CPU Scheduling - Chapter 5 PDF

Chapter 5: CPU Scheduling Dr. Shadi Banitaan Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018  Outline  Basic Concepts  Scheduling...

Chapter 5: CPU Scheduling Dr. Shadi Banitaan Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018  Outline  Basic Concepts  Scheduling Criteria  Scheduling Algorithms  Thread Scheduling  Multi-Processor Scheduling  Real-Time CPU Scheduling  Linux scheduling Operating System Concepts – 10th Edition 5.2 Silberschatz, Galvin and Gagne ©2018  Objectives  Describe various CPU scheduling algorithms  Assess CPU scheduling algorithms based on scheduling criteria  Explain the issues related to multiprocessor and multicore scheduling  Describe various real-time scheduling algorithms  Describe the scheduling algorithms used in the Windows, Linux, and Solaris operating systems Operating System Concepts – 10th Edition 5.3 Silberschatz, Galvin and Gagne ©2018  Basic Concepts  Maximum CPU utilization obtained with multiprogramming  CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait  CPU burst followed by I/O burst  CPU burst distribution is of main concern Operating System Concepts – 10th Edition 5.4 Silberschatz, Galvin and Gagne ©2018  Histogram of CPU-burst Times Large number of short bursts Small number of longer bursts Operating System Concepts – 10th Edition 5.5 Silberschatz, Galvin and Gagne ©2018  CPU Scheduler  The CPU scheduler selects from among the processes in ready queue, and allocates a CPU core to one of them Queue may be ordered in various ways  CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state 2. Switches from running to ready state 3. Switches from waiting to ready 4. Terminates  For situations 1 and 4, there is no choice in terms of scheduling. A new process (if one exists in the ready queue) must be selected for execution.  For situations 2 and 3, however, there is a choice. Operating System Concepts – 10th Edition 5.6 Silberschatz, Galvin and Gagne ©2018  Preemptive and Nonpreemptive Scheduling  When scheduling takes place only under circumstances 1 and 4, the scheduling scheme is nonpreemptive.  Otherwise, it is preemptive.  Under Nonpreemptive scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases it either by terminating or by switching to the waiting state.  Virtually all modern operating systems including Windows, MacOS, Linux, and UNIX use preemptive scheduling algorithms. Operating System Concepts – 10th Edition 5.7 Silberschatz, Galvin and Gagne ©2018  Preemptive Scheduling and Race Conditions  Preemptive scheduling can result in race conditions when data are shared among several processes.  Consider the case of two processes that share data. While one process is updating the data, it is preempted so that the second process can run. The second process then tries to read the data, which are in an inconsistent state.  This issue will be explored in detail in Chapter 6. Operating System Concepts – 10th Edition 5.8 Silberschatz, Galvin and Gagne ©2018  Dispatcher  Dispatcher module gives control of the CPU to the process selected by the CPU scheduler; this involves: Switching context Switching to user mode Jumping to the proper location in the user program to restart that program  Dispatch latency – time it takes for the dispatcher to stop one process and start another running Operating System Concepts – 10th Edition 5.9 Silberschatz, Galvin and Gagne ©2018  Scheduling Criteria  CPU utilization – keep the CPU as busy as possible  Throughput – # of processes that complete their execution per time unit  Turnaround time – amount of time to execute a particular process  Waiting time – amount of time a process has been waiting in the ready queue  Response time – amount of time it takes from when a request was submitted until the first response is produced. Operating System Concepts – 10th Edition 5.10 Silberschatz, Galvin and Gagne ©2018  Scheduling Algorithm Optimization Criteria  Max CPU utilization  Max throughput  Min turnaround time  Min waiting time  Min response time Operating System Concepts – 10th Edition 5.11 Silberschatz, Galvin and Gagne ©2018  First- Come, First-Served (FCFS) Scheduling Process Burst Time P1 24 P2 3 P3 3  Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: P1 P2 P3 0 24 27 30  Waiting time for P1 = 0; P2 = 24; P3 = 27  Average waiting time: (0 + 24 + 27)/3 = 17 Operating System Concepts – 10th Edition 5.12 Silberschatz, Galvin and Gagne ©2018  FCFS Scheduling (Cont.) Suppose that the processes arrive in the order: P2 , P3 , P1  The Gantt chart for the schedule is: P2 P3 P1 0 3 6 30  Waiting time for P1 = 6; P2 = 0; P3 = 3  Average waiting time: (6 + 0 + 3)/3 = 3  Much better than previous case  Convoy effect - short process behind long process Consider one CPU-bound and many I/O-bound processes Operating System Concepts – 10th Edition 5.13 Silberschatz, Galvin and Gagne ©2018  Shortest-Job-First (SJF) Scheduling  Associate with each process the length of its next CPU burst Use these lengths to schedule the process with the shortest time  SJF is optimal – gives minimum average waiting time for a given set of processes  Preemptive version called shortest-remaining-time-first  How do we determine the length of the next CPU burst? Could ask the user Estimate Operating System Concepts – 10th Edition 5.14 Silberschatz, Galvin and Gagne ©2018  Example of SJF Process Burst Time P1 6 P2 8 P3 7 P4 3  SJF scheduling chart P4 P1 P3 P2 0 3 9 16 24  Average waiting time = (3 + 16 + 9 + 0) / 4 = 7 Operating System Concepts – 10th Edition 5.15 Silberschatz, Galvin and Gagne ©2018  Determining Length of Next CPU Burst  Can only estimate the length – should be similar to the previous one Then pick process with shortest predicted next CPU burst  Can be done by using the length of previous CPU bursts, using exponential averaging  Commonly, α set to ½ Operating System Concepts – 10th Edition 5.16 Silberschatz, Galvin and Gagne ©2018  Examples of Exponential Averaging   =0 n+1 = n Recent history does not count   =1 n+1 =  tn Only the actual last CPU burst counts  If we expand the formula, we get: n+1 =  tn+(1 - ) tn -1 + … +(1 -  )j  tn -j + … +(1 -  )n +1 0 Operating System Concepts – 10th Edition 5.17 Silberschatz, Galvin and Gagne ©2018  Shortest Remaining Time First Scheduling  Preemptive version of SJN  Whenever a new process arrives in the ready queue, the decision on which process to schedule next is redone using the SJN algorithm.  Is SRT more “optimal” than SJN in terms of the minimum average waiting time for a given set of processes? Operating System Concepts – 10th Edition 5.18 Silberschatz, Galvin and Gagne ©2018  Example of Shortest-remaining-time- first  Now we add the concepts of varying arrival times and preemption to the analysis Process i Arrival TimeT Burst Time P1 0 8 P2 1 4 P3 2 9 P4 3 5  Preemptive SJF Gantt Chart P1 P2 P4 P1 P3 0 1 5 10 17 26  Average waiting time = [(10-1)+(1-1)+(17-2)+(5-3)]/4 = 26/4 = 6.5 Operating System Concepts – 10th Edition 5.19 Silberschatz, Galvin and Gagne ©2018  Round Robin (RR)  Each process gets a small unit of CPU time (time quantum q), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready 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.  Timer interrupts every quantum to schedule next process  Performance q large  FIFO (FCFS) q small  RR  Note that q must be large with respect to context switch, otherwise overhead is too high Operating System Concepts – 10th Edition 5.20 Silberschatz, Galvin and Gagne ©2018  Example of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3 P3 3  The Gantt chart is: P1 P2 P3 P1 P1 P1 P1 P1 0 4 7 10 14 18 22 26 30  Typically, higher average turnaround than SJF, but better response  q should be large compared to context switch time q usually 10 milliseconds to 100 milliseconds, Context switch < 10 microseconds Operating System Concepts – 10th Edition 5.21 Silberschatz, Galvin and Gagne ©2018  Time Quantum and Context Switch Time Operating System Concepts – 10th Edition 5.22 Silberschatz, Galvin and Gagne ©2018  Turnaround Time Varies With The Time Quantum 80% of CPU bursts should be shorter than q Operating System Concepts – 10th Edition 5.23 Silberschatz, Galvin and Gagne ©2018  Priority Scheduling  A priority number (integer) is associated with each process  The CPU is allocated to the process with the highest priority (smallest integer  highest priority) Preemptive Nonpreemptive  SJF is priority scheduling where priority is the inverse of predicted next CPU burst time  Problem  Starvation – low priority processes may never execute  Solution  Aging – as time progresses increase the priority of the process Operating System Concepts – 10th Edition 5.24 Silberschatz, Galvin and Gagne ©2018  Example of Priority Scheduling Process Burst Time Priority P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2  Priority scheduling Gantt Chart  Average waiting time = 8.2 Operating System Concepts – 10th Edition 5.25 Silberschatz, Galvin and Gagne ©2018  Priority Scheduling w/ Round-Robin  Run the process with the highest priority. Processes with the same priority run round-robin  Example: Processa Burst Time Priority P1 4 3 P2 5 2 P3 8 2 P4 7 1 P5 3 3  Gantt Chart with time quantum = 2 Operating System Concepts – 10th Edition 5.26 Silberschatz, Galvin and Gagne ©2018  Multilevel Queue  The ready queue consists of multiple queues  Multilevel queue scheduler defined by the following parameters: Number of queues Scheduling algorithms for each queue Method used to determine which queue a process will enter when that process needs service Scheduling among the queues Operating System Concepts – 10th Edition 5.27 Silberschatz, Galvin and Gagne ©2018  Multilevel Queue  With priority scheduling, have separate queues for each priority.  Schedule the process in the highest-priority queue! Operating System Concepts – 10th Edition 5.28 Silberschatz, Galvin and Gagne ©2018  Multilevel Queue  Prioritization based upon process type Operating System Concepts – 10th Edition 5.29 Silberschatz, Galvin and Gagne ©2018  Multilevel Feedback Queue  A process can move between the various queues.  Multilevel-feedback-queue scheduler defined by the following parameters: Number of queues Scheduling algorithms for each queue Method used to determine when to upgrade a process Method used to determine when to demote a process Method used to determine which queue a process will enter when that process needs service  Aging can be implemented using multilevel feedback queue Operating System Concepts – 10th Edition 5.30 Silberschatz, Galvin and Gagne ©2018  Example of Multilevel Feedback Queue  Three queues: Q0 – RR with time quantum 8 milliseconds Q1 – RR time quantum 16 milliseconds Q2 – FCFS  Scheduling A new process enters queue Q0 which is served in RR  When it gains CPU, the process receives 8 milliseconds  If it does not finish in 8 milliseconds, the process is moved to queue Q1 At Q1 job is again served in RR and receives 16 additional milliseconds  If it still does not complete, it is preempted and moved to queue Q2 Operating System Concepts – 10th Edition 5.31 Silberschatz, Galvin and Gagne ©2018  Thread Scheduling  Distinction between user-level and kernel-level threads  When threads supported, threads scheduled, not processes  Many-to-one and many-to-many models, thread library schedules user- level threads to run on LWP Known as process-contention scope (PCS) since scheduling competition is within the process Typically done via priority set by programmer  Kernel thread scheduled onto available CPU is system-contention scope (SCS) – competition among all threads in system Operating System Concepts – 10th Edition 5.32 Silberschatz, Galvin and Gagne ©2018  Pthread Scheduling  API allows specifying either PCS or SCS during thread creation PTHREAD_SCOPE_PROCESS schedules threads using PCS scheduling PTHREAD_SCOPE_SYSTEM schedules threads using SCS scheduling  Can be limited by OS – Linux and macOS only allow PTHREAD_SCOPE_SYSTEM Operating System Concepts – 10th Edition 5.33 Silberschatz, Galvin and Gagne ©2018  Pthread Scheduling API #include #include #define NUM_THREADS 5 int main(int argc, char *argv[]) { int i, scope; pthread_t tid[NUM THREADS]; pthread_attr_t attr; pthread_attr_init(&attr); if (pthread_attr_getscope(&attr, &scope) != 0) fprintf(stderr, "Unable to get scheduling scope\n"); else { if (scope == PTHREAD_SCOPE_PROCESS) printf("PTHREAD_SCOPE_PROCESS"); else if (scope == PTHREAD_SCOPE_SYSTEM) printf("PTHREAD_SCOPE_SYSTEM"); else fprintf(stderr, "Illegal scope value.\n"); } Operating System Concepts – 10th Edition 5.34 Silberschatz, Galvin and Gagne ©2018  Pthread Scheduling API pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM); for (i = 0; i < NUM_THREADS; i++) pthread_create(&tid[i],&attr,runner,NULL); for (i = 0; i < NUM_THREADS; i++) pthread_join(tid[i], NULL); } void *runner(void *param) { pthread_exit(0); } Operating System Concepts – 10th Edition 5.35 Silberschatz, Galvin and Gagne ©2018  Multiple-Processor Scheduling  CPU scheduling more complex when multiple CPUs are available  Multiprocess may be any one of the following architectures: Multicore CPUs Multithreaded cores NUMA systems Heterogeneous multiprocessing Operating System Concepts – 10th Edition 5.36 Silberschatz, Galvin and Gagne ©2018  Multiple-Processor Scheduling  Symmetric multiprocessing (SMP) is where each processor is self scheduling.  All threads may be in a common ready queue (a)  Each processor may have its own private queue of threads (b) Operating System Concepts – 10th Edition 5.37 Silberschatz, Galvin and Gagne ©2018  Multicore Processors  Recent trend to place multiple processor cores on same physical chip  Faster and consumes less power  Multiple threads per core also growing Takes advantage of memory stall to make progress on another thread while memory retrieve happens  Figure Operating System Concepts – 10th Edition 5.38 Silberschatz, Galvin and Gagne ©2018  Multithreaded Multicore System  Each core has > 1 hardware threads.  If one thread has a memory stall, switch to another thread!  Figure Operating System Concepts – 10th Edition 5.39 Silberschatz, Galvin and Gagne ©2018  Multithreaded Multicore System  Chip-multithreading (CMT) assigns each core multiple hardware threads. (Intel refers to this as hyperthreading.)  On a quad-core system with 2 hardware threads per core, the operating system sees 8 logical processors. Operating System Concepts – 10th Edition 5.40 Silberschatz, Galvin and Gagne ©2018  Multithreaded Multicore System  Two levels of scheduling: 1. The operating system deciding which software thread to run on a logical CPU 2. How each core decides which hardware thread to run on the physical core. Operating System Concepts – 10th Edition 5.41 Silberschatz, Galvin and Gagne ©2018  Multiple-Processor Scheduling – Load Balancing  If SMP, need to keep all CPUs loaded for efficiency  Load balancing attempts to keep workload evenly distributed  Push migration – periodic task checks load on each processor, and if found pushes task from overloaded CPU to other CPUs  Pull migration – idle processors pulls waiting task from busy processor Operating System Concepts – 10th Edition 5.42 Silberschatz, Galvin and Gagne ©2018  Multiple-Processor Scheduling – Processor Affinity  When a thread has been running on one processor, the cache contents of that processor stores the memory accesses by that thread.  We refer to this as a thread having affinity for a processor (i.e., “processor affinity”)  Load balancing may affect processor affinity as a thread may be moved from one processor to another to balance loads, yet that thread loses the contents of what it had in the cache of the processor it was moved off of.  Soft affinity – the operating system attempts to keep a thread running on the same processor, but no guarantees.  Hard affinity – allows a process to specify a set of processors it may run on. Operating System Concepts – 10th Edition 5.43 Silberschatz, Galvin and Gagne ©2018  NUMA and CPU Scheduling If the operating system is NUMA-aware, it will assign memory closes to the CPU the thread is running on. Operating System Concepts – 10th Edition 5.44 Silberschatz, Galvin and Gagne ©2018  Real-Time CPU Scheduling  Can present obvious challenges  Soft real-time systems – Critical real-time tasks have the highest priority, but no guarantee as to when tasks will be scheduled  Hard real-time systems – task must be serviced by its deadline Operating System Concepts – 10th Edition 5.45 Silberschatz, Galvin and Gagne ©2018  Real-Time CPU Scheduling  Event latency – the amount of time that elapses from when an event occurs to when it is serviced.  Two types of latencies affect performance 1. Interrupt latency – time from arrival of interrupt to start of routine that services interrupt 2. Dispatch latency – time for schedule to take current process off CPU and switch to another Operating System Concepts – 10th Edition 5.46 Silberschatz, Galvin and Gagne ©2018  Interrupt Latency Operating System Concepts – 10th Edition 5.47 Silberschatz, Galvin and Gagne ©2018  Dispatch Latency  Conflict phase of dispatch latency: 1. Preemption of any process running in kernel mode 2. Release by low- priority process of resources needed by high- priority processes Operating System Concepts – 10th Edition 5.48 Silberschatz, Galvin and Gagne ©2018  Priority-based Scheduling  For real-time scheduling, scheduler must support preemptive, priority- based scheduling But only guarantees soft real-time  For hard real-time must also provide ability to meet deadlines  Processes have new characteristics: periodic ones require CPU at constant intervals Has processing time t, deadline d, period p 0≤t≤d≤p Rate of periodic task is 1/p Operating System Concepts – 10th Edition 5.49 Silberschatz, Galvin and Gagne ©2018  Rate Monotonic Scheduling  A priority is assigned based on the inverse of its period  Shorter periods = higher priority;  Longer periods = lower priority  We have two processes, P1 and P2. The periods for P1 and P2 are 50 and 100, respectively—that is, p1 = 50 and p2 = 100. The processing times are t1 = 20 for P1 and t2 = 35 for P2. The deadline for each process requires that it complete its CPU burst by the start of its next period.  P1 is assigned a higher priority than P2. Operating System Concepts – 10th Edition 5.50 Silberschatz, Galvin and Gagne ©2018  Missed Deadlines with Rate Monotonic Scheduling  Assume that process P1 has a period of p1 = 50 and a CPU burst of t1 = 25. For P2, the corresponding values are p2 = 80 and t2 = 35.  Process P2 misses finishing its deadline at time 80 Operating System Concepts – 10th Edition 5.51 Silberschatz, Galvin and Gagne ©2018  Earliest Deadline First Scheduling (EDF)  Priorities are assigned according to deadlines: The earlier the deadline, the higher the priority The later the deadline, the lower the priority  Process P1 has a period of p1 = 50 and a CPU burst of t1 = 25. For P2, the corresponding values are p2 = 80 and t2 = 35. Operating System Concepts – 10th Edition 5.52 Silberschatz, Galvin and Gagne ©2018  Exercise  Consider two processes, P1 and P2, where p1 = 50, t1 = 25, p2 = 75, and t2 = 30. Can these two processes be scheduled using rate- monotonic scheduling? Illustrate your answer using a Gantt chart. Illustrate the scheduling of these two processes using the earliest deadline first (EDF) scheduling. Operating System Concepts – 10th Edition 5.53 Silberschatz, Galvin and Gagne ©2018  POSIX Real-Time Scheduling  The POSIX.1b standard  API provides functions for managing real-time threads  Defines two scheduling classes for real-time threads: 1. SCHED_FIFO - threads are scheduled using a FCFS strategy with a FIFO queue. There is no time-slicing for threads of equal priority 2. SCHED_RR - similar to SCHED_FIFO except time-slicing occurs for threads of equal priority  Defines two functions for getting and setting scheduling policy: 1. pthread_attr_getsched_policy(pthread_attr_t *attr, int *policy) 2. pthread_attr_setsched_policy(pthread_attr_t *attr, int policy) Operating System Concepts – 10th Edition 5.54 Silberschatz, Galvin and Gagne ©2018  POSIX Real-Time Scheduling API #include #include #define NUM_THREADS 5 int main(int argc, char *argv[]) { int i, policy; pthread_t_tid[NUM_THREADS]; pthread_attr_t attr; pthread_attr_init(&attr); if (pthread_attr_getschedpolicy(&attr, &policy) != 0) fprintf(stderr, "Unable to get policy.\n"); else { if (policy == SCHED_OTHER) printf("SCHED_OTHER\n"); else if (policy == SCHED_RR) printf("SCHED_RR\n"); else if (policy == SCHED_FIFO) printf("SCHED_FIFO\n"); } Operating System Concepts – 10th Edition 5.55 Silberschatz, Galvin and Gagne ©2018  POSIX Real-Time Scheduling API (Cont.) if (pthread_attr_setschedpolicy(&attr, SCHED_FIFO) != 0) fprintf(stderr, "Unable to set policy.\n"); for (i = 0; i < NUM_THREADS; i++) pthread_create(&tid[i],&attr,runner,NULL); for (i = 0; i < NUM_THREADS; i++) pthread_join(tid[i], NULL); } void *runner(void *param) { pthread_exit(0); } Operating System Concepts – 10th Edition 5.56 Silberschatz, Galvin and Gagne ©2018  Linux Scheduling Through Version 2.5  Prior to kernel version 2.5, ran variation of standard UNIX scheduling algorithm  Version 2.5 moved to constant order O(1) scheduling time Preemptive, priority based Two priority ranges: time-sharing and real-time Real-time range from 0 to 99 and nice value from 100 to 140 Map into global priority with numerically lower values indicating higher priority Higher priority gets larger q Task run-able as long as time left in time slice (active) If no time left (expired), not run-able until all other tasks use their slices All run-able tasks tracked in per-CPU runqueue data structure Two priority arrays (active, expired)  Tasks indexed by priority  When no more active, arrays are exchanged Worked well, but poor response times for interactive processes Operating System Concepts – 10th Edition 5.57 Silberschatz, Galvin and Gagne ©2018  Linux Scheduling in Version 2.6.23 +  Completely Fair Scheduler (CFS)  Scheduling classes Each has specific priority Scheduler picks highest priority task in highest scheduling class Rather than quantum based on fixed time allotments, based on proportion of CPU time Two scheduling classes included, others can be added 1. default 2. real-time Operating System Concepts – 10th Edition 5.58 Silberschatz, Galvin and Gagne ©2018  Linux Scheduling in Version 2.6.23 + (Cont.)  Quantum calculated based on nice value from -20 to +19 Lower value is higher priority Calculates target latency – interval of time during which task should run at least once Target latency can increase if say number of active tasks increases  CFS scheduler maintains per task virtual run time in variable vruntime Associated with decay factor based on priority of task – lower priority is higher decay rate Normal default priority yields virtual run time = actual run time  To decide next task to run, scheduler picks task with lowest virtual run time Operating System Concepts – 10th Edition 5.59 Silberschatz, Galvin and Gagne ©2018  CFS Performance Operating System Concepts – 10th Edition 5.60 Silberschatz, Galvin and Gagne ©2018  Linux Scheduling (Cont.)  Real-time scheduling according to POSIX.1b Real-time tasks have static priorities  Real-time plus normal map into global priority scheme  Nice value of -20 maps to global priority 100  Nice value of +19 maps to priority 139 Operating System Concepts – 10th Edition 5.61 Silberschatz, Galvin and Gagne ©2018  End of Chapter 5 Dr. Shadi Banitaan Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018