Info-Grade: Operating System

FCFS Job Scheduling Algorithm in OS (Operating System)

A process scheduler schedules different process to be assigned to the CPU based on particular scheduling algorithms. In this section we will discuss about FCFS (First come First Server) Scheduling algorithm.

First Come First Serve (FCFS)
In this algorithm jobs are executed on first come, first serve basis. It is a non preemptive, preemptive scheduling algorithm. This algorithm is easy to understand and implement. Its implementation is based on FIFO queue. It is poor in performance as average wait time is high.

Example :
Consider following process and calculate average turnover FCFS algorithm.

= Solution
Gaint Chart

Average Waiting Time = Finished time - Arrival time - Brush time
= (0+25+32)/3
= 19
Average Turnaround Time = Finished time - Arrival time
= (27+34+34)/3
= 31.67

Device Driver (Operating System)

Device driver is the software that is responsible for communicating with device controller and reset of the operating system. Device drivers are vendor specific software and are provided by I/O device manufactures. It place vital role in making operating systems independent of I/O devices. Device manufactures are responsible for providing different drivers for different operating systems. This means separate device driver is needed for Windows, Linux, Sun Solaries, Unix etc.

Fragmentation (Operating System)

Fragmentation refers to the condition of disk in which files are divided into pieces scattered around the disk. Fragmentation occurs naturally when we use a disk frequently, creating, deleting and modifying files.
There are two types of fragmentation

1. Internal Fragmentation

It occurs with all memory allocation strategies. This is caused by the fact that memory is allocated in blocks of a fixed size, wheres the actual memory needed will rarely be that exact size. For a random distribution of memory requests, on the average 1/2 block will be wasted per memory request, because on the average the last allocated block will be only half null.
Note that the same effect happens with hard drives, and that modern hardware gives us increasingly larger drives and memory at the expense of ever larger block size, which translate to more memory lost to internal fragmentation.
Some system use variable size blocks to minimize losses due to internal fragmentation.

2. External Fragmentation

External fragmentation means that the available memory is broken up into lots of little pieces, none of which is big enough to satisfy the next memory requirement, although the same total could.
All the memory allocation strategies suffer from external fragmentation, though first and best fits experience the problems more so than worst fit. The amount of memory lost to fragmentation may very with algorithm, usage patterns, and some design decision such as which end of a hole to allocate and which end to save on the free list.
Statistical analysis of first fit, for example, shows that for N blocks of allocated memory, another 0.5 N will be lost to fragmentation.
If the program in memory are relocatable, then the external fragmentation problem can be reduced via compaction, i.e. moving all processes down to one end of physical memory. This only involves updating the relocation register for each process, as all internal work is done using logical addresses.

Virtual Memory in Computer

virtual memory is a section of a hard disk that's set up to emulate the computer's RAM.

A computer can address more memory than the amount physically installed on the system. This extra memory is actually called virtual memory and it is a section of a hard disk that's set up to emulate the computer's RAM.

Virtual Memory is a storage allocation scheme in which secondary memory can be addressed as thought it were part of main memory. Virtual memory is a memory management technique that is implemented using both hardware and software. It maps memory addresses used by a program, called virtual address, into physical addresses in computer memory. It gives an illusion to the programmer that programs which are larger in size than actual memory can be executed. Virtual memory can be implemented with demand paging.
The main visible advantage of this scheme is that programs can be larger than physical memory. Virtual memory serves two purposes. First, it allows us to extend the use of physical memory by using disk. Second, it allows us to have memory protection, because each virtual address is translated to a physical address.

Modern microprocessors intended for general-purpose unit, or MMU, is built into the hardware. The MMU's job is to translate virtual addresses into physical addresses.

Virtual memory also allows the sharing of files and memory by multiple processes with several benefits:

System libraries can be shared by mapping them into the virtual address space of more than one process.
Processes can also share virtual memory by mapping the same block of memory to more than one process.
Process pages can be shared during fork() system call, eliminating the need to copy all of the page of the original (parent) process.

Semaphore Concept (Operating System)

Concept of semaphore was devised by Dijkastra in 1965. Semaphore is an integer variable that is used to record number of weakups and had been saved. If it is equal to zero it indicates that no wakeup's are saved. A positive value shows that one or more wakeup's are pending. The DOWN operation (sleep) checks the semaphore to see if it is greater than zero. If it is, it decrements the value (UP) and continues. If the semaphore is zero the process sleeps. The UP operation (weakup) increments the value of the semaphore. If one or more proces were sleeping on that semaphore then one of the process is chosen and allowed to compute its DOWN. Checking and updating the semaphore must be done as an atomic action to avoid race conditions. Product Customer problem can be solved using semaphore as below.

 
#define N 100 /* number of slots in the buffer */
typedef int semaphore; /* semaphores are a special kind of int */
semaphore mutex = 1; /* controls access to critical region */
semaphore empty = N; /* counts empty buffer slots */
semaphore null = 0; /* counts full buffer slots */
void producer(void){
    int item; 
    message m;      // message buffer. 
    while(TRUE){
        item = produce_item(); // generate something to put in buffer 
        receive(consumer, &m); // Receive an empty message, if any 
        build_message(&m, item); // construct a message to send 
        send(consumer, &m);  // send item to consumer 
    }
} 
void consumer(void){
    int item, i; 
    message m; 
    for(i=0; i<N; i++)
        send(producer, &m);  // send N empty messages 
    while(TRUE){
        receive(produce, &m);  // get message containing item   
        item = extract_item(&m); // extract item from message
        send(producer, &m); //send back empty replay 
        consumer_item(term); //do something with the item
    }
}

The Shell (Operating System)

The shell is an interface between user and the operating system

The operating system shell is the mechanism that carries out the system calls requested by the various parts of the system.
Shell is not part of the operating system kernel. The shell is the part of operating systems such as UNIX and MS-DOS where we can type commands to the operating system and receive a response.
Shell is also called the Command Line Interpreter (CLI) or the “C” prompt. Shell is the primary interface between a user sitting at his terminal and the operating system. Many shells exist, including sh, csh, ksh, and bash.
It starts out by typing the prompt, which tells the user that the shell is waiting to accept a command. If the user now types “date” command, the shell creates a child process and runs the date program as the child.

While the child process is running, the shell waits for it to terminate. When the child finishes, the shell types the prompt again and tries to read the next input line.
A more complicated command is:

cat file1 file2 file3 | sort > /dev/lp &

This command concatenates three files and pipes them to the sort program. It then redirects the sorted file to a line printer. The ampersand “&” at the end of the command instructs UNIX to issue the command as a background job. This results in the command prompt being returned immediately, whilst another process carries out the requested work.
Above command makes a series of system calls to the operating system in order to satisfy the whole request.

Types of Operating System

Mainframe Operating Systems:

A mainframe with 1000 disks and thousands of gigabytes of data is not unusual. Mainframes are normally used as web servers, servers for large-scale electronic commerce sites etc.
The operating systems for mainframes typically offer three kinds of services: batch, transaction processing, and time sharing.
A batch system is one that processes routine jobs without any interactive user present. For example, claim processing in an insurance company
Transaction processing systems handle large numbers of small requests per second; for example, check processing at a bank or airline reservations.
Timesharing systems allow multiple remote users to run jobs on the computer at once, such as querying a big database.
These functions are closely related: mainframe operating systems often perform all of them. An example mainframe operating system is OS/390, a descendant of OS/360

Personal Computer Operating Systems:

Job of personal computer operating system is to provide a good interface to a single user. They are widely used for word processing, spreadsheets, Internet access etc.
Personal computer operating systems are so widely known to the people who use computers but only few computer users knows about other types of operating systems.
Common examples of PC operating systems are Windows 2008, Windows 2007, the Macintosh operating system, Linux, Ubuntu etc.

Server Operating Systems:

Server operating systems run on servers, which are very large personal computers, workstations, or even mainframes.
They serve multiple users at once over a network and allow the users to share hardware and software resources. Servers can provide print service, file service, or Web service.
Internet providers run many server machines to support their customers and Web sites use servers to store the Web pages and handle the incoming requests. Some Examples of typical server operating systems are UNIX and Windows 2007 server, Sun Solaris etc.

Real-Time Operating Systems:

Another type of operating system is the real-time system. These systems are characterized by having time as a key parameter.
Deadline slip may cause huge disaster sometimes. Two Types: hard real-time system, soft real-time
If the action absolutely must occur at a certain moment (or within a certain range), we have a hard real-time system. For example, if a car is moving down an assembly line, certain actions must take place at certain instants of time, if a welding robot welds too early or too late, the car will be ruined
Another kind of real-time system is a soft real-time system, in which missing an occasional deadline is acceptable. Digital audio or multimedia systems fall in this category.

Time Sharing Systems:

Time sharing is a technique which enables many people, located at various terminals, to use a particular computer system at the same time.
Time-sharing or multitasking is a logical extension of multiprogramming. Processor's time which is shared among multiple users simultaneously is termed as time-sharing. In Time-Sharing Systems objective is to minimize response time.
Multiple jobs are executed by the CPU by switching between them, but the switches occur so frequently. Thus, the user can receives an immediate response.

History of Operating System

In this section we take a brief look at the history of operating system which is almost the same as looking at the history of computers.

First Generation (1945-1955)

During second second world war many people were developing automatic calculating machines. These first generation computers filled entire rooms with thousands of vacuum tubes.
They did not have an operating system, they did not even have programming languages and programmers had to physically wire the computer to carry out their intended instructions. The programmers also had to book time on the computer as a programmer had to have dedicated use of the machine.

Second Generation (1955-1965)

Vacuum Tubes provide very unreliable and a programmer, wishing to run his program, could quite easily spend all his/her time searching for an replacing tubes that had blown.
Development of Transistor : Now, instead of programmers booking time on the machine, the computers were under submitted on punched cards that were placed onto a magnetic tape. This tape was given to the operators who ran the job through the computer and delivered the output to the expectant programmer.
Concept of batch-processing (jobs): Instead of submitting one job at a time, many jobs were placed onto a single tape and these were processed one after another by the computer. The ability to do this can be seen as the first real operating system.

Third Generation (1965-1980)

IC (Integrated circuit) as a replacement for transistors : The third generation saw the start of multi programming. This is the computer could give the illusion of running more than one task at a time.
When one job had to wait for I/O request, another program could use the CPU. The concept of multi-programming led to a need for a more complex operating system.
Another feature of third generation machine was that they implemented spooling. This allowed reading of punch cards onto disc as soon as they were brought into the computer room. This eliminated the need to store the jobs stored to disc, thus allowing programs that produced output to tun at the speed of the disc, and not the printer.
Up until these point programmers were used to giving their job to an operator and watching it run.
This concept led to the concept of time sharing. This allowed programmers to access the computer from a terminal and work in an interactive manner.
Obviously, with the advent of multi programming, spooling and time sharing, operating system had to become a lot more complex in order to deal with all these issues.

Fourth Generation (1980-present):

The late seventies saw the development of Large Scale Integration (LSI). This led directly to the development of the personal computer (PC).
These computers were (originally) designed to be single user, highly interactive and provide graphics capability. One of the requirements for the original PC produced by IBM was an operating system and, Bill Gates supplied MS-DOS on which he made his fortune.
In addition, mainly on non-Intel processors, the UNIX operating system was being used.
Mainly, we can say that Graphical User Interface (GUI) became popular in 4th generation computers.

Fifth Generation (Sometime in the future):

We can notice that each generation have been influenced by new hardware. The fifth generation of computers may be the first that breaks with this tradition and the advances in software will be as important as advances in hardware.
Able to interact with humans in a way that is natural to us. No longer will we use mice and keyboards but we will be able to talk to computers in the same way that we communicate with each other.
In addition, we will be able to talk in any language and the computer will have the ability to convert to any other language. Computers will also be able to reason in a way that imitates humans. Advances need to be made in AI (Artificial Intelligence).
It is also likely that computers will need to be more powerful. Maybe parallel processing will be required. Maybe a computer based on a non-silicon substance may be needed to fulfill that requirement (as silicon has a theoretical limit as to how fast it can go).

What is Segmentation? (Operating System)

Paged virtual memory is one-dimensional in which virtual addresses goes from 0 to some maximum address
A segment is a logical entity. It might contain a procedure, or an array, or a stack, or a collection of scalar variables, but usually it doesn’t contain a mixture of different types.
Segmentation is the mechanism that provides the machine with multiple completely independent address spaces. Thus segmentation provides two-dimensional address space. Each segment consists of linear sequence of addresses, from 0 to some maximum

To specify an address in this segmented or 2-dimensional memory, the program must supply a 2-part virtual address: 1) segment number(s) and 2) an address within the segment called offset (s).
Virtual address to physical address translation process is shown in figure below:

Deadlock Detection and Recovery (Operating System)

If a system does not employ either a deadlock-prevention or a deadlock-avoidance algorithm, then a deadlock situation may occur. In this environment, the system must provide:
An algorithm that examines the state of the system to determine whether a deadlock has occurred.
An algorithm to recover from the deadlock.

Algorithm for detecting deadlock:

For each node, N in the graph, perform the following five steps with N as the starting node.
Initialize L to the empty list, designate all arcs as unmarked.
Add current node to end of L, check to see if node now appears in L two times. If it does, graph contains a cycle (listed in L), algorithm terminates.
From given node, see if any unmarked outgoing arcs. If so, go to step 5; if not, go to step 6.
Pick an unmarked outgoing arc at random and mark it. Then follow it to the new current node and go to step 3.
If this is initial node, graph does not contain any cycles, algorithm terminates. Otherwise, dead end. Remove it, go back to previous node, make that one current node, go to step 3.

Recovery From Deadlock
When a detection algorithm determines that a deadlock exists, there are two options for breaking a deadlock. One solution is simply to abort one or more processes to break the circular wait. The second option is to preempt some resources from one or more of the deadlocked processes.

Process Termination: To eliminate deadlocks by aborting a process, we use one of two methods. In both methods, the system reclaims all resources allocated to the terminated processes.

Abort all deadlocked processes
Abort one process at a time until the deadlock cycle is eliminated –overhead, a deadlock-detection after terminating a process

Resource Preemption: To eliminate deadlocks using resource preemption, we successively preempt some resources from processes and give these resources to other processes until the deadlock cycle is broken. Some issues need to be addressed are:

Selecting a victim: Some process will have to rolled back to break deadlock. Select that process as victim that will incur minimum cost.
Rollback: Determine how far to roll back the process

Total rollback: Abort the process and then restart it. More effective is to roll back the process only as far as necessary to break deadlock

Starvation: Starvation happens if same process is always chosen as victim. To avoid this, we include the number of rollbacks in the cost factor.

Critical Section (Operating System)

One way to avoid race conditions is not to allow two processes to be in their critical sections at the same time.
Critical section is the part of the process that accesses a shared variable.
That is, we need a mechanism of mutual exclusion. Mutual exclusion is the way of ensuring that one process, while using the shared variable, does not allow another process to access that variable.

Race Condition (Operating System)

Race conditions are situations where two or more processes reads or writes some shared data at the same time and the final result is incorrect.
Final result may be different according to order of completion of competing processes.
let us consider a simple but common example: a print spooler. When a process wants to print a file, it enters the file name in a special spooler directory.
Another process periodically checks to see if there are any files to be printed, and if there are, it prints them and then removes their names from the directory.
Consider the situation given in figure below, where in and out are two shared variables.

Process A reads value of in(i.e. 7) but before inserting the name of the document to be printed in index 7, the CPU decides to schedule process B.
Now, process B also reads in, and also gets 7. Process B now continues to run and stores the name of its file in slot 7 and updates in to be an 8.

After some time process A runs again. It looks at value of in stored in its local variable and finds a 7 there, and writes its file name in slot 7.Then it increments value of in and sets in to 8.The printer daemon will not notice anything wrong, but process B will never receive any output. Situations like this are called race conditions.

System Calls (Operating System)

System calls are the interface between the operating system and the user programs. Access to the operating system is done through system calls.
Each system call has a procedure associated with it so that calls to the operating system can be done in a similar way as that of normal procedure call.
When we call one of these procedures it places the parameters into registers and informs the operating system that there is some work to be done. When a system call is made TRAP instruction is executed. This instruction switches the CPU from user mode to kernel (or supervisor) mode.

Eventually, the operating system will have completed the work and will return a result.
Making a system call is like making a special kind of procedure call, only system calls enter the kernel and procedure calls do not.
Example: count = read(file, buffer, nbytes);

To make this concept clearer, let us examine the read call written above. Calling program first pushes the parameters onto stack (steps 1-3) before calling the read library procedure (step 4), which actually makes the read system call.

The library procedure, possibly written in assembly language, typically puts the system call number in specified register (step 5). Then it executes a TRAP instruction to switch from user mode to kernel mode and start execution at a fixed address within the kernel (step 6).
The kernel code examines the system call number and then dispatches to the correct system call handler (step 7). At that point the system call handler runs (step 8).

Once the system call handler has completed its work, control may be returned to the user-space library procedure at the instruction following the TRAP instruction (step 9).
This procedure then returns to the user program in the usual way procedure calls return (step 10).
To finish the job, the user program has to clean up the stack, as it does after any procedure call (step 11).

Process and Thread its Difference in CPU (Operating System)

Process

Process is a program that is ready for execution in CPU. When a program is loaded into memory, it becomes ready for execution and competes with other process to access CPU. Thus when a program is loaded into memory it becomes process. Computers nowadays can do many things at the same time. They can be writing to a printer, reading from a disc and scanning an image. Operating system is responsible for running many processes, usually, on the same CPU. In fact, only one process can be run at a time so the operating system has to share the CPU between the processes that are available to be run, while giving the illusion that the computer is doing many things at the same time. This approach can be directly contrasted with the first computers. They could only run one program at a time.

Now, the main point of this part is to consider how an operating system deals with processes when we allow many to run. To achieve true parallelism we must have multiprocessor system when n processors can execute n programs simultaneously. True parallelism cannot be achieved with singl CPU. In single processor CPU switched from one process to another process rapidly. In 1 sec CPU can serve 100's of processes giving the illusion of parallelism to a user which is called pseudo-parallelism.

Threads

Threads are like mini-processes that operate within a single process. Each thread has its own program counter and stack so that it knows where it is. Apart from this they can be considered the same as process, with the exception that they share the same address space. This means that all threads from the same process have access to the same global variable and the same files. Threads are also called light weight process. The table given below shows the various items that a process has compared to the items that each thread has.

Uses of Thread

Some application need to perform multiple activities at once that share common data and files. Thus reason behind need of multiple threads.
Ability of parallel entities, within a process to all share common variables and data.
Since threads do not have their own address space, it is easier to create and destroy threads than processes. In fact thread creation is around 100 times faster than process creation.
Thread switching has much less overhead than process switching because threads have few information attached with them.
Performance gain when there is a lot of computing and lot of I/O as tasks can overlap. Pure CPU bound thread application no advantage through
Threads are very effective on multiple CPU system for concurrent processing because with multiple CPU's true parallelism can be achieved

**Difference between Process and Threads**
Thread	Process
- Threads are lightweight	- Processes are heavyweight
- Threads are runs in address space of processes	- Processes have their own address space
- Threads shares files and other data	- Processes do not share files and other data
- Thread switching is faster	- Process switching is slower
Threads are easy to create and destroy	Process are difficult to create and destroy

Deadlock, Necessary Conditions of Deadlock/Characterization, Deadlocks Handling, Prevention, Avoidance in OS

In a multiprogramming environment, several processes may compete for a finite number of resources. A process requests resources; if the resources are not available at that time, the process enters into wait state. Waiting processes may never again change state, because the resources they have requested are held by other waiting processes. This situation is called a deadlock. System is deadlocked if there is a set of processes such that every process in the set is waiting for another process in the set.

Example 1
System has 2 disk drives.
P1 and P2 each hold one disk drive and each needs another one.

Example 2
- Porcess A makes a request to use the scanner
- Porcess A given the scanner
- Process B request thee CD writer
- Process B given the CD writer
- Now a requrest the CD writer
- A is denied permission until B releases the CD writer
- Also now B ask for the scanner
- Result is DEADLOCK !!

Necessary conditions for deadlock/Deadlock Characterization

A deadlock can arise if the following four conditions hold simultaneously (Necessary Condition).
1. Mutual exclusion:
Only one process at time can use the resource.
.
2. Hold and wait:
A process must be holding at least one resource and waiting to acquire additional resources that are currently being held by other processes.

3. No preemption:
A resource can be released only voluntarily by the process holding it, after that process has completed its task.

4. Circular wait:
A set {P0, P1, …, Pn} of waiting processes must exist such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and Pn is waiting for a resource that is held by P0.

Deadlocks Handling

Mainly there are four methods of handling deadlock
1. Deadlock avoidance
To avoid deadlocks the system dynamically considers every request and decides whether it is safe to grant it at this point. The system requires additional prior information regarding the overal potential use of each resource for each process.

2. Deadlock Prevention
It means that we design such a system where there is no chance of having a deadlock. The goal is to ensure that at least one of the Coffman's necessary conditions for deadlock can never hold. Deadlock can be prevented deadlocks by constraining how request for resources can be made in the system and how they are handled.

3. Deadlock Detection and Recovery
We can allow the system to enter a deadlock state, detect it and recover - If a system does not employ either a deadlock prevention or a deadlock avoidance algorithm, then a deadlock situation may occur. In this environment, the system can provide an algorithm that examines the state of the system to determine whether a deadlock has occurred and an algorithm to recover from the deadlock.

4. Ignore the Problem (Ostrich Algorithm) :
We can ignore the deadlock problem altogether. If the deadlock occurs, the system will stop functioning and will need to be restarted manually. This approach is used in systems in which deadlocks occur infrequently and if it is not a life critical system. This method is cheaper than other methods. This solution is used by most operating systems, including UNIX.

Deadlock Prevention

If we simulate deadlock with a table which is standing on its four legs then we can also simulate four legs with the four conditions which when occurs simultaneously, cause the deadlock.

However, if we break one of the legs of the table then the table will fall definitely. The same happens with deadlock, if we can be able to violate one of the four necessary conditions and don't let them occur together then we can prevent the deadlock.

Deadlock prevention methods are given below
1. Attack mutual Exclusion
Must hold only for non-sharable resources. Sharable resources, on the other hand, do not require mutually exclusive access, and thus cannot be involved in a deadlock.

2. Attack Hold and Wait
Ensure that whenever a process requests a resource, it does not hold any other resources. Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none. There is a drawback of low resource utilization in this method.

3. Attack No Preemption
If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are preempted (or released). If a process requests some resources and if they are not available, we check whether they are allocated to some other process that is waiting for additional resources. If so, we preempt the desired resources from the waiting process and allocate them to the requesting process.

4. Attack Circular Wait
One way to ensure that circular wait never holds is to Impose a total ordering of all resource types, and require that each process requests resources in an increasing or decreasing order of enumeration.

Deadlock Avoidance

Deadlock can be avoided if certain information about processes are available to the operating system before allocation of resources, such as which resources a process will consume in its lifetime. For every resource request, the system sees whether granting the request will cause the system to enter an unsafe state. The system then only grants request that will lead to safe states. In order for the system to be able to determine whether the next state will be safe or unsafe, it must know in advance at any time.

Resources currently available
Resources currently allocated to each process
Resources that will be required and released by these process in the future

It is possible for a process to be in an unsafe state but for this not result in a deadlock. The notion of safe/unsafe state only refers to the ability of the system to enter a deadlock state or not. For example, if a process requests A which would result in an unsafe state but releases B which would prevent circular wait then the state is but the system is not in deadlock. One known algorithm that is used for deadlock avoidance is the Banker's algorithm, which requests resource usage limit to be known in advance. However, for many systems it is impossible to know in advance what every process will request. This means that deadlock avoidance is impossible practically.

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