1 ADVANCED OPERATING SYSTEMS Lecture 7 (Week 9) Resource Management – I (Process Scheduling) by: Syed Imtiaz Ali

What is Process Scheduling? In a multiprogrammed enviornment, multiple processes or threads competing for the CPU at the same time. This situation occurs whenever two or more of them are simultaneously in the ready state. If only one CPU is available, a choice has to be made which process to run next. Part of operating system that makes choice is scheduler, and algorithm is scheduling algorithm. 2

Introduction to Scheduling (1) Process scheduling does not matter much on simple PCs When we turn to networked servers, the situation changes appreciably. Here multiple processes often do compete for the CPU, so scheduling matters. –For example, when the CPU has to choose between running a process that gathers the daily statistics and one that serves user requests, the users will be a lot happier if the latter gets first crack at the CPU. 3

Introduction to Scheduling (2) The scheduler also has to worry about: 1.Picking the right process to run 2.Making efficient use of the CPU because process switching is expensive. During process switching following activities occur: –Switch from user mode to kernel mode –State of the current process must be saved –New process must be selected by running the scheduling algorithm. –New process must be started. 4

5 Process Behavior (1) Bursts of CPU usage alternate with periods of I/O wait a)a CPU-bound process b)an I/O bound process

Process Behavior (2) Nearly all processes alternate bursts of computing with (disk) I/O requests Typically the CPU runs for a while without stopping, then a system call is made to read from a file or write to a file. When the system call completes, the CPU computes again until it needs more data or has to write more data, and so on. I/O in this sense is when a process enters the blocked state waiting for an external device to complete its work. 6

Process Behavior (3) Nearly all processes alternate bursts of computing with (disk) I/O requests Typically the CPU runs for a while without stopping, then a system call is made to read from a file or write to a file. When the system call completes, the CPU computes again until it needs more data or has to write more data, and so on. I/O in this sense is when a process enters the blocked state waiting for an external device to complete its work. 7

When to take a Scheduling Decision? (1) There are a variety of situations in which scheduling decision is needed. First, when a new process is created: –a decision needs to be made whether to run the parent process or the child process. Since both processes are in ready state, the scheduler can legitimately choose to run either the parent or the child next. Second, when a process exits: –That process can no longer run (since it no longer exists), so some other process must be chosen from the set of ready processes. »Continue 8

When to take a Scheduling Decision? (2) Third, when a process blocks on I/O: –another process has to be selected to run. –reason for blocking plays a role in the choice. For example, if A is an important process and it is waiting for B to exit its critical region, letting B run next will allow it to exit its critical region and thus let A continue. Fourth, when an I/O interrupt occurs: –It is up to the scheduler to decide to run the: newly ready process process that was running at the time of the interrupt (if the interrupt occurs to exit from I/O) some third process. 9

Non-preemptive Scheduling This algorithm picks a process to run and then just lets it run until: –it blocks (either on I/O or waiting for another process) –it voluntarily releases the CPU. In effect, no scheduling decisions are made during clock interrupts. After clock interrupt processing has been completed, the process that was running before the interrupt is resumed, unless a higher-priority process was waiting for a now- satisfied timeout. 10

Preemptive Scheduling Preemptive scheduling algorithm picks a process and lets it run for a maximum of some fixed time. If it is still running at the end of the time interval, it is suspended and the scheduler picks another process to run Doing preemptive scheduling requires, a clock interrupt at the end of the time interval to give control of the CPU back to the scheduler. If no clock is available, nonpreemptive scheduling is the only option. 11

Categories of Scheduling (1) In different environments different scheduling algorithms are needed. –Different application areas have different goals. –What the scheduler should optimize for is not the same in all systems. Three environments worth distinguishing are: 1. Batch. 2. Interactive. 3. Real time. 12

Categories of Scheduling (2) Batch & Interactive Systems In batch systems, there are no users impatiently waiting at their terminals for a quick response to a short request. –Nonpreemptive algorithms, or preemptive algorithms with long time periods for each process, are often acceptable. –This approach reduces process switches and thus improves performance. In interactive systems, preemption is essential to keep one process from hogging the CPU and denying service to the others. 13

Categories of Scheduling (3) Real-Time Systems In systems with real-time constraints, preemption is not needed –Processes know that they may not run for long periods of time and usually do their work and block quickly. The difference with interactive systems is that real-time systems run only programs that are intended to further the application at hand. –Interactive systems are general purpose and may run arbitrary programs that are not cooperative or even malicious. 14

15 Scheduling Algorithm Goals

Scheduling in Batch Systems (1) First-Come First-Served(1) The simplest of all scheduling algorithms is non-preemptive first-come first-served. There is a single queue of ready processes. When first job enters, it is started immediately and allowed to run as long as it wants to. –As other jobs come in, they are put onto the end of the queue. When the running process blocks, the first process on the queue is run next. –Once blocked process is ready, like a newly arrived job, it is put on the end of the queue. 16

Scheduling in Batch Systems (2) First-Come First-Served(2) The strength of this algorithm is that it is: –easy to understand –easy to program With this algorithm, a single linked list keeps track of all ready processes. Picking a process to run just requires removing one from the front of the queue. Adding a new job or unblocked process just requires attaching it to the end of the queue. 17

Scheduling in Batch Systems (3) First-Come First-Served(3) A powerful disadvantage. –Suppose that there is: many l/O-bound processes that use CPU for 1 msec but each have to perform 1000 disk reads to complete. one compute-bound process that runs for 1 sec/time –The compute-bound process runs for 1 sec, then it reads a disk block. All the I/O processes now run and start disk reads. –When the compute-bound process gets its disk block, it runs for another 1 sec, followed by all the I/O-bound processes in quick succession. >>Continue 18

Scheduling in Batch Systems (4) First-Come First-Served(4) The net result is that each I/O-bound process gets to read 1 block per second and will take 1001 sec to finish. With a scheduling algorithm that preempted the compute-bound process every 10 msec, the I/O-bound processes would finish in 11 sec instead of 1001 sec, and without slowing down the compute-bound process very much. 19

Scheduling in Batch Systems (5) Shortest Job First (1) Another non-preemptive batch algorithm that assumes the run times are known in advance. When several equally important jobs are sitting in the input queue waiting to be started, the scheduler picks the shortest job first. 20

Scheduling in Batch Systems (6) Shortest Job First (2) Look at Figure: –Four jobs A, B, C, and D with run times of 8,4,4, and 4 minutes, respectively. By running them in that order, the turnaround time for: –A is 8 minutes –C is 16 minutes 21 –B is 12 minutes –D is 20 minutes An average of 14 minutes.

Scheduling in Batch Systems (7) Shortest Job First (3) Now let us consider running these four jobs using shortest job first, as shown in Fig. (b). The turnaround times are now 4, 8, 12, and 20 minutes for an average of 11 minutes. Shortest job first is only optimal when all the jobs are available simultaneously. 22

Scheduling in Batch Systems (8) Shortest Remaining Time Next With this algorithm: –Scheduler always chooses the process whose remaining run time is the shortest. The run time has to be known in advance. When a new job arrives, its total time is compared to current process' remaining time. –If new job needs less time to finish than current process, it is suspended and new job started This scheme allows new short jobs to get good service. 23

Scheduling in Interactive Systems (1) Round-Robin Scheduling (1) One of the most widely used algorithms Each process is assigned a time interval, called its quantum –If the process is still running at the end of the quantum, the CPU is preempted and given to another process. –If the process has blocked or finished before the quantum has elapsed, the CPU switching is done 24

Scheduling in Interactive Systems (2) Round-Robin Scheduling (2) Round robin is easy to implement. –All the scheduler needs to do is maintain a list –When the process uses up its quantum, it is put on the end of the list 25 a)list of runnable processes b)list of runnable processes after B uses up its quantum

Scheduling in Interactive Systems (3) Round-Robin Scheduling (3) Issue with round robin is length of quantum Switching from one process to another requires a certain amount of time –Suppose that process switch takes 1 msec, –Suppose that the quantum is set at 4 msec. After doing 4 msec of useful work, CPU will have to spend 1 msec on process switching. 20% of the CPU time will be thrown away on administrative overhead (this is too much) 26

Scheduling in Interactive Systems (4) Round-Robin Scheduling (4) To improve the CPU efficiency: –we could set the quantum to, say, 100 msec. –Now the wasted time is only 1%. But consider what happens on a server system if 50 requests come in within a very short time –They will be put on list of runnable processes. –If the CPU is idle, the first one will start immediately, the second one may not start until 100 msec later, and so on. With short quantum they may have better service. 27

Scheduling in Interactive Systems (5) Round-Robin Scheduling (5) The conclusion can be formulated as follows: –setting the quantum too short causes too many process switches and lowers the CPU efficiency –setting it too long may cause poor response to short interactive requests. A quantum around msec is often a reasonable compromise. 28

Scheduling in Interactive Systems (6) Priority Scheduling (1) Round-robin scheduling thinks that all processes are equally important. The need to take external factors into account leads to priority scheduling. The basic idea is straightforward: –each process is assigned a priority, and runnable process with highest priority is allowed to run. –E.g. sending electronic mail has lower priority than a video film in real time. 29

Scheduling in Interactive Systems (7) Priority Scheduling (2) To prevent high-priority processes from running indefinitely: –the scheduler decreases the priority of the currently running process at each clock tick. –Alternatively, each process may be assigned a maximum time quantum that it is allowed to run. When this quantum is used up, the next highest priority process is given a chance to run. Priorities can be assigned to processes statically or dynamically. 30

Scheduling in Interactive Systems (8) Priority Scheduling (3) It is often convenient to group processes into priority classes and use priority scheduling among the classes but round-robin scheduling within each class. Figure shows a system with four priority classes. 31

Scheduling in Interactive Systems (8) Priority Scheduling (3) The scheduling algorithm is as follows: –In presence of runnable processes in priority class 4, just run each one for one quantum, round-robin fashion. –If priority class 4 is empty, then run the class 3 processes round robin. –If classes 4 and 3 are both empty, then run class 2 round robin, and so on. If priorities are not adjusted occasionally, lower priority classes may all starve to death. 32

33 Scheduling in Real-Time Systems (1) A real-time system is one in which time plays an essential role. –For example, the computer in a compact disc player gets the bits as they come off the drive and must convert them into music within a very tight time interval. –If the calculation takes too long, the music will sound peculiar. Other real-time systems are: –patient monitoring in a hospital ICU –the autopilot in an aircraft –robot control in an automated factory.

34 Scheduling in Real-Time Systems (2) Real-time systems ate generally categorized as: 1.Hard real time: –there are absolute deadlines that must be met, or else 2.Soft real time: –missing an occasional deadline is undesirable, but nevertheless tolerable. In both, real-time behavior is achieved by dividing program into a number of processes –When an external event is detected, it is job of the scheduler to schedule the processes in such a way that all deadlines are met.

35 Scheduling in Real-Time Systems (3) The events that a real-time system may have to respond to can be further categorized as: 1.Periodic (occurring at regular intervals) 2.Aperiodic (occurring unpredictably). If in a periodic real time systems –Given m periodic events event i occurs within period P i and requires C i seconds –Then the load can only be hand if –A real-time system that meets this criterion is said to be schedulable.

36 Scheduling in Real-Time Systems (4) As an example, consider soft real-time system: –Periodic events, m = 3 –These periods have 100, 200, and 500 msec –these events require 50, 30, and 100 msec of CPU time per event, respectively, Is this system is schedulable? –Yes! because 50/ / /500 = < 1. If a fourth event with a period of 1 sec is added, system schedulable as long as this event does not need more than 150 msec of CPU time per event.

37 Scheduling in Real-Time Systems (5) Real-time scheduling algorithms can be: 1.Static It makes their scheduling decisions before the system starts running. 2.Dynamic It makes their scheduling decisions at run time. Static scheduling only works when there is: –information available in advance about the work to be done and the deadlines that have to be met. Dynamic scheduling algorithms do not have these restrictions.