Lesson 3: Processes, Threads & CPU Scheduling

Transcription

1 Lesson 3: Processes, Threads & CPU Scheduling Contents The concept of computing Process Process states and life-cycle CPU Scheduling considerations Processes hierarchy Process creation and termination Inter-process communication schemes Synchronization among processes Threads, relationship to processes Multithreading models CPU Scheduling Criteria & Optimization Priority Scheduling Queuing and Queues Organization Deadline Real-Time CPU Scheduling AE3B33OSD Lesson 3/Page 2

2 Process Concept An operating system executes a variety of programs: Batch system jobs Time-shared systems user programs or tasks Textbooks use the terms job and process almost interchangeably Process is a program in execution; process execution must progress in sequential fashion A process includes: program counter stack data section code ( text ) controls the execution does not necessarily be a part of a single process can be shared AE3B33OSD Lesson 3/Page 3 Process States As a process executes, it changes its state new: The process is being created running: Instructions are being executed waiting: The process is waiting for some event to occur ready: The process is waiting to be assigned to a CPU terminated: The process has finished execution AE3B33OSD Lesson 3/Page 4

3 Process Control Block (PCB) Information associated with each process Process state Program counter CPU registers CPU scheduling information Memory-management information Accounting information I/O status information Environment variables AE3B33OSD Lesson 3/Page 5 CPU Switch from Process to Process Sometimes call context switch AE3B33OSD Lesson 3/Page 6

4 Process Scheduling Queues Why Scheduling? Necessity to decide which process can get CPU cycles Job queue set of all processes in the system Ready queue set of all processes residing in main memory, ready and waiting to execute Device queues set of processes waiting for an I/O device Processes migrate among the various queues AE3B33OSD Lesson 3/Page 7 Ready Queue and Various I/O Device Queues AE3B33OSD Lesson 3/Page 8

5 Simplified Model of Process Scheduling AE3B33OSD Lesson 3/Page 9 Schedulers Long-term scheduler (or job scheduler) selects which processes should be brought into the ready queue Long-term scheduler is invoked very infrequently (seconds, minutes) (may be slow) The long-term scheduler controls the degree of multiprogramming Short-term scheduler (or CPU scheduler) selects which process should be executed next and allocates CPU Short-term scheduler is invoked very frequently (milliseconds) (must be fast) Processes can be described as either: I/O-bound process spends more time doing I/O than computations, has many short CPU bursts CPU-bound process computationally intensive spends much time doing computations; few very long CPU bursts usually able to eat all CPU capacity AE3B33OSD Lesson 3/Page 10

6 Context Switch When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process Context-switch time is overhead; the system does no useful work while switching Should be very fast Time dependent on hardware support Hardware designers try to support routine context-switch actions like saving/restoring all CPU registers by one pair of machine instructions AE3B33OSD Lesson 3/Page 11 Process Creation Parent process create children processes Children, in turn create other processes, forming a tree of processes Resource sharing Parent and children share all resources Children share subset of parent s resources Parent and child share no resources Execution Parent and children can execute concurrently, or Parent can wait until children terminate Address space Child duplicate of parent Child has a program loaded into it POSIX examples fork system call creates new process exec system call used after a fork to replace the process memory space with a new program AE3B33OSD Lesson 3/Page 12

7 Process Creation Illustrated Tree of processes POSIX parent process waiting for its child to finish AE3B33OSD Lesson 3/Page 13 C Program Forking Separate Process int main() { Pid_t pid; /* fork another process */ pid = fork(); if (pid < 0) { /* error occurred */ fprintf(stderr, "Fork Failed"); exit(-1); } else if (pid == 0) { /* child process */ execl ("/bin/ls", "ls", NULL); } else { /* parent process */ /* parent will wait for the child to complete */ wait (NULL); printf ("Child Complete"); exit(0); } } AE3B33OSD Lesson 3/Page 14

8 Process Termination Process executes last statement and asks the operating system to delete it (_exit) Output data from child to parent (via wait) Process resources are deallocated by operating system Parent may terminate execution of children processes (abort) Child has exceeded allocated resources Task assigned to child is no longer required If parent is exiting Some operating system do not allow children to continue if the parent terminates the problem of zombie All children terminated - cascading termination AE3B33OSD Lesson 3/Page 15 Cooperating Processes Independent process cannot affect or be affected by the execution of another process Cooperating process can affect or be affected by the execution of another process Advantages of process cooperation Information sharing Computation speed-up Modularity Convenience Producer-Consumer Problem Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process unbounded-buffer places no practical limit on the size of the buffer bounded-buffer assumes that there is a fixed buffer size AE3B33OSD Lesson 3/Page 16

9 Inter-Process Communication (IPC) Mechanism for processes to communicate and to synchronize their actions Message system processes communicate with each other without resorting to shared variables IPC facility provides two operations: send(message) message size fixed or variable receive(message) If P and Q wish to communicate, they need to: establish a communication link between them exchange messages via send/receive Implementation of communication link physical (e.g., shared memory, hardware bus) logical (e.g., logical properties) AE3B33OSD Lesson 3/Page 17 Direct & Indirect Communication Direct Communication Processes must name each other explicitly: send (P, message) send a message to process P receive(q, message) receive a message from process Q Properties of communication link Links are established automatically A link is associated with exactly one pair of communicating processes Between each pair there exists exactly one link The link may be unidirectional, but is usually bi-directional Indirect Communication Messages are directed and received from mailboxes (also referred to as ports) Each mailbox has a unique id and is created by the kernel on request Processes can communicate only if they share a mailbox Properties of communication link Link established only if processes share a common mailbox A link may be associated with many processes Each pair of processes may share several communication links Link may be unidirectional or bi-directional AE3B33OSD Lesson 3/Page 18

10 Process Synchronization Message passing may be either blocking or nonblocking Blocking is considered synchronous Blocking send: the sender blocks until the message is received by the other party Blocking receive: the receiver block until a message is available Non-blocking is considered asynchronous Non-blocking send: the sender sends the message and continues executing Non-blocking receive: the receiver gets either a valid message or a null message (when nothing has been sent to the receiver) Often a combination is used: Non-blocking send and blocking receive AE3B33OSD Lesson 3/Page 19 Single and Multithreaded Processes Benefits of Multithreading Responsiveness Easy Resource Sharing Economy Utilization of Multi-processor Architectures (?) AE3B33OSD Lesson 3/Page 20

11 User & Kernel Threads User-level Threads All thread management done by user-level threads library application programmer can influence the scheduling strategies Kernel knows nothing about threads (considers only processes) OSK cannot manage and schedule threads Three primary thread libraries: POSIX Pthreads Win32 threads Java threads Kernel-level Threads Threads supported by the OS Kernel Examples Windows XP/2000 Linux Mac OS X Tru64 UNIX AE3B33OSD Lesson 3/Page 21 Multithreading Models Many-to-One Many user-level threads mapped to single kernel thread Examples: Solaris Green Threads POSIX Threads (PThreads) One-to-One Each user-level thread maps to kernel thread Examples Windows NT/XP/2000 Linux PThreads if OSK supports multi-threading Many-to-Many Complicated, with few benefits, seldom used AE3B33OSD Lesson 3/Page 22

12 Basic Concepts of CPU Scheduling 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 distribution AE3B33OSD Lesson 3/Page 23 CPU Scheduler Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state 2. Switches from waiting to ready 3. Switches from running to ready state 4. Terminates Switches under 1, 2 and 4 are non-preemptive If switch No. 3 is included, scheduling is preemptive Any of these situations is caused by some interrupt scheduler is interrupt driven AE3B33OSD Lesson 3/Page 24

13 Process state diagram revisited r New s Ready Running x Terminated p e w Waiting CPU short-time Scheduler Transition s r w e x p Process is created start Meaning Process gets CPU (can execute) run Process requests a service and must wait for its completion An event causing service completion occurred the process can continue Process terminates (either by itself or by abort ) exit Process forcibly removed from a CPU even though it is capable further execution (e.g., time quantum exhausted) preemption. This transition does NOT exist in non-preemptive scheduling! AE3B33OSD Lesson 3/Page 25 Dispatcher Dispatcher module gives control of the CPU to the process (thread) selected by the short-term 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 overhead AE3B33OSD Lesson 3/Page 26

14 Scheduling Criteria & Optimization CPU utilization keep the CPU as busy as possible Maximize CPU utilization Throughput # of processes that complete their execution per time unit Maximize throughput Turnaround time amount of time to execute a particular process Minimize turnaround time Waiting time amount of time a process has been waiting in the ready queue Minimize waiting time Response time amount of time it takes from when a request was submitted until the first response is produced, not output (for timesharing and interactive environment ) Minimize response time AE3B33OSD Lesson 3/Page 27 First-Come, First-Served (FCFS) Scheduling Process Burst Time P 1 24 P 2 3 P 3 3 Suppose that the processes arrive in the order: P 1, P 2, P 3 The Gantt Chart for the schedule is: P 1 P 2 P Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 = 17 AE3B33OSD Lesson 3/Page 28

15 FCFS Scheduling (Cont.) Suppose that the processes arrive in the order P 2, P 3, P 1 The Gantt chart for the schedule is: P 2 P 3 P Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3 Average waiting time: ( )/3 = 3 Much better than previous case Convoy effect short process behind long process AE3B33OSD Lesson 3/Page 29 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 Two schemes: nonpreemptive once CPU given to the process it cannot be preempted until completes its CPU burst preemptive if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF) SJF is optimal gives minimum average waiting time for a given set of processes AE3B33OSD Lesson 3/Page 30

16 Example of Non-Preemptive SJF Process Arrival Time Burst Time P P P P SJF (non-preemptive) P 1 P 3 P 2 P Average waiting time = ( )/4 = 4 AE3B33OSD Lesson 3/Page 31 Example of Preemptive SJF Process Arrival Time Burst Time P P P P SJF (preemptive) P 1 P 2 P 3 P 2 P 4 P Average waiting time = ( )/4 = 3 16 AE3B33OSD Lesson 3/Page 32

17 Problem: Determining Length of Next CPU Burst Can only estimate the length Can be done by using the length of previous CPU bursts, using exponential averaging t n = actual lenght of n τ n + 1 CPU burst = predicted value for the next CPU 3. α, 0 α 1 4. Define : τ n+ 1 = α tn + 1 α τ n th ( ). burst AE3B33OSD Lesson 3/Page 33 α =0 Examples of Exponential Averaging τ n+1 = τ n History does not count at all α =1 τ n+1 = α t n Only the actual last CPU burst counts If we expand the formula, we get: τ n+1 = α t n +(1 - α)α t n (1 - α ) j α t n -j + +(1 - α ) n +1 τ 0 Since both α and (1 - α) are less than or equal to 1, each successive term has less weight than its predecessor AE3B33OSD Lesson 3/Page 34

18 Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (usually smallest integer highest priority) Preemptive Nonpreemptive SJF is a priority scheduling where the priority is the predicted next CPU burst time Problem: Starvation low priority processes may never execute Solution: Aging as time progresses increase the priority of the process AE3B33OSD Lesson 3/Page 35 Round Robin (RR) Each process gets a small unit of CPU time (time quantum), usually 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. Performance how to choose q? q large FCFS q small q must be large with respect to context switch, otherwise overhead is too high AE3B33OSD Lesson 3/Page 36

19 Example of RR with Time Quantum = 20 The Gantt chart is: Process Burst Time P 1 53 P 2 17 P 3 68 P 4 24 P 1 P 2 P 3 P 4 P 1 P 3 P 4 P 1 P 3 P Typically, higher average turnaround then SJF, but better response AE3B33OSD Lesson 3/Page 37 Time Quantum and Context Switch Time Turnaround Time Varies With the Time Quantum AE3B33OSD Lesson 3/Page 38

20 Multilevel Queue Ready queue is partitioned into separate queues: foreground (interactive) background (batch) Each queue has its own scheduling algorithm foreground RR background FCFS Scheduling must be done between the queues Fixed priority scheduling; (i.e., serve all from foreground then from background). Danger of starvation. Time slice each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR 20% to background in FCFS AE3B33OSD Lesson 3/Page 39 Multilevel Queue Scheduling AE3B33OSD Lesson 3/Page 40

21 Multilevel Feedback Queue A process can move between the various queues aging can be treated this way 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 AE3B33OSD Lesson 3/Page 41 Example of Multilevel Feedback Queue Three queues: Q 0 RR with time quantum 8 milliseconds Q 1 RR time quantum 16 milliseconds Q 2 FCFS Scheduling A new job enters queue Q 0. When it gains CPU, job receives 8 milliseconds. If it exhausts 8 milliseconds, job is moved to queue Q 1. At Q 1 the job receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2. AE3B33OSD Lesson 3/Page 42

22 Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available Multiple-Processor Scheduling has to decide not only which process to execute but also where (i.e. on which CPU) to execute it Homogeneous processors within a multiprocessor Asymmetric multiprocessing only one processor accesses the system data structures alleviating the need for data sharing Symmetric multiprocessing (SMP) each processor is self-scheduling, all processes in common ready queue, or each has its own private queue of ready processes Processor affinity process has affinity for the processor on which it has been recently running Reason: Some data might be still in cache Soft affinity is usually used the process can migrate among CPUs AE3B33OSD Lesson 3/Page 43 Thread Scheduling Local Scheduling: How the threads library decides which thread to put onto an available LWP Global Scheduling: How the kernel decides which kernel thread to run next Pthreads library has calls to choose different scheduling policies and parameters AE3B33OSD Lesson 3/Page 44

23 Windows XP Priorities Priority classes (assigned to each process) Relative priorities within each class Relative priority normal is a base priority for each class starting priority of the thread When the thread exhausts its quantum, the priority is lowered When the thread comes from a wait-state, the priority is increased depending on the reason for waiting A thread released from waiting for keyboard or mouse gets more boost than a thread having been waiting for disk I/O AE3B33OSD Lesson 3/Page 45 Linux Scheduling Two algorithms: time-sharing and real-time Time-sharing Prioritized credit-based process with most credits is scheduled next Credit subtracted when timer interrupt occurs When credit = 0, another process chosen When all processes have credit = 0, recrediting occurs Based on factors including priority and history Real-time Soft real-time POSIX.1b compliant two classes FCFS and RR Highest priority process always runs first AE3B33OSD Lesson 3/Page 46

24 Real-Time Systems A real-time system requires that results be not only correct but in time produced within a specified deadline period An embedded system is a computing device that is part of a larger system automobile, airliner, dishwasher,... A safety-critical system is a real-time system with catastrophic results in case of failure e.g., railway traffic control system A hard real-time system guarantees that real-time tasks be completed within their required deadlines mainly single-purpose systems A soft real-time system provides priority of real-time tasks over non real-time tasks a standard computing system with a real-time part that takes precedence AE3B33OSD Lesson 3/Page 47 Real-Time CPU Scheduling Periodic processes require the CPU at specified intervals (periods) p is the duration of the period d is the deadline by when the process MUST be serviced (must finish within d) often equal to p t is the processing time AE3B33OSD Lesson 3/Page 48

25 Scheduling of N Tasks Can be scheduled if r CPU utilization r N = i= 1 ti p i 1 (N = number of processes) Process P 1 : service time = 20, period = 50, deadline = 50 Process P 2 : service time = 35, period = 100, deadline = 100 r = + = < schedulable When P 2 has a higher priority than P 1, a failure occurs: AE3B33OSD Lesson 3/Page 49 Rate Monotonic Scheduling (RMS) A process priority is assigned based on the inverse of its period Shorter periods = higher priority; Longer periods = lower priority P 1 is assigned a higher priority than P 2. Process P 1 : service time = 20, period = 50, deadline = 50 Process P 2 : service time = 35, period = 100, deadline = 100 works well AE3B33OSD Lesson 3/Page 50

26 Missed Deadlines with RMS failure Process P 1 : service time = 25, period = 50, deadline = 50 Process P 2 : service time = 35, period = 80, deadline = r = + = 0,9375 < 1 schedulable RMS is guaranteed to work if N = number of processes sufficient condition r = lim N N i= 1 N ti p i N N ( 2 1) N ( 2 1) = ln ; N N N ( 2 1 ) 2 0, , , , , , AE3B33OSD Lesson 3/Page 51 Earliest Deadline First (EDF) Scheduling Priorities are assigned according to deadlines: the earlier the deadline, the higher the priority; the later the deadline, the lower the priority. Process P 1 : service time = 25, period = 50, deadline = 50 Process P 2 : service time = 35, period = 80, deadline = 80 Works well even for the case when RMS failed PREEMPTION may occur AE3B33OSD Lesson 3/Page 52

27 RMS and EDF Comparison RMS: Deeply elaborated algorithm Deadline guaranteed if the condition is satisfied (sufficient condition) Used in many RT OS r N ( 2 1) N EDF: Periodic processes deadlines kept even at 100% CPU load Consequences of the overload are unknown and unpredictable When the deadlines and periods are not equal, the behaviour is unknown AE3B33OSD Lesson 3/Page 53 End of Lesson 3