CSC 2/456 29 9/20/2018 CSC 2/456 30 CPU Scheduling on Multi- Processors Multiprocessor
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