Unit 6 Operating System and Real Time Programme

6.1 Operating System An “operating system” is piece of software that acts as a layer between the

computer hardware and the application software. It enables the computer hardware to communicate and operate with the computer software. Without a computer operating system, a computer would be useless.

Some people broaden their definition of operating system to include the supporting applications. In this case the core piece is known as the kernel.

Operating system types

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As computers have progressed and developed so have the types of operating systems. Below is a basic list of the different types of operating systems and a few examples of operating systems that fall into each of the categories. Many computer operating systems will fall into more than one of the below categories.

a) GUI - Short for Graphical User Interface, a GUI Operating System contains graphics and icons and is commonly navigated by using a computer mouse Below are some examples of GUI Operating Systems.

System 7.xWindows 98Windows CE

b) Multi-user - A multi-user operating system allows for multiple users to use the same computer at the same time and/or different times. Below are some examples of multi-user operating systems.

LinuxUnixWindows 2000

c) Multiprocessing - An operating system capable of supporting and utilizing more than one computer processor. Below are some examples of multiprocessing operating systems.

LinuxUnixWindows 2000

Explanation:

Multiprocessing refers to an operating situation where the simultaneous processing of programs takes place. This state of ongoing and coordinated processing is usually achieved by interconnecting two or more computer processors that make it possible to use the available resources to best advantage. Many operating systems today are equipped with a multiprocessing capability, although multiprogramming tends to be the more common approach today.

The basic platform for multiprocessing allows for more than one computer to be engaged in the used of the same programs at the same time. This means that persons working at multiple work stations can access and work with data contained within a given program. It is this level of functionality that makes it possible for users in a work environment to effectively interact via a given program.

There are essentially two different types of multiprocessing. Symmetric multiprocessing, more than one computer processor will share memory capacity and data path protocols. While the process may involve more than one computer station, only one copy or the operating system will be used to initiate all the orders executed by the processors involved in the connection.

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The second approach to multiprocessing is known as massively parallel processing. Within this structure, it is possible harness and make use of large numbers of processors in order to handle tasks. Often, this type of multiprocessing will involve over two hundred processors. Within the environment of MPP, each processor works with individual operating systems and memory resources, but will connect with the other processors in the setup to divide tasks and oversee different aspects of transmissions through data paths.

Multiprocessing is a common situation with corporations that function with multiple locations and a large number of employees. The combination of resources that can result from the use of multiple computer processors make it possible to transmit data without regard to distance or location, as well as allow large numbers of users to work with a program simultaneously. While the actual creation of a multiprocessing system can be somewhat complicated, the approach ultimately saves a great deal of time and money for larger companies.

What is multiprogramming?

Multiprogramming is one of the more basic types of parallel processing that can be employed in many different environments. Essentially, multiprogramming makes it possible for several programs to be active at the same time, while still running through a single processor. The functionality of multiprogramming in this environment involves a continual process of sequentially accomplishing tasks associated with the function of one program, then moving on to run a task associated with the next program.

Multiprogramming is very different from the multiprocessing because even though there may be several programs currently active, the uniprocessor is not simultaneously executing commands for all the programs. Instead, the processor addresses each program, executes a single command, then moves on to the next program in the queue. The previous program remains active, but enters into a passive state until the uniprocessor returns to the front of the queue and executes a second command.

From an end user standpoint, the process of multiprogramming is seamless. As far as actual functionality, the user appears to be using several different applications at the same time. This is because multiprogramming utilizes the uniprocessor to execute commands quickly. The end result is that a user notices little if any lag time when minimizing one application in order to perform a task associated with a different application.

The mechanism within multiprogramming is known as an interrupt. Each task is granted a specific amount of time for processing before the operating systems will move on to the next program and the next task. In a sense, multiprogramming is about juggling several tasks at one time, quickly performing one piece of the required action, then moving to do something with a different task before returning to the former job.

Memory is important to the proper function of multiprogramming. Capacity should be ample enough to ensure that if one program within the rotating queue encounters a problem, it does not prevent delays or impact the operation of other open applications. At the same time, some type of memory protection should be in place. If this is not the case, then a problem with one application can create a cascading effect that shuts down or at least slows down the other open applications.

d) Multitasking - An operating system that is capable of allowing multiple software processes to run at the same time. Below are some examples of multitasking operating systems.

UnixWindows 2000

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Explanation:

Multitasking is the act of doing multiple things at once. It is often encouraged among office workers and students, because it is believed that multitasking is more efficient than focusing on a single task at once. Numerous studies on multitasking have been carried out, with mixed results. It would appear that in some cases, multitasking is indeed an effective way to utilize time, while in other instances, the quality of the work suffers as a result of split attention.

The term initially emerged in the tech industry, to describe a computer's single central processing unit performing multiple tasks. Early computers were capable of performing only one function at once, although sometimes very quickly. Later computers were able to run a wide assortment of programs; in fact, your computer is multitasking right now as it runs your web browser and any other programs you might have open, along with the basic programs which start every time you log on to your operating system.

In the late 1990s, people began to use “multitasking” to describe humans, especially in office environments. A secretary might be said to be multitasking when she or he answers phones, responds to emails, generates a report, and edits a form letter simultaneously. The ability of the human mind to focus on multiple tasks at once is rather amazing; the American Psychological Association calls this the “executive control” of the brain. The executive control allows the brain to delegate tasks while skimming material and determining the best way to process it.

While accomplishing multiple things at once appears more efficient on the surface, it can come with hidden costs. Certain complex higher order tasks, for example, demand the full function of the brain; most people wouldn't want brain surgeons multitasking, for example. Insufficient attention can cause errors while multitasking, and switching between content and different media formats can have a detrimental effect as well.

A certain amount of multitasking has become necessary and expected in many industries, and job seekers often list the ability to multitask as a skill on their resumes. Students also find this skill very valuable, since it allows them to take notes while processing lecture information, or work on homework for one course while thinking about another. When you do decide to multitask, make sure to check your work carefully, to ensure that it is of high quality, and consider abandoning multitasking for certain tasks if you notice a decline.

e) Multithreading - Operating systems that allow different parts of a software program to run concurrently. Operating systems that would fall into this category are:

LinuxUnixWindows 2000

Explanation:

In the world of computing, multithreading is the task of creating a new thread of execution within an existing process rather than starting a new process to begin a function. Essentially, the task of multithreading is intended to make wiser use of computer resources by allowing resources that are already in use to be simultaneously utilized by a slight variant of the same process. The basic concept of multithreading has been around for some time, but gained wider attention as computers became more commonplace during the decade of the 1990’s.

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This form of time-division multiplexing creates an environment where a program is configured to allow processes to fork or split into two or more threads of execution. The parallel execution of threads within the same program is often touted as a more efficient use of the resources of the computer system, especially with desktop and laptop systems. By allowing a program to handle multiple tasks with a multithreading model, the system does not have to allow for two separate programs to initiate two separate processes and have to make use of the same files at the same time.

While there are many proponents of multithreading, there are also those that understand the process as being potentially harmful to the task of computing. The time slicing that is inherent in allowing a fork or thread to split off from a running process is thought by some to set up circumstances where there may be some conflict between threads when attempting to share caches or other hardware resources. There is also some concern that the action of multithreading could lower the response time of each single thread in the process, effectively negating any time savings that is generated by the configuration.

However, multithreading remains one of the viable options in computer multitasking. It is not unusual for a processor to allow for both multithreading as well as the creation of new processes to handle various tasks. This allows the end user all the benefits of context switching while still making the best use of available resources.

Operating system listing

Below is a listing of many of the different types of operating systems available today, the dates they were released, the platforms they have been developed for and who developed them.

Operating system

Platform Developer

AIX / AIXL Various IBM

AmigaOS Amiga Commodore

BSD Various BSD

Caldera Linux

Various SCO

Corel Linux Various Corel

Debian Linux`

Various GNU

DUnix Various Digital

DYNIX/ptx Various IBM

HP-UX VariousHewlett Packard

IRIX Various SGI

Kondara Linux

Various Kondara

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Linux VariousLinus Torvalds

MAC OS 8Apple Macintosh

Apple

MAC OS 9Apple Macintosh

Apple

MAC OS 10Apple Macintosh

Apple

MAC OS XApple Macintosh

Apple

Mandrake Linux

Various Mandrake

MINIX Various MINIX

MS-DOS 1.x IBM / PC Microsoft

MS-DOS 2.x IBM / PC Microsoft

MS-DOS 3.x IBM / PC Microsoft

MS-DOS 4.x IBM / PC Microsoft

MS-DOS 5.x IBM / PC Microsoft

MS-DOS 6.x IBM / PC Microsoft

NEXTSTEP Various Apple

OSF/1 Various OSF

QNX Various QNX

Red Hat Linux

Various Red Hat

SCO Various SCO

Slackware Linux

Various Slackware

Sun Solaris Various Sun

SuSE Linux Various SuSE

System 1Apple Macintosh

Apple

System 2Apple Macintosh

Apple

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System 3Apple Macintosh

Apple

System 4Apple Macintosh

Apple

System 6Apple Macintosh

Apple

System 7Apple Macintosh

Apple

System V Various System V

Tru64 Unix Various Digital

Turbolinux Various Turbolinux

Ultrix Various Ultrix

Unisys Various Unisys

Unix Various Bell labs

UnixWare Various UnixWare

VectorLinux Various VectorLinux

Windows 2000

IBM / PC Microsoft

Windows 2003

IBM / PC Microsoft

Windows 3.X IBM / PC Microsoft

Windows 95 IBM / PC Microsoft

Windows 98 IBM / PC Microsoft

Windows CE PDA Microsoft

Windows ME IBM / PC Microsoft

Windows NT IBM / PC Microsoft

Windows Vista

IBM / PC Microsoft

Windows XP IBM / PC Microsoft

Xenix Various Microsoft

6.2 System Resources

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System resources are the parts within a computer that are available to be used by the operating system and other applications. The most notable of the system resources is the amount of memory in use, but CPU time should be considered here as well. Each time an application starts, the application will request memory from the operating system and a slice of CPU time to perform its function. For example, when a computer user starts the word processing application on the computer, they will click the icon for the application and shortly thereafter, the program starts. During the time while the user is waiting for the program to start, the operating system is provisioning system resources to handle this application. It is essentially making room for it among the other processes and applications that may be running at the time the program is started. When the word processor application starts, it sends a request to the operating system to provision the necessary system resources for it to function

Depending on the amount of memory available, the application may open quickly, or may open a bit slower if less memory is available when the application starts. Sometimes there is not enough memory to get an application running right away, in which case the operating system recognizes the lack of system resources and will make an attempt to store some things in a swap file to allow more memory to be available for the active applications.

The swap file acts like memory but is contained on the hard disk of the computer. When the RAM memory within a computer becomes full, the operating system will page (or write) things out to the computers swap file, freeing up RAM memory for programs in use. As the swap file continues to grow, it can become full. This will cause the operating system to produce warning messages indicating that the swap file or virtual memory is full and the user will be instructed to close some programs to free up system resources, allowing the computer to function better. Many times, restarting the computer is the best way to alleviate these warning messages.

If a peripheral is needed, like a printer or disk drive, the hardware being requested will send an Interrupt Request (IRQ) to the CPU. The IRQ is the signal that the peripheral device uses to let the CPU know that it needs to do something. Hardware resources are the memory and CPU time used when peripheral devices, like printers, scanners, and modems are used. Each time one of these devices is accessed by the user, the device sends a signal to the motherboard to interrupt the CPU so it can operate. Once it is finished performing the requested tasks, the device signals again that it has completed. These signals are known as Interrupt Requests (IRQs), and each device has a specific channel or set of channels that it can use to communicate with the motherboard. If all of the channels for a specified device are used, the device cannot function. Each IRQ channel can only use one device, or have one device assigned to it in a computing system. This helps the motherboard know which devices it should expect on which IRQs. System resources are monitored by the computers operating system to ensure that the computer runs as efficiently as possible, given the resources available at any time.

Type of resources

CPU time Random access memory and virtual memory Hard disk space Network throughput Electrical power External Devices Input/output operations

Resource management

A resource handle is an identifier for a resource that is currently being accessed. Resource handles can be opaque, in which case they are often integer numbers, or they

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can be pointers that allow access to further information. Common resource handles are file descriptors and sockets.

A situation when a computer program allocates a resource and fails to deallocate it after use is called a resource leak. Resource tracking is the ability of an operating system, virtual machine or other program to terminate the access to a resource that has been allocated but not deallocated after use. When implemented by a virtual machine this is often done in the form of garbage collection. Without resource tracking programmers must take care of proper manual resource deallocation. They may also use the RAII technique to automate this task.

Access to memory areas is often controlled by semaphores, which allows a pathological situation called a deadlock, when different threads or processes try to allocate resources already allocated by each other. A deadlock usually leads to a program becoming partially or completely unresponsive.

Access to resources is also sometimes regulated by queuing; in the case of computing time on a CPU the controlling algorithm of the task queue is called a scheduler.

6.3 Computer Program

A computer program, or just a program) is a sequence of instructions written to perform a specified task for a computer.[1] A computer requires programs to function, typically executing the program's instructions in a central processor.[2] The program has an executable form that the computer can use directly to execute the instructions. The same program in its human-readable source code form, from which executable programs are derived (e.g., compiled), enables a programmer to study and develop its algorithms.

Computer source code is often written by professional computer programmers. Source code is written in a programming language that usually follows one of two main paradigms: imperative or declarative programming. Source code may be converted into an executable file (sometimes called an executable program or a binary) by a compiler and later executed by a central processing unit. Alternatively, computer programs may be executed with the aid of an interpreter, or may be embedded directly into hardware.

Computer programs may be categorized along functional lines: system software and application software. Many computer programs may run simultaneously on a single computer, a process known as multitasking.

6.4 Process

In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system

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(OS), a process may be made up of multiple threads of execution that execute instructions concurrently.[1][2]

A computer program is a passive collection of instructions, a process is the actual execution of those instructions. Several processes may be associated with the same program; for example, opening up several instances of the same program often means more than one process is being executed.

Multitasking is a method to allow multiple processes to share processors (CPUs) and other system resources. Each CPU executes a single task at a time. However, multitasking allows each processor to switch between tasks that are being executed without having to wait for each task to finish. Depending on the operating system implementation, switches could be performed when tasks perform input/output operations, when a task indicates that it can be switched, or on hardware interrupts.

A common form of multitasking is time-sharing. Time-sharing is a method to allow fast response for interactive user applications. In time-sharing systems, context switches are performed rapidly. This makes it seem like multiple processes are being executed simultaneously on the same processor. The execution of multiple processes seemingly simultaneously is called concurrency.

For security and reliability reasons most modern operating systems prevent direct communication between independent processes, providing strictly mediated and controlled inter-process communication functionality.

Process management in multi-tasking operating systems

A multitasking* operating system may just switch between processes to give the appearance of many processes executing concurrently or simultaneously, though in fact only one process can be executing at any one time on a single-core CPU (unless using multi-threading or other similar technology).[3]

It is usual to associate a single process with a main program, and 'daughter' ('child') processes with any spin-off, parallel processes, which behave like asynchronous subroutines. A process is said to own resources, of which an image of its program (in memory) is one such resource. (Note, however, that in multiprocessing systems, many processes may run off of, or share, the same reentrant program at the same location in memory— but each process is said to own its own image of the program.)

Processes are often called tasks in embedded operating systems. The sense of 'process' (or task) is 'something that takes up time', as opposed to 'memory', which is 'something that takes up space'. (Historically, the terms 'task' and 'process' were used interchangeably, but the term 'task' seems to be dropping from the computer lexicon.)

The above description applies to both processes managed by an operating system, and processes as defined by process calculi.

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If a process requests something for which it must wait, it will be blocked. When the process is in the Blocked State, it is eligible for swapping to disk, but this is transparent in a virtual memory system, where blocks of memory values may be really on disk and not in main memory at any time. Note that even unused portions of active processes/tasks (executing programs) are eligible for swapping to disk. All parts of an executing program and its data do not have to be in physical memory for the associated process to be active.

______________________________

*Tasks and processes refer essentially to the same entity. And, although they have somewhat different terminological histories, they have come to be used as synonyms. Today, the term process is generally preferred over task, except when referring to 'multitasking', since the alternative term, 'multiprocessing', is too easy to confuse with multiprocessor (which is a computer with two or more CPUs).

Process States

An operating system kernel that allows multi-tasking needs processes to have certain states. Names for these states are not standardised, but they have similar functionality.

First, the process is "created" - it is loaded from a secondary storage device (hard disk or CD-ROM...) into main memory. After that the process scheduler assigns it the state "waiting".

While the process is "waiting" it waits for the scheduler to do a so-called context switch and load the process into the processor. The process state then becomes "running", and the processor executes the process instructions.

If a process needs to wait for a resource (wait for user input or file to open ...), it is assigned the "blocked" state. The process state is changed back to "waiting" when the process no longer needs to wait.

Once the process finishes execution, or is terminated by the operating system, it is no longer needed. The process is removed instantly or is moved to the "terminated" state. When removed, it just waits to be removed from main memory.

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Primary process states

The following typical process states are possible on computer systems of all kinds. In most of these states, processes are "stored" on main memory.

a) Created

(Also called New) When a process is first created, it occupies the "created" or "new" state. In this state, the process awaits admission to the "ready" state. This admission will be approved or delayed by a long-term, or admission, scheduler. Typically in most desktop computer systems, this admission will be approved automatically, however for real-time operating systems this admission may be delayed. In a real time system, admitting too many processes to the "ready" state may lead to oversaturation and overcontention for the systems resources, leading to an inability to meet process deadlines.

b) Ready or Running

(Also called waiting or runnable) A "ready" or "waiting" process has been loaded into main memory and is awaiting execution on a CPU (to be context switched onto the CPU by the dispatcher, or short-term scheduler). There may be many "ready" processes at any one point of the systems execution - for example, in a one processor system, only one process can be executing at any one time, and all other "concurrently executing" processes will be waiting for execution.

A ready queue is used in computer scheduling. Modern computers are capable of running many different programs or processes at the same time. However, the CPU is only capable of handling one process at a time. Processes that are ready for the CPU are kept

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in a queue for "ready" processes. Other processes that are waiting for an event to occur, such as loading information from a hard drive or waiting on an internet connection, are not in the ready queue.

c) Blocked

A process that is waiting for some event (such as I/O operation completion or a signal).

d) Terminated

A process may be terminated, either from the "running" state by completing its execution or by explicitly being killed. In either of these cases, the process moves to the "terminated" state. If a process is not removed from memory after entering this state, this state may also be called zombie.

Additional process states

Two additional states are available for processes in systems that support virtual memory. In both of these states, processes are "stored" on secondary memory (typically a hard disk).

e) Swapped out and waiting

(Also called suspended and waiting.) In systems that support virtual memory, a process may be swapped out, that is removed from main memory and placed in virtual memory by the mid-term scheduler. From here the process may be swapped back into the waiting state.

f) Swapped out and blocked

(Also called suspended and blocked.) Processes that are blocked may also be swapped out. In this event the process is both swapped out and blocked, and may be swapped back in again under the same circumstances as a swapped out and waiting process (although in this case, the process will move to the blocked state, and may still be waiting for a resource to become available).

6.5 Real time process

In computing, real-time refers to a time frame that is very brief, appearing to be immediate. When a computer processes data in real time, it reads and handles data as it is received, producing results without delay. For example, a website that is updated in real-time will allow its viewers to see changes as soon as they occur, rather than waiting for updates to be visible at some later date.

A non-real-time computer process does not have a deadline. Such a process can be considered non-real-time, even if fast results are preferred. A real-time system, on the other hand, is expected to

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respond not just quickly, but also within a predictable period of time. A good example of a real-time computer system is a car’s anti-lock break system. An anti-lock brake system is expected to release a vehicle’s brakes, preventing dangerous wheel locking, in a predictably short time frame.

Unfortunately, there are times when real-time systems fail to respond as desired. A Real-time process fails when its task is not completed before its deadline. In computing, there is no grace period given because of other demands on a system. Real-time deadlines must be kept without regard to other factors; they are considered mission critical.

When a process is considered hard real-time, it must complete its operation by a specific time. If it fails to meet its deadline, its operation is without value and the system for which it is a component could face failure. When a system is considered soft, real-time, however, there is some room for lateness. For example, in a soft, real-time system, a delayed process may not cause the entire system to fail. Instead, it may lead to a decrease in the usual quality of the process or system.

Hard, real-time systems are often used in embedded systems. Consider, for example, a car engine control system. Such a system is considered hard, real–time because a late process could cause the engine to fail. Hard real-time systems are employed when it is crucial that a task or event is handled by a strict deadline. This is typically necessary when damage or the loss of life may occur as a result of a system failure.

Soft real-time systems are usually employed when there are multiple, connected systems that must be maintained despite shifting events and circumstances. These systems are also used when concurrent access requirements are present. For example, the software used to maintain travel schedules for major transportation companies is often soft real-time. It is necessary for such software to update schedules with little delay. However, a delay of a few seconds is not likely to cause the kind of mayhem possible when a hard, real-time system fails.

6.6 Scheduling

Scheduling is a key concept in computer multitasking, multiprocessing operating system and real-time operating system designs. Scheduling refers to the way processes are assigned to run on the available CPUs, since there are typically many more processes running than there are available CPUs. This assignment is carried out by softwares known as a scheduler and dispatcher.

The scheduler is concerned mainly with:

CPU utilization - to keep the CPU as busy as possible. Throughput - number of processes that complete their execution per time unit. Turnaround - total time between submission of a process and its completion. 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. Fairness - Equal CPU time to each thread.

In real-time environments, such as mobile devices for automatic control in industry (for example robotics), the scheduler also must ensure that processes can meet deadlines; this is crucial for keeping the system stable. Scheduled tasks are sent to mobile devices and managed through an administrative back end.

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Types of operating system schedulers

Operating systems may feature up to 3 distinct types of schedulers: a long-term scheduler (also known as an admission scheduler or high-level scheduler), a mid-term or medium-term scheduler and a short-term scheduler . The names suggest the relative frequency with which these functions are performed.

a)Long-term Scheduler

The long-term, or admission, scheduler decides which jobs or processes are to be admitted to the ready queue; that is, when an attempt is made to execute a program, its admission to the set of currently executing processes is either authorized or delayed by the long-term scheduler. Thus, this scheduler dictates what processes are to run on a system, and the degree of concurrency to be supported at any one time - ie: whether a high or low amount of processes are to be executed concurrently, and how the split between IO intensive and CPU intensive processes is to be handled. In modern OS's, this is used to make sure that real time processes get enough CPU time to finish their tasks. Without proper real time scheduling, modern GUI interfaces would seem sluggish. [Stallings, 399].1

Long-term scheduling is also important in large-scale systems such as batch processing systems, computer clusters, supercomputers and render farms. In these cases, special purpose job scheduler software is typically used to assist these functions, in addition to any underlying admission scheduling support in the operating system.

b) Mid-term Scheduler

The mid-term scheduler temporarily removes processes from main memory and places them on secondary memory (such as a disk drive) or vice versa. This is commonly referred to as "swapping out" or "swapping in" (also incorrectly as "paging out" or "paging in"). The mid-term scheduler may decide to swap out a process which has not been active for some time, or a process which has a low priority, or a process which is page faulting frequently, or a process which is taking up a large amount of memory in order to free up main memory for other processes, swapping the process back in later when more memory is available, or when the process has been unblocked and is no longer waiting for a resource. [Stallings, 396] [Stallings, 370]

In many systems today (those that support mapping virtual address space to secondary storage other than the swap file), the mid-term scheduler may actually perform the role of the long-term scheduler, by treating binaries as "swapped out processes" upon their execution. In this way, when a segment of the binary is required it can be swapped in on demand, or "lazy loaded". [Stallings, 394]

c) Short-term Scheduler

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The short-term scheduler (also known as the CPU scheduler) decides which of the ready, in-memory processes are to be executed (allocated a CPU) next following a clock interrupt, an IO interrupt, an operating system call or another form of signal. Thus the short-term scheduler makes scheduling decisions much more frequently than the long-term or mid-term schedulers - a scheduling decision will at a minimum have to be made after every time slice, and these are very short. This scheduler can be preemptive, implying that it is capable of forcibly removing processes from a CPU when it decides to allocate that CPU to another process, or non-preemptive (also known as "voluntary" or "co-operative"), in which case the scheduler is unable to "force" processes off the CPU. [Stallings, 396].

d) Dispatcher

Another component involved in the CPU-scheduling function is the dispatcher. The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler. This function involves the following:

Switching context Switching to user mode Jumping to the proper location in the user program to restart that program

The dispatcher should be as fast as possible, since it is invoked during every process switch. The time it takes for the dispatcher to stop one process and start another running is known as the dispatch latency. [Silberschatz, 157]

Scheduling criteria

Different CPU scheduling algorithms have different properties, and the choice of a particular algorithm may favor one class of processes over another. In choosing which algorithm to use in a particular situation, we must consider the properties of the various algorithms. Many criteria have been suggested for comparing CPU scheduling algorithms. Which characteristics are used for comparison can make a substantial difference in which algorithm is judged to be best. The criteria include the following:

CPU Utilization. We want to keep the CPU as busy as possible. 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, this rate may be one process per hour; for short transactions, it may be 10 processes per second.

Turnaround time. From the point of view of a particular 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. 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 the time during which a process executes or does I/O; it affects only the amount of

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time that a process spends waiting in the ready queue. Waiting time is the sum of periods spend waiting in the ready queue.

Response time. In an interactive system, turnaround time may not be the best criterion. Often, a process can produce some output fairly early and can continue computing new results while previous results are being output to the user. Thus, another measure is the time from the submission of a request until the first response is produced. This measure, called response time, is the time it 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.

It is desirable to maximize CPU utilization and throughput and to minimize turnaround time, waiting time, and response time. In most cases, we optimize the average measure. However, under some circumstances, it is desirable to optimize the minimum or maximum values rather than the average. For example, to guarantee that all users get good service, we may want to minimize the maximum response time. Investigators have suggested that, for interactive systems, it is more important to minimize the variance in the response time than to minimize the average response time. 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. However, little work has been done on CPU-scheduling algorithms that minimize variance.

Fundamental scheduling algorithms

CPU scheduling deals with the problem of deciding which of the processes in the ready queue is to be allocated the CPU. There are many different CPU scheduling algorithms. In this section, we describe several of them.

a) First In First Out

Also known as First Come, First Served (FCFS), its the simplest scheduling algorithm, FIFO simply queues processes in the order that they arrive in the ready queue.

Since context switches only occur upon process termination, and no reorganization of the process queue is required, scheduling overhead is minimal.

Throughput can be low, since long processes can hog the CPU Turnaround time, waiting time and response time can be low for the same reasons

above No prioritization occurs, thus this system has trouble meeting process deadlines. The lack of prioritization does permit every process to eventually complete, hence

no starvation.

b) Shortest remaining time

Also known as Shortest Job First (SJF). With this strategy the scheduler arranges processes with the least estimated processing time remaining to be next in the queue. This

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requires advanced knowledge or estimations about the time required for a process to complete.

If a shorter process arrives during another process' execution, the currently running process may be interrupted, dividing that process into two separate computing blocks. This creates excess overhead through additional context switching. The scheduler must also place each incoming process into a specific place in the queue, creating additional overhead.

This algorithm is designed for maximum throughput in most scenarios. Waiting time and response time increase as the process' computational

requirements increase. Since turnaround time is based on waiting time plus processing time, longer processes are significantly affected by this. Overall waiting time is smaller than FIFO, however since no process has to wait for the termination of the longest process.

No particular attention is given to deadlines, the programmer can only attempt to make processes with deadlines as short as possible.

Starvation is possible, especially in a busy system with many small processes being run.

c) Fixed priority pre-emptive scheduling

The O/S assigns a fixed priority rank to every process, and the scheduler arranges the processes in the ready queue in order of their priority. Lower priority processes get interrupted by incoming higher priority processes.

Overhead is not minimal, nor is it significant. FPPS has no particular advantage in terms of throughput over FIFO scheduling. Waiting time and response time depend on the priority of the process. Higher

priority processes have smaller waiting and response times. Deadlines can be met by giving processes with deadlines a higher priority. Starvation of lower priority processes is possible with large amounts of high

priority processes queuing for CPU time.

d) Round-robin scheduling

The scheduler assigns a fixed time unit per process, and cycles through them.

RR scheduling involves extensive overhead, especially with a small time unit. Balanced throughput between FCFS and SJN, shorter jobs are completed faster

than in FCFS and longer processes are completed faster than in SJN. Fastest average response time, waiting time is dependent on number of processes,

and not average process length. Because of high waiting times, deadlines are rarely met in a pure RR system. Starvation can never occur, since no priority is given. Order of time unit

allocation is based upon process arrival time, similar to FCFS.

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e) Multilevel Queue Scheduling

This is used for situations in which processes are easily classified into different groups. For example, a common division is made between foreground (interactive) processes and background (batch) processes. These two types of processes have different response-time requirements and so may have different scheduling needs.

Overview

Scheduling algorithm

CPU Utilization

ThroughputTurnaround

timeResponse

timeDeadline handling

Starvation free

First In First Out

Low Low High Low No Yes

Shortest remaining time

Medium High Medium Medium No No

Fixed priority pre-emptive scheduling

Medium Low High High Yes No

Round-robin scheduling

High Medium Medium Low No Yes

How to choose a scheduling Algorithm

When designing an operating system, a programmer must consider which scheduling algorithm will perform best for the use the system is going to see. There is no universal “best” scheduling algorithm, and many operating systems use extended or combinations of the scheduling algorithms above. For example, Windows NT/XP/Vista uses a Multilevel feedback queue, a combination of fixed priority preemptive scheduling, round-robin, and first in first out. In this system, processes can dynamically increase or decrease in priority depending on if it has been serviced already, or if it has been waiting extensively. Every priority level is represented by its own queue, with round-robin scheduling amongst the high priority processes and FIFO among the lower ones. In this sense, response time is short for most processes, and short but critical system processes

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get completed very quickly. Since processes can only use one time unit of the round robin in the highest priority queue, starvation can be a problem for longer high priority processes.

Operating system scheduler implementations

Windows

Very early MS-DOS and Microsoft Windows systems were non-multitasking, and as such did not feature a scheduler. Windows 3.1x used a non-preemptive scheduler, meaning that it did not interrupt programs. It relied on the program to end or tell the OS that it didn't need the processor so that it could move on to another process. This is usually called cooperative multitasking. Windows 95 introduced a rudimentary preemptive scheduler; however, for legacy support opted to let 16 bit applications run without preemption.

Windows NT-based operating systems use a multilevel feedback queue. 32 priority levels are defined, 0 through to 31, with priorities 0 through 15 being "normal" priorities and priorities 16 through 31 being soft real-time priorities, requiring privileges to assign. 0 is reserved for the Operating System. Users can select 5 of these priorities to assign to a running application from the Task Manager application, or through thread management APIs. The kernel may change the priority level of a thread depending on its I/O and CPU usage and whether it is interactive (i.e. accepts and responds to input from humans), raising the priority of interactive and I/O bounded processes and lowering that of CPU bound processes, to increase the responsiveness of interactive applications. The scheduler was modified in Windows Vista to use the cycle counter register of modern processors to keep track of exactly how many CPU cycles a thread has executed, rather than just using an interval-timer interrupt routine. Vista also uses a priority scheduler for the I/O queue so that disk defragmenters and other such programs don't interfere with foreground operations.

Mac OS

Mac OS 9 uses cooperative scheduling, where one process controls multiple cooperative threads. The kernel schedules the process using a Round-robin scheduling algorithm. Then, each process has its own copy of the thread manager that schedules each thread. The kernel then, using a preemptive scheduling algorithm, schedules all tasks to have processor time. Mac OS X[5] uses Mach (kernel) threads, and each thread is linked to its own separate process. If threads are being cooperative, then only one can run at a time. The thread must give up its right to the processor for other processes to run.

Linux

Since version 2.5 of the kernel, Linux has used a multilevel feedback queue with priority levels ranging from 0-140. 0-99 are reserved for real-time tasks and 100-140 are considered nice task levels. For real-time tasks, the time quantum for switching processes

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is approximately 200 ms, and for nice tasks approximately 10 ms. The scheduler will run through the queue of all ready processes, letting the highest priority processes go first and run through their time slices, after which they will be placed in an expired queue. When the active queue is empty the expired queue will become the active queue and vice versa. From versions 2.6 to 2.6.23, the kernel used an O(1) scheduler. In version 2.6.23, they replaced this method with the Completely Fair Scheduler that uses red-black trees instead of queues.[6]

FreeBSD

FreeBSD uses a multilevel feedback queue with priorities ranging from 0-255. 0-63 are reserved for interrupts, 64-127 for the top half of the kernel, 128-159 for real-time user threads, 160-223 for time-shared user threads, and 224-255 for idle user threads. Also, like Linux, it uses the active queue setup, but it also has an idle queue.[7]

NetBSD

NetBSD uses a multilevel feedback queue with priorities ranging from 0-223. 0-63 are reserved for time-shared threads (default, SCHED_OTHER policy), 64-95 for user threads which entered kernel space, 96-128 for kernel threads, 128-191 for user real-time threads (SCHED_FIFO and SCHED_RR policies), and 192-223 for software interrupts.

Solaris

Solaris uses a multilevel feedback queue with priorities ranging from 0-169. 0-59 are reserved for time-shared threads, 60-99 for system threads, 100-159 for real-time threads, and 160-169 for low priority interrupts. Unlike Linux, when a process is done using its time quantum, it's given a new priority and put back in the queue.

Summary

Operating System Preemption Algorithm

Windows 3.1x None Cooperative Scheduler

Windows 95,98,ME Half Only for 32 bit operations

Windows NT,XP,Vista Yes Multilevel Feedback Queue

Mac OS pre 9 None Cooperative Scheduler

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Mac OS X Yes Mach (kernel)

Linux pre 2.5 Yes Multilevel Feedback Queue

Linux 2.5-2.6.23 Yes O(1) scheduler

Linux post 2.6.23 Yes Completely Fair Scheduler

Solaris Yes Multilevel feedback queue

NetBSD Yes Multilevel feedback queue

FreeBSD Yes Multilevel feedback queue

6.7 Virtual memory

A virtual memory system denotes an area located on a computer’s hard drive that allows programs to operate without the need to load them into the physical memory. Computers have basically two kinds of memory systems: random access memory (RAM) and virtual memory (VM). When there is not an adequate amount of physical memory or RAM, available to run all the applications that a user may have opened at any one time, the system uses virtual memory to make up the difference.

If the computer did not have the ability to access the virtual memory when it exhausted the RAM, the user would receive an error message indicating that other applications would have to be closed in order to load a new program. The virtual memory process works by seeking locations on the physical memory that have not been accessed for a certain period of time. This information is then copied to an area on the hard drive. The available space that is freed up can now be used to load the new program.

This feature is one of the many operations performed automatically by your computer that goes unnoticed by the average user. The virtual memory is not only a way the computer creates additional memory for utilizing applications, but also takes advantage of the available system memory resources. This is cheaper than purchasing additional RAM chips. The hard drive of every computer system has an area that is used for virtual memory.

This secondary source of storage, where information is stored and retrieved, is called a paging file. The area where data is exchanged back and forth between the physical memory and the virtual memory system, in equal sized blocks, is called the pages. Virtual memory is basically a small paging file, which is located on the hard drive. Simply adding to the size of the paging file can increase the size of the virtual memory system storage capacity. In contrast, the only way to create more RAM is by purchasing and installing chips with larger memory capacities.

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One of the disadvantages of virtual memory is that the read and write processing speed is noticeably slower when compared to random access memory. Users who depend significantly on the virtual memory system to run their applications will suffer a decline in the overall performance of their computer system. The fact is hard disks are not built for handling tiny bits of information. The key to optimal system performance is to have more than enough RAM to handle your routine program processing workloads. This will ensure that accessing VMS is an exception and not the rule.

Memory Management Unit

The computer hardware that is responsible for managing the computer’s memory system is called the memory management unit (MMU). This component serves as a buffer between the CPU and system memory. The functions performed by the memory management unit can typically be divided into three areas: hardware memory management, operating system memory management and application memory management. Although the memory management unit can be a separate chip component, it is usually integrated into the central processing unit (CPU).

Generally, the hardware associated with memory management includes random access memory (RAM) and memory caches. RAM is the physical storage compartment that is located on the hard disk. It is the main storage area of the computer where data is read and written. Memory caches are used to hold copies of certain data from the main memory. The CPU accesses this information held in the memory cache, which helps speed up the processing time.

When the physical memory, or RAM, runs out of memory space, the computer automatically uses virtual memory from the hard disk to run the requested program. The memory management unit allocates memory from the operating system to various applications. The virtual address area, which is located within the central processing unit, is comprised of a range of addresses that are divided into pages. Pages are secondary storage blocks that are equal in size. The automated paging process allows the operating system to utilize storage space scattered on the hard disk.

Instead of the user receiving an error message that there is not enough memory, the MMU automatically instructs the system to build enough virtual memory to execute the application. Contiguous virtual memory space is created out of a pool of equal size blocks of virtual memory for running the application. This feature is a major key to making this process work effectively and efficiently because the system is not required to create one chunk of virtual memory to handle the program requirements. Creating various sizes of memory space to accommodate different size programs cause a problem known as fragmentation. This could lead to the possibility of not having enough free space for larger programs when the total space available is actually enough.

Application memory management entails the process of allocating the memory required to run a program from the available memory resources. In larger operating systems, many copies of the same application can be running. The memory management unit often assigns an application the memory address that best fits its need. It’s simpler to assign these programs the same addresses. Also, the memory management unit can distribute memory resources to programs on an as needed basis. When the operation is completed, the memory is recycled for use elsewhere.

One of the main challenges for memory management unit is to sense when data is no longer needed and can be discarded. This frees up memory for use on other processes. Automatic and manual memory management has become a separate field of study because of this issue. Inefficient memory management presents a major issue when it comes to optimal performance of computer systems.

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Does Adding RAM improve your computer speed?

Computer speed is one of the most sought after features of both desktops and laptops. Whether gaming, surfing the Web, executing code, running financial reports or updating databases, a computer can never be too fast. Will adding more random access memory (RAM) increase computer speed? In some cases it will, but not in all.

If RAM is the only bottleneck in an otherwise fast system, adding RAM will improve computer speed, possibly dramatically. If there are other problems aside from a shortage of RAM, however, adding memory might help, but the other factors will need to be addressed to get the best possible performance boost. In some cases a computer might simply be too old to run newer applications efficiently, if at all.

In Windows™ systems you can check RAM usage several ways. One method is to hold down the keys, Ctrl + Alt + Del to bring up the Task Manager. Click the Performance tab to see a graph of RAM resources. Third party freeware memory managers will also check memory usage for you. Some even monitor memory to free up RAM when necessary, though this is a stopgap measure.

If your system is low on RAM or routinely requires freeing RAM, installing more memory should improve computer speed. Check your motherboard before heading to your favorite retailer, however. The board might be maxed out in terms of the amount of RAM it will support. It can also happen that existing memory might need to be replaced if all ports are occupied by 1-gigabyte sticks, for example, on a motherboard that will support greater sticks.

If you are a gamer or work with video applications a slow graphics card might be a contributor to poor performance. A good graphics card should have its own on-board RAM and graphics processor (GPU), otherwise it will use system RAM and CPU resources. Consult the motherboard manual to see if you can improve performance by upgrading to a better card. If your present card is top notch and RAM seems fine, the central processing unit (CPU) is another upgrade that can drastically improve computer speed.

Maintenance issues also affect performance and might need to be addressed to remove bottlenecks. A lack of sufficient hard disk space will slow performance, as will a fragmented drive. Spyware, adware, keyloggers, root kits, Trojans and viruses can also slow a computer by taking up system resources as they run background processes.

In some cases a computer serves fine except for one specific application. Most software advertises minimum requirements but these recommendations are generally insufficient for good performance. One rule of thumb is to double the requirements for better performance. If your system can only meet minimal requirements this is likely the problem.

Taking the measures outlined should improve computer speed unless the system is already running at peak and the motherboard cannot be upgraded further. If so, the only alternative is to invest in a new computer that supports newer, faster technology. With prices falling all the time it should be easy to find an affordable buy that will reward you each time you boot up.

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