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Chapter 6 Scheduling

Chapter 6 Scheduling. Overview. CPU scheduling is the basis of multiprogrammed operating systems By switching the CPU among processes The OS can make the computer more productive On operating systems that support threads

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Chapter 6 Scheduling

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  1. Chapter 6 Scheduling

  2. Overview • CPU scheduling is the basis of multiprogrammed operating systems • By switching the CPU among processes • The OS can make the computer more productive • On operating systems that support threads • It is kernel-level threads that are in fact being scheduled by the operating system • Not processes

  3. Basic Concepts • In a single-processor system • Only one process can run at a time • Others must wait until the CPU is free and can be rescheduled • The objective of multiprogramming • To have some process running at all times • To maximize CPU utilization • The idea is relatively simple • A process is executed until it must wait • Typically for the completion of some I/O request • In a simple computer system • The CPU just sits idle

  4. Basic Concepts (cont.) • All this waiting time is wasted • No useful work is accomplished • With multiprogramming • Try to use this time productively • Several processes are kept in memory at one time • When one process has to wait • The OS takes the CPU away from that process • Gives the CPU to another process • This pattern continues • Every time one process has to wait • Another process can take over use of the CPU • Scheduling of this kind is a fundamental operating-system function

  5. Basic Concepts (cont.) • Almost all computer resources are scheduled before use • The CPU is certainly one of the primary computer resources • The scheduling is central to operating-system design

  6. CPU–I/O Burst Cycle • The success of CPU scheduling depends on an observed property of processes • Process execution consists of a cycle of CPU execution and I/O wait • Processes alternate between these two states • Process execution begins with a CPU burst • Followed by an I/O burst, which is followed by another CPU burst, then another I/O burst, and so on • Eventually, the final CPU burst ends with a system request to terminate execution • The durations of CPU bursts have been measured extensively • Vary greatly from process to process

  7. CPU–I/O Burst Cycle (cont.)

  8. CPU–I/O Burst Cycle (cont.) • And from computer to computer • Tend to have a frequency curve • Characterized as exponential or hyperexponential • With a large number of short CPU bursts and a small number of long CPU bursts • An I/O-bound program typically has many short CPU bursts • A CPU-bound program may have a few long CPU bursts • This distribution can be important in the selection of an appropriate CPU-scheduling algorithm

  9. CPU–I/O Burst Cycle (cont.)

  10. CPU Scheduler • Whenever the CPU becomes idle • The operating system must select one of the processes in the ready queue to be executed • The selection process is carried out by the short-term scheduler, or CPU scheduler • The scheduler selects a process from the processes in memory ready to execute • Allocates the CPU to that process • The ready queue is not necessarily a first-in, first-out (FIFO)queue • Can be implemented as a FIFO queue, a priority queue, a tree, or simply an unordered linked list • All the processes in the ready queue are lined up

  11. CPU Scheduler (cont.) • Waiting for a chance to run on the CPU • The records in the queues are generally process control blocks (PCBs) of the processes

  12. Preemptive Scheduling • CPU-scheduling decisions may take place under the following four circumstances • When a process switches from the running state to the waiting state • e.g., As the result of an I/O request or an invocation of wait() for the termination of a child process • When a process switches from the running state to the ready state • e.g., When an interrupt occurs • When a process switches from the waiting state to the ready state • e.g., At completion of I/O • When a process terminates

  13. Preemptive Scheduling (cont.) • For situations 1 and 4 • There is no choice in terms of scheduling • A new process must be selected for execution • There is a choice for situations 2 and 3 • When scheduling takes place only under circumstances 1 and 4 • The scheduling scheme is nonpreemptive or cooperative • Otherwise, it is preemptive • Under nonpreemptive scheduling • Once the CPU has been allocated to a process • The process keeps the CPU until it releases the CPU either by terminating or by switching to the waiting state

  14. Preemptive Scheduling (cont.) • Cooperative scheduling is the only method that can be used on certain hardware platforms • Does not require the special hardware needed for preemptive scheduling • e.g., A timer • Preemptive scheduling can result in race conditions • When data are shared among several processes • Consider that 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

  15. Preemptive Scheduling (cont.) • Preemption also affects the design of the operating-system kernel • During the processing of a system call • The kernel may be busy with an activity on behalf of a process • Such activities may involve changing important kernel data, e.g., I/O queues • If the process is preempted in the middle of these changes • The kernel or the device driver needs to read or modify the same structure • Chaos ensues • Certain OSes, including most versions of UNIX

  16. Preemptive Scheduling (cont.) • Deal with this by waiting for a system call to complete • Or for an I/O block to take place before doing a context switch • Ensures that the kernel structure is simple • The kernel will not preempt a process while the kernel data structures are in an inconsistent state • This kernel-execution model is a poor one for supporting real-time computing • Tasks must complete execution within a given time frame • Interrupts can occur at any time • Cannot always be ignored by the kernel

  17. Preemptive Scheduling (cont.) • The sections of code affected by interrupts must be guarded from simultaneous use • The operating system needs to accept interrupts at almost all times • Otherwise, input might be lost or output overwritten • So that these sections of code are not accessed concurrently by several processes • Disable interrupts at entry • Reenable interrupts at exit • Sections of code that disable interrupts do not occur very often • Typically contain few instructions

  18. Dispatcher • The dispatcher is the module • Gives control of the CPU to the process selected by the short-term scheduler • This function involves • Switching context • Switching to user mode • Jumping to the proper location in the user program to restart that program • Should be as fast as possible • Invoked during every process switch • The dispatch latency • The time the dispatcher takes to stop one process and start another running

  19. Scheduling Criteria • Different CPU-scheduling algorithms have different properties • The choice of a particular algorithm may favor one class of processes over another • Must consider the properties of the various algorithms in choosing which algorithm to use • Many criteria have been suggested for comparing CPU-scheduling algorithms • How algorithm is judged to be best • CPU utilization • To keep the CPU as busy as possible • Conceptually, CPU utilization can range from 0 to 100 percent

  20. Scheduling Criteria (cont.) • In a real system, it should range from 40 percent for a lightly loaded system to 90 percent for a heavily loaded system • Throughput • If the CPU is busy executing processes, then work is being done • One measure of work is the number of processes that are completed per time unit, called throughput • For long processes, may be one process per hour • For short ones, may be 10 processes per second • Turnaround time • From the point of view of a process, the important criterion is how long it takes to execute that process • The interval from the time of submission of a process to the time of completion is the turnaround time

  21. Scheduling Criteria (cont.) • Turnaround time is the sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O • Waiting time • The CPU-scheduling algorithm does not affect the amount of time during which a process executes or does I/O • Affects only the amount of time that a process spends waiting in the ready queue • Waiting time is the sum of the periods spent waiting in the ready queue • Response time • In an interactive system, turnaround time may not be the best criterion • A process can produce some output fairly early

  22. Scheduling Criteria (cont.) • And can continue computing new results while previous results are being output to the user • Another measure is the time from the submission of a request until the first response is produced • Response time is the time a process takes to start responding • Not the time it takes to output the response • The turnaround time is generally limited by the speed of the output device • Desire to maximize CPU utilization and throughput • And to minimize turnaround time, waiting time, and response time • In most cases, optimize the average measure

  23. Scheduling Criteria (cont.) • Under some circumstances • Prefer to optimize the minimum or maximum values rather than the average • e.g., To guarantee that all users get good service, may want to minimize the maximum response time • For interactive systems • Such as desktop systems • More important to minimize the variance in the response time than to minimize the average • A system with reasonable and predictable response time may be considered more desirable than a system that is faster on the average but is highly variable • Little work has been done on CPU-scheduling algorithms that minimize variance

  24. Scheduling Criteria (cont.) • The first problem is defining the criteria to be used in selecting an algorithm • To select an algorithm • Must first define the relative importance of these elements • The criteria may include several measures • Maximizing CPU utilization under the constraint that the maximum response time is 1 second • Maximizing throughput such that turnaround time is on averag) linearly proportional to total execution time

  25. Thread Scheduling • User-level threads are managed by a thread library • The kernel is unaware of them • To run on a CPU • User-level threads must ultimately be mapped to an associated kernel-level thread • This mapping may be indirect and may use a lightweight process (LWP)

  26. Contention Scope • One distinction between user-level and kernel-level threads • How they are scheduled • On systems implementing the many-to-one and many-to-many models • The thread library schedules user-level threads to run on an available LWP • Known as process-contention scope (PCS) • Competition for the CPU takes place among threads belonging to the same process • The thread library schedules user threads onto available LWPs • Do not mean that the threads are actually running on a CPU

  27. Contention Scope (cont.) • That will require the operating system to schedule the kernel thread onto a physical CPU • To decide which kernel-level thread to schedule onto a CPU • The kernel uses system-contention scope (SCS) • Competition for the CPU takes place among all threads in the system • Systems using the one-to-one model schedule threads using only SCS • Such as Windows, Linux, and Solaris • PCS is done according to priority • The scheduler selects the runnable thread with the highest priority to run

  28. Contention Scope (cont.) • User-level thread priorities are set by the programmer • Not adjusted by the thread library • Some thread libraries may allow the programmer to change the priority of a thread • PCS will typically preempt the thread currently running in favor of a higher-priority thread • There is no guarantee of time slicing among threads of equal priority

  29. Pthread Scheduling • The POSIX Pthread API allows specifying PCS or SCS during thread creation • Pthreads identifies two contention scope values • PTHREAD_SCOPE_PROCESS schedules threads using PCS scheduling • PTHREAD_SCOPE_SYSTEM schedules threads using SCS scheduling • On systems implementing the many-to-many model • The PTHREAD_SCOPE_PROCESS policy schedules user-level threads onto available LWPs • The number of LWPs is maintained by the thread library, perhaps using scheduler activations

  30. Pthread Scheduling (cont.) • The PTHREAD_SCOPE_SYSTEM scheduling policy will create and bind an LWP for each user-level thread on many-to-many systems • Effectively mapping threads using the one-to-one policy • The Pthread IPC provides two functions • For getting and setting the contention scope policy pthread_attr_setscope(pthread_attr_t* attr, int scope) pthread_attr_getscope(pthread_attr_t* attr, int *scope) • The first parameter for both functions contains a pointer to the attribute set for the thread

  31. Pthread Scheduling (cont.) • The second parameter for the pthread_attr_setscope() function is passed either the PTHREAD_SCOPE_ SYSTEM or the PTHREAD_SCOPE_PROCESS value • Indicating how the contention scope is to be set • For pthread_attr_getscope() • This second parameter contains a pointer to an int value set to the current value of the contention scope • If an error occurs, each of these functions returns a nonzero value • A Pthread scheduling API • The program first determines the existing contention scope • Then sets it to PTHREAD_SCOPE_SYSTEM • Creates five separate threads

  32. Pthread Scheduling (cont.) • Will run using the SCS scheduling policy • On some systems, only certain contention scope values are allowed • e.g., Linux and Mac OS X systems allow only PTHREAD_SCOPE_SYSTEM

  33. Pthread Scheduling (cont.)

  34. POSIX Real-Time Scheduling • The POSIX standard also provides extensions for real-time computing • POSIX.1b • Related to scheduling real-time threads • POSIX defines two scheduling classes for real-time threads • SCHED_FIFO • SCHED_FIFO schedules threads according to a first-come, first-served policy using a FIFO queue • No time slicing among threads of equal priority • The highest-priority real-time thread at the front of the FIFO queue will be granted the CPU until it terminates or blocks

  35. POSIX Real-Time Scheduling (cont.) • SCHED_RR • Uses a round-robin policy • Similar to SCHED_FIFO except that it provides time slicing among threads of equal priority • POSIX provides one more scheduling class • SCHED_ OTHER • Its implementation is undefined and system specific • May behave differently on different systems • The POSIX API specifies the two functions • For getting and setting the scheduling policy Pthread_attr_getsched_policy(pthread_attr_t *attr, int *policy) pthread_attr_setsched_policy(pthread_attr_t *attr, int policy)

  36. POSIX Real-Time Scheduling (cont.) • The first parameter to both functions is a pointer to the set of attributes for the thread • The second parameter is either a pointer to an integer that is set to the current scheduling policy for pthread_attr_getsched_policy() • Or an integer value: SCHED_FIFO, SCHED_RR, or SCHED_OTHER for the pthread_attr_setsched_policy() function • Both functions return nonzero values if an error occurs • A POSIX Pthread program using this API • This program first determines the current scheduling policy • Then sets the scheduling algorithm to SCHED_FIFO

  37. POSIX Real-Time Scheduling (cont.)

  38. POSIX Real-Time Scheduling (cont.)

  39. Linux Scheduling • To schedule tasks with the Linux scheduler • Prior to Version 2.5, Linux ran a variation of the traditional UNIX scheduling algorithm • Resulted in poor performance for systems with a large number of runnable processes • With Version 2.5 of the kernel, the scheduler was overhauled to include a scheduling algorithm • Known as O(1) • Ran in constant time regardless of the number of tasks in the system • Led to poor response times for the interactive processes common on many desktop systems • During development of the 2.6 kernel, the scheduler was again revised

  40. Linux Scheduling (cont.) • In release 2.6.23 of the kernel • The Completely Fair Scheduler (CFS) became the default Linux scheduling algorithm • Scheduling in the Linux system is based on scheduling classes • Each class is assigned a specific priority • By using different scheduling classes • The kernel can accommodate different scheduling algorithms based on the needs of the system and its processes • e.g., The scheduling criteria for a Linux server may be different from those for a mobile device running Linux • To decide which task to run next • The scheduler selects the highest-priority task belonging to the highest-priority scheduling class

  41. Linux Scheduling (cont.) • Standard Linux kernels implement two scheduling classes • A default scheduling class using the CFS scheduling algorithm • A real-time scheduling class • New scheduling classes can be added • The CFS scheduler assigns a proportion of CPU processing time to each task • Rather than using strict rules to associate a relative priority value with the length of a time quantum • This proportion is calculated based on the nice value assigned to each task

  42. Linux Scheduling (cont.) • Nice values range from -20 to +19 • A numerically lower nice value indicates a higher relative priority • Tasks with lower nice values receive a higher proportion of CPU processing time than tasks with higher nice values • The default nice value is 0 • The term nice comes from the idea • If a task increases its nice value from, say, 0 to +10, it is being nice to other tasks in the system by lowering its relative priority • CFS does not use discrete values of time slices • Instead identifies a targeted latency

  43. Linux Scheduling (cont.) • An interval of time during which every runnable task should run at least once • Proportions of CPU time are allocated from the value of targeted latency • Having default and minimum values • Targeted latency can increase • If the number of active tasks in the system grows beyond a certain threshold • The CFS scheduler does not directly assign priorities • It records how long each task has run by maintaining the virtual run time of each task • Using the per-task variable vruntime

  44. Linux Scheduling (cont.) • The virtual run time is associated with a decay factor based on the priority of a task • Lower-priority tasks have higher rates of decay than higher-priority tasks • For tasks at normal priority with nice values of 0 • Virtual run time is identical to actual physical run time • e.g., If a task with default priority runs for 200 milliseconds, its vruntime will also be 200 milliseconds • If a lower-priority task runs for 200 milliseconds • Its vruntime will be higher than 200 milliseconds • If a higher-priority task runs for 200 milliseconds • Its vruntime will be less than 200 milliseconds • To decide which task to run next • The scheduler simply selects the task that has the smallest vruntime value

  45. Linux Scheduling (cont.) • A higher-priority task that becomes available to run can preempt a lower-priority task • The CFS scheduler provides an efficient algorithm for selecting a task to run next • Each runnable task is placed in a red-black tree • A balanced binary search tree whose key is based on the value of vruntime

  46. Linux Scheduling (cont.) • As a task is runnable, it is added to the tree • If a task is not runnable, it is removed • e.g., If it is blocked while waiting for I/O • Tasks that have been given less processing time are toward the left side of the tree • Smaller values of vruntime • Tasks that have been given more processing time are on the right side • Based on the properties of a binary search tree • The leftmost node has the smallest key value • The task with the highest priority • The red-black tree is balanced • Discover the leftmost node will require O(lg N) operations, and N is the number of nodes in the tree

  47. Linux Scheduling (cont.) • For efficiency reasons, the Linux scheduler caches this value in the variable rb_leftmost • Thus determining which task to run next requires only retrieving the cached value • Two tasks have the same nice values • One task is I/O-bound • Will run only for short periods before blocking for additional I/O • The other is CPU-bound • Will exhaust its time period whenever it has an opportunity to run on a processor • The value of vruntime will eventually be lower for the I/O-bound task than for the CPU-bound task • Giving the I/O-bound task higher priority than the CPU-bound task

  48. Linux Scheduling (cont.) • If the CPU-bound task is executing when the I/O-bound task becomes eligible to run • The I/O-bound task will preempt the CPU-bound task • e.g., AS I/O the task is waiting for becomes available • Linux also implements real-time scheduling using the POSIX standard • Any task scheduled using either the SCHED_FIFO or the SCHED_RR real-time policy runs at a higher priority than normal tasks • Linux uses two separate priority ranges • One for real-time tasks and a second for normal tasks • Real-time tasks are assigned static priorities within the range of 0 to 99 • Normal tasks are assigned priorities from 100 to 139

  49. Linux Scheduling (cont.) • The two ranges map into a global priority scheme • Numerically lower values indicate higher relative priorities • Normal tasks are assigned a priority based on their nice values • A value of -20 maps to priority 100 • A nice value of +19 maps to 139 • Another aspect of scheduling is also important to Linux • The running of the various kernel tasks

  50. Linux Scheduling (cont.) • Kernel tasks encompass both tasks • Tasks requested by a running process • Tasks that execute internally on behalf of the kernel itself, like tasks spawned by Linux’s I/O subsystem • Linux has two separate process-scheduling algorithms • One is a time-sharing algorithm for fair, preemptive scheduling among multiple processes • The other is designed for real-time tasks • Absolute priorities are more important than fairness • Earlier schedulers does not maintain fairness among interactive tasks • Particularly on systems like desktops and mobile devices

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