Chapter-2Process ManagementProcess Concept When the OS runs a program (i.e., a binary executable), this program is loaded into memory and the control is transferred to this programs first instruction. Then, the program starts to run.

A process is a program in execution.

A process is more than a program, because the former has a program counter, stack, data section and so on (i.e., the runtime stuffs).

Moreover, multiple processes may be associated with one program (e.g., runs the same program, a web browser, twice).

An operating system executes a variety of programs: Batch system jobs Time-shared systems user programs or tasks

(Process in Memory)

Process StateAt any moment, a process can be in one of the five states: new, running, waiting, ready and terminated.

1. New: The process is being created2. Running: The process is being executed3. Waiting: The process is waiting for some event to occur (e.g., waiting for I/O completion)4. Ready: The process is waiting to be assigned to a processor.5. Terminated: The process has finished execution.

(Process State Diagram)Process Control Block

Information associated with each process Process state: The state may be new, ready running, waiting, halted, and so on. Program counter: The counter indicates the address of the next instruction to be executed for this process. CPU registers: The registers vary in number and type, depending on the computer architecture. They include accumulators, index registers, stack pointers, and general-purpose registers, plus any condition-code information. Along with the program counter, this state information must be saved when an interrupt occurs, to allow the process to be continued correctly afterward CPU scheduling information: This information includes a process priority, pointers to scheduling queues, and any other scheduling parameters. Memory-management information: This information may include such information as the value of the base and limit registers, the page tables, or the segment tables, depending on the memory system used by the operating system Accounting information: This information includes the amount of CPU and real time used, time limits, account numbers, job or process numbers, and so on. I/O status information: This information includes the list of I/O devices allocated to the process, a list of open files, and so on.

ThreadsA process is a program that performs a single thread of execution.

Process Scheduling

Since the number of processes is always larger than the number of CPUs, the OS must maintain maximum CPU utilization.

To determine which process can do what, processes are chained into a number of Scheduling queues.

For example, in addition to the ready queue, each event may have its own scheduling queue (i.e., waiting queue).

Process Scheduling Queues

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

Ready Queue And Various I/O Device Queues

Queuing Diagram for Scheduling

SchedulersThere are three types of schedulers

1. Long-Term (Job) Scheduler: Selects which processes should be brought into the ready queue. Invoked very infrequently (seconds, minutes); may be slow. controls the degree of multiprogramming2. Short-Term (CPU) scheduler: Selects which process should execute next and allocates CPU. invoked very frequently (milliseconds) - must be very fast

3. Medium-Term Scheduler: swaps out process temporarily balances load for better throughput

Addition of Medium Term Scheduling

Short-term scheduler is invoked very frequently (milliseconds) (must be fast) Long-term scheduler is invoked very infrequently (seconds, minutes) (may be slow) The long-term scheduler controls the degree of multiprogramming Processes can be described as either: I/O-bound process spends more time doing I/O than computations, many short CPU bursts CPU-bound process spends more time doing computations; few very long CPU bursts

Context Switch

What is a process context? The context of a process includes the values of CPU registers, the process state, the program counter, and other memory/file management information.

What is a context switch? After the CPU scheduler selects a process (from the ready queue) and before allocates CPU to it, the CPU scheduler must save the context of the currently running process, put it into a queue, load the context of the selected process, and Let it run.

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 Time dependent on hardware support

Operations on Processes

There are three commonly seen operations: Process Creation: Create a new process. The newly created is the child of the original. Use fork() or vfork() in Unix to create new processes. Process Termination: Terminate the execution of a process. Under Unix, use exit(). Process Join: Wait for the completion of a child process. Under Unix use wait(). fork(), vfork(), exit() and wait() are system calls. Use ps aux to see all running processes

Process Creation Parent process create children processes, which, in turn create other processes, forming a tree of processes

Resource sharing Parent and children share all resources Children share subset of parents resources Parent and child share no resources Execution Parent and children execute concurrently Parent waits until children terminate Address space Child duplicate of parent Child has a program loaded into it UNIX examples fork system call creates new process exec system call used after a fork to replace the process memory space with a new program

(Process creation using fork() system call.)

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 */execlp("/bin/ls", "ls", NULL);}else { /* parent process *//* parent will wait for the child to complete */wait (NULL);printf ("Child Complete");exit(0);}}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 child to continue if its parent terminates All children terminated - cascading termination

Interprocess Communications (IPC)

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

Cooperating Processes Importance

Information sharing: Multiple processes can use the same set of information (e.g., files). Computation Speedup: One may split a process into multiple processes to use multiple processors. Modularity: A system can be divided into separate processes. Use the ps command to see how many processes are not yours! Convenience: A user may have multiple tasks to work at the same time (e.g., editing, browsing, printing).However, handling cooperating processes is difficult.

Cooperating processes must communicate to get the job done. There are two types of communications:a) Processes that share the same memory: locks, semaphores, monitors, and others.(Shared Systems)b) Processes that are running in a distributed environment: message passing, sockets, remote procedure calls (RPC).(Message Passing) 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:a) send(message) message size fixed or variable b) receive(message) If P and Q wish to communicate, they need to:a) establish a communication link between themb) exchange messages via send/receive Implementation of communication linka) physical (e.g., shared memory, hardware bus)b) logical (e.g., logical properties)

(a) Message passing (b) Shared System

Shared Memory System

Message passing systemMessage Passing system are divided into three types:

Naming

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-directionalIndirect Communication

Messages are directed and received from mailboxes (also referred to as ports) Each mailbox has a unique id 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 Operations create a new mailbox send and receive messages through mailbox destroy a mailbox Primitives are defined as: send(A, message) send a message to mailbox A receive(A, message) receive a message from mailbox A Mailbox sharing P1, P2, and P3 share mailbox A P1, sends; P2 and P3 receive Who gets the message? Solutions Allow a link to be associated with at most two processes Allow only one process at a time to execute a receive operation Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was.

Indirect Communication using mailboxes

Synchronization

Message passing may be either blocking or non-blocking Blocking is considered synchronous Blocking send has the sender block until the message is received Blocking receive has the receiver block until a message is available Non-blocking is considered asynchronous Non-blocking send has the sender send the message and continue Non-blocking receive has the receiver receive a valid message or null

Buffering

Queue of messages attached to the link; implemented in one of three ways1.Zero capacity 0 messages Sender must wait for receiver (rendezvous)2.Bounded capacity finite length of n messages Sender must wait if link full3.Unbounded capacity infinite length Sender never waits

Process Scheduling: Basic concepts

Maximum CPU utilization obtained with multiprogramming CPUI/O Burst Cycle Process execution consists of a cycle of CPU execution and I/O wait CPU burst distribution

CPU-I/O Burst Cycle

Alternating Sequence of CPU and I/O Bursts

Process execution repeats the CPU burst and I/O burst cycle. When a process begins an I/O burst, another process can use the CPU for a CPU burst.

Histogram of CPU-burst durations.The durations of CPU bursts have been measured extensively. Although they vary greatly from process to process and from computer to computer they tend to have a frequency curve similar to that shown in the above Figure. The curve is generally characterized as exponential or hyper exponential, 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 might have a few long CPU bursts. This distribution can be important in the selection of an appropriate CPU-scheduling algorithm.

CPU-bound and I/O-bound A process is CPU-bound if it generates I/O requests infrequently, using more of its time doing computation. A process is I/O-bound if it spends more of its time to do I/O than it spends doing computation. A CPU-bound process might have a few very long CPU bursts. An I/O-bound process typically has many short CPU bursts.

Levels of Scheduling High Level Scheduling or Job Scheduling Selects jobs allowed to compete for CPU and other system resources. Intermediate Level Scheduling or Medium Term Scheduling Selects which jobs to temporarily suspend/resume to smooth fluctuations in system load. Low Level (CPU) Scheduling or Dispatching Selects the ready process that will be assigned the CPU Ready Queue contains PCBs of processes.

CPU Scheduler What does a CPU scheduler do? When the CPU is idle, the OS must select another process to run. This selection process is carried out by the short-term scheduler (or CPU scheduler). The CPU scheduler selects a process from the ready queue, and allocates the CPU to it. The ready queue does not have to be a FIFO one. There are many ways to organize the ready queue.

Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them. Non-preemptive Scheduling Once CPU has been allocated to a process, the process keeps the CPU until Process exits OR Process switches to waiting state Preemptive Scheduling Process can be interrupted and must release the CPU. Need to coordinate access to shared data CPU scheduling decisions may take place when a process: switches from running state to waiting state switches from running state to ready state switches from waiting to ready terminates Scheduling under 1 and 4 is non-preemptive. All other scheduling is preemptive.

Preemptive vs. Non-preemptive Non-preemptive scheduling: scheduling occurs when a process voluntarily enters the wait state (case 1) or terminates (case 4). Simple, but very inefficient Preemptive scheduling: scheduling occurs in all possible cases. What if the kernel is in its critical section modifying some important data? Mutual exclusion may be violated. The kernel must pay special attention to this situation and, hence, is more complex.

DispatcherScheduling Flow and Dispatcher

The dispatcher is the last step in scheduling. It Switches context Switches to user mode Braches to the stored program counter to resume the programs execution.

Dispatcher module gives control of the CPU to the process selected by the short-term scheduler Dispatch Latency: time it takes for the dispatcher to stop one process and start another running. Or the time to switch two processes. Dispatcher must be fast.

Scheduling CriteriaThere are many criteria for comparing different scheduling algorithms. Here are five common ones:

1. CPU Utilization2. Throughput3. Turnaround Time4. Waiting Time5. Response Time

CPU Utilization: Keep the CPU and other resources as busy as possible CPU utilization ranges from 0 to 100 percent. Normally 40% is lightly loaded and 90% or higher is heavily loaded.

Throughput: Number of processes that complete their execution per time unit. Higher throughput means more jobs get done. However, for long processes, this rate may be one job per hour, and, for short (student) jobs, this rate may be 10 per minute.

Turnaround time: Amount of time to execute a particular process from its entry time. From a users point of view, turnaround time is more important than CPU utilization and throughput. Turnaround time is the sum of waiting time before entering the system waiting time in the ready queue waiting time in all other events (e.g., I/O) time the process actually running on the CPU

Waiting time : Amount of time a process has been waiting in the ready queue. Why only ready queue? CPU scheduling algorithms do not affect the amount of time during which a process is waiting for I/O and other events. However, CPU scheduling algorithms do affect the time that a process stays in the ready queue.

Response Time (in a time-sharing environment) amount of time it takes from when a request was submitted until the first response is produced, NOT output. For example: in front of your workstation, you perhaps care more about the time between hitting the Return key and getting your first output than the time from hitting the Return key to the completion of your program (e.g., turnaround time).

Optimization Criteria1. Max CPU utilization2. Max throughput3. Min turnaround time 4. Min waiting time 5. Min response timeScheduling AlgorithmsWe will discuss a number of scheduling algorithms:1. First-Come, First-Served (FCFS)2. Shortest-Job-First (SJF)3. Priority4. Round-Robin5. Multilevel Queue6. Multilevel Feedback Queue

1. First-Come, First-Served: Policy: Process that requests the CPU FIRST is allocated the CPU FIRST. FCFS is a non-preemptive algorithm. Implementation - using FIFO queues Incoming process is added to the tail of the queue. Process selected for execution is taken from head of queue. Performance metric - Average waiting time in queue. FCFS is not preemptive. Once a process has the CPU, it will occupy the CPU until the process completes or voluntarily enters the wait state.

Example

Process Burst TimeP124P2 3P3 3Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is:

Waiting time for P1 = 0; P2 = 24; P3 = 27Average waiting time: (0 + 24 + 27)/3 = 17

Example 2Suppose that the processes arrive in the orderP2 , P3 , P1Then Gantt chart for the schedule is:

Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3Much better than previous caseConvoy effect short process behind long process

FCFS: Problems It is easy to have the convoy effect: all the processes wait for the one big process to get off the CPU. CPU utilization may be low. Consider a CPU-bound process running with many I/O-bound process. It is in favor of long processes and may not be fair to those short ones. What if your 1-minute job is behind a 10-hour job? It is troublesome for time-sharing systems, where each user needs to get a share of the CPU at regular intervals.

Shortest-Job-First (SJF) Scheduling

Each process in the ready queue is associated with the length of its next CPU burst. When a process must be selected from the ready queue, the process with the smallest next CPU burst is selected. Thus, the processes in the ready queue are sorted in CPU burst length. Two schemes: non preemptive 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 known as the Shortest-Remaining-Time-First (SRTF) SJF is optimal gives minimum average waiting time for a given set of processes

Example of Non-Preemptive SJF

Process Arrival TimeBurst Time P1 0.07 P22.04 P34.01 P45.04SJF (non-preemptive)

Average waiting time = (0 + 6 + 3 + 7)/4 = 4 Example for Preemptive SJFIf the process arrive in this order:ProcessArrivalTimeBurstTime

A08

B14

C29

D35

How the Scheduling done:

Gang chart

Avg wait =(9+0+15+2)/4 =26/4 =6.5

Other way: (10- 1) + (1 - 1) + (17- 2) +(5-3)]/ 4 = 26/4 = 6.5

How do we know the next CPU burst? Without a good answer to this question, SJF cannot be used for CPU scheduling.qWe try to predict the next CPU burst! Let tn be the length of the nth CPU burst and pn+1 be the prediction of the next CPU burst where a is a weight value in [0,1]. If a = 0, then pn+1 = pn and recent history has no effect. If a = 1, only the last burst matters. If a is , the actual burst and predict values are equally important.

Estimating the next burst: Example: Initially, we have to guess the value of p1 because we have no history value. The following is an example with a = .

Define

SJF Problems: It is difficult to estimate the next burst time value accurately. SJF is in favor of short jobs. As a result, some long jobs may not have a chance to run at all. This is starvation. The preemptive version is usually referred to as shortest-remaining-time-first scheduling, because scheduling is based on the remaining time of a process.

Priority Scheduling

A priority number (integer) is associated with each process. Can be based on Cost to user Importance to user Aging %CPU time used in last X hours.

The CPU is allocated to the process with the highest priority (smallest integer highest priority) Preemptive nonpreemptive SJF is a priority scheduling where 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 The scheduler always picks the process (in ready queue) with the highest priority to run.

Priority Scheduling: ExampleFour jobs A, B, C and D come into the system in this order at about the same time. Subscripts are priority. Smaller means higher.

Then:

Average wait time = (0+7+17+23)/4 = 47/4 = 11.75Average turnaround time= (7+17+23+28)/4 = 75/4 = 18.75

Priority Scheduling: Starvation

Priority scheduling can be non-preemptive or preemptive. With preemptive priority scheduling, if the newly arrived process has a higher priority than the running one, the latter is preempted. Indefinite block (or starvation) may occur: a low priority process may never have a chance to run.

Priority Scheduling: Aging Aging is a technique to overcome the starvation problem. Aging: gradually increases the priority of processes that wait in the system for a long time. Example: If 0 is the highest (resp., lowest) priority, then we could decrease (resp., increase) the priority of a waiting process by 1 every fixed period (e.g., every minute).

Round-Robin (RR) Scheduling

RR is similar to FCFS, except that each process is assigned a time quantum. All processes in the ready queue is a FIFO list. When the CPU is free, the scheduler picks the first and lets it run for one time quantum. If that process uses CPU for less than one time quantum, it is moved to the tail of the list. Otherwise, when one time quantum is up, that process is preempted by the scheduler and moved to the tail of the list.

Example of RR with Time Quantum = 20

Process Burst TimeP153P217 P368 P4 24The Gantt chart is:

Typically, higher average turnaround than SJF, but better response

Other Examples:

Third example:

Time Quantum and Context Switch Time

Turnaround Time Varies With The Time Quantum

RR Scheduling: Some Issues If time quantum is too large, RR reduces to FCFS If time quantum is too small, RR becomes processor sharing Context switching may affect RRs performance Shorter time quantum means more context switches Turnaround time also depends on the size of time quantum. In general, 80% of the CPU bursts should be shorter than the time quantumMultilevel Queue Scheduling

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). Possibility 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 A priority is assigned to each queue. A higher priority process may preempt a lower priority process.

A process P can run only if all queues above the queue that contains P are empty. When a process is running and a process in a higher priority queue comes in, the running process is preempted.

Multilevel Feedback Queue

Multilevel queue with feedback scheduling is similar to multilevel queue; however, it allows processes to move between queues. If a process uses more (resp., less) CPU time, it is moved to a queue of lower (resp., higher) priority. As a result, I/O-bound (resp., CPU-bound) processes will be in higher (resp., lower) priority 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

Multilevel Feedback Queues

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 job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.

At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.