What is the Process Control Block? What are its fields?

The task controlling block, switch frame or task struct are the names of one and the same thing that we commonly called as the PCB or the process control block. 

- This data structure belongs to the kernel of the operating system and consists of the information that is required for managing a specific process. 

- The process control block is responsible for manifesting the processes in the operating system. 

- The operating system needs to be regularly informed about resources’ and processes’ statuses since managing the resources of the computer system for the processes is a part of its purpose. 

- The common approach to this issue is the creation and updating of the status table for every process and resource and objects which are relevant such as the files, I/O devices and so on:

1.  Memory tables are one such example as they consist of information regarding how the main memory and the virtual or the secondary memory has been allocated to each of the processes. It may also contain the authorization attributes given to each process for accessing the shared memory areas.

2.   I/O tables are another such example of the tables. The entries in these tables state about the availability of the device required for the process or of what has been assigned to the process. the status of the I/O operations taking place is also mentioned here along with address of the memory buffers they are using.

3.   Then we have the file tables that contain the information regarding the status of the files and their locations in memory.

4. Lastly, we have the process tables for storing the data that the operating systems require for the management of the processes. The main memory contains at least a part of the process control block even though its configuration and location keeps on varying with the operating system and the techniques it uses for memory management.

- Physical manifestation of a process consists of program data areas both dynamic and static, instructions, task management info etc. and this is what that actually forms the process control block. 

- PCB has got a central role to play in process management. 

- Operating system utilities access and modify it such as memory utilities, performance monitoring utilities, resource access utilities and scheduling utilities etc. 

- The current state of the operating system is defined by the set of process control blocks. 

- It is in the terms of PCBs that the data structuring is carried out. 

- In today’s sophisticated operating systems that are capable of multi-tasking, many different types of data items are stored in process control block. 

- These are the data items that are necessary for efficient and proper process management. 

- Even though the details of the PCBs depend up on the system, the common parts can still be identified and classified in to the following three classes:

1.  Process identification data: This includes the unique identifier of the process that is usually a number. In multi-tasking systems it may consists of user group identifier, parent process identifier, user identifier and so on. These IDs are very much important since they let the OS cross check with the tables.

2.   Process state data: This information defines the process status when it is not being executed. This makes it easy for the operating system to resume the process from appropriate point later. Therefore, this data consists of CPU process status word, CPU general purpose registers, stack pointer, frame pointers and so on.

3.   Process control data: This includes process scheduling state, priority value and amount of time elapsed since its suspension. 

What is meant by Time sharing system?

In the field of computer science, sharing resources of a computer through techniques of multi-tasking and multi-programming by many other system users is termed as a time sharing system. 

- It was first introduced in the year of 1960 and eventually emerged as the most popular computing model of the 1970s. 

- With it, occurred a major shift in the technology of designing the efficient computers. 

- These types of systems allowed quite a large number of users for interacting with the same computer system at the same time. 

- Providing computing capabilities was a costly affair at that time. 

- Time sharing greatly brought down this cost by providing these capabilities at a very less cost. 

- Since time sharing allows multiple users to interact simultaneously with the same system, it has actually made it possible for the organizations and the individuals to use a system that they do not even own. 

- This has further led to the promotion of the computers to be used interactively and development of other applications with an interactive interface. 

- The earlier systems apart from being expensive were quite slow. 

- This was the reason why the systems could be dedicated only to one task at a time. 

- The task was carried out through the control panels from where the operator would enter small programs manually through switches so as to load and execute a new program series. 

- These programs would take even up to weeks for completing execution. 

- The realization of the interaction pattern was what that led to the development of time sharing systems. 

- Usually, the data entered by a single user was in small bursts of info and then a long pause. 

- But if there would have been multiple number of users working concurrently on the same system, there activities could fill up the pauses of the single user. 

- The overall process could be made very efficient for a given size of the use group. 

- In the same way, the slice or share of time that was engaged in waiting for network input or tape or disk could be utilized by activities of other users. 

- A system that would be able to harness this potential advantage was difficult to be implemented.

- Even though batch processing was a high at that time, it could only make use of the time delay between two programs. 

- In the early times, the multiplexing of computer terminals in to main frame computer systems was seen.

- Such implementations were capable of sequentially polling those terminals to check for additional action and data requests made by the user of the system.

- Later, came the interconnection technology that was interrupt driven and made use of the IEEE 488 i.e., parallel data transfer technologies.

- Time sharing faded for some time with the advent of the micro computing but again it came back in to the scene with the rise of internet. 

- The corporate server farms cost in millions and are capable of hosting a large number of customers sharing the same resources.

- The operation of the websites using the early serial terminals was in bursts of activity that were followed by idle periods. 

- However, it is because of this bursting that the services of the web sites could be used by a large number of users simultaneously and with the advantage that the delays in communications won’t be noticed by them.

- However, if the server gets too damn busy they will surely start coming in to the notice.

- Earlier some time sharing services such as the service bureaus were offered by many companies. 

- Some examples of common systems that are used for time sharing are:

1. SDS 940
2. PDP – 10
3. IBM 360

What is a CPU Scheduler?

Scheduling is a very important concept when it comes to the multi-tasking operating systems. 

- It is the method via which the data flows, threads and processes are provided access to the shared resources of the computer system. 

- These resources include communications bandwidth, processor time, and memory and so on. 

- Scheduling is important as it helps in striking a balance the system processor and its resources effectively. 

- It helps in achieving the target QoS or quality of service. 

But what gave rise to scheduling? 

- Almost all the modern systems require to carry out multiple tasks i.e., multi-tasking and multiplexing as well which require a scheduling algorithm. 

- Multiplexing means transmission of the multiple data flows at the same time. - There are some other things also with which the scheduler is concerned. They are:

1. Throughput: It is the ratio of total number of processes executed to a given amount of time.
2. Latency: This factor can be sub – divided in to two namely response time and the turnaround time. Response time is the time taken from the submission of the process till its output is produced by the processor. The latter i.e., the turnaround time is the time period elapsed between the process submission and its completion.
3. Waiting/ fairness time: This is the equal CPU time given to each process or we can say that the time is allocated as per the priority of the processes. The time for which the processes wait in the ready queue is also counted in this.

- But in practical, conflicts may arise between these goals such as in case of latency versus throughput. 

- If such a case occurs, a suitable compromise has to be implemented by the scheduler. 

- The needs and the objectives of the user are used for deciding to who (of the above concerns) the preference is to be given. 

- In robotics or embedded systems i.e., in the real time environments, it becomes a duty of the scheduler for ensuring that all the processes meet their deadlines. 

- This is important for maintaining the stability of the system. 

- The mobile devices are given the scheduled tasks which are then managed by an administrative back end.

Types of CPU Schedulers

There are many types of CPU schedulers as discussed below:

1. Long-term Schedulers: 

- These schedulers facilitate the long term scheduling and are also known as the high level schedulers and the admission schedulers. 

- It is up to them to determine which processes and jobs are to be sent to the ready queue. 

- When the CPU makes an attempt for executing a program, the long term scheduler has the right to decide whether this program will be admitted to the currently executing set of processes. 

- Thus, it is dictated by this scheduler what processes are to be run and the extent of the concurrency has to be there.

- It also decides what amounts of processes have to be concurrently executed. 

- It also decides the handling of the split between the CPU intensive and I/O processes.  

2. Medium-term Schedulers: 

- The processes are temporarily removed from the main memory and placed up on the secondary memory by this scheduler. 

- This process is called “swapping in” and “swapping out”. 

- Usually this scheduler swaps out the following processes:

a)   processes that have been inactive since some time

b)   the processes that has raised a frequent page faulting

c)   processes having a low priority

d) processes that take up large memory chunks for releasing the main memory to other processes

- The scheduler later swaps in these processes whenever sufficient memory is available and if the processes are unblocked and not in waiting state.

3. Short-term Schedulers: 

It takes decision regarding the processes to be executed after clock interrupt. 

What is multi-tasking, multi-programming and multi-threading?

When it comes to computing, there are 3 important tasks that are inter-related concepts namely multi-programming, multitasking and multi-threading. 

What is Multitasking?

- This has actually emerged out of the need of multitasking since while the system performed one task a lot of time was wasted. 

- As their needs grew,people wanted the computer to perform many tasks at the same time. Multi-tasking is what we call it. 

- Here, multiple tasks or processes are carried out simultaneously.

- The common processing resources i.e., the main memory and the CPU are shared by these processes. 

- If the system has only one CPU to work with, then it can only run one task at a time. 

- Such systems seek to multi-task by scheduling all the processes required to be carried out. 

- It runs one task and the other one waits in the pipeline.

- The CPU is reassigned to all the tasks turn by turn and this is termed as a context switch. 

- When this happens very frequently, it gives an illusion that the processes are being executed in parallel. 

- There are other systems called multi-processor machines which have more than one CPU and can perform a number of tasks greater than the number of CPUs. 

- There are a number of scheduling strategies that might be adopted by the operating systems and they are:

Ø  Multi – programming

Ø  Time – sharing

Ø  Real – time systems

What is Multi-Programming?

- Earlier we had very slow peripheral devices and therefore the CPU time was a luxury and so expensive. 

- Whenever a program was being executed for accessing a peripheral, the CPU was to keep waiting for the peripheral to finish with processing the data. 

- It is very inefficient. 

- Then came the concept of multi–programming which was a very good solution. 

-  When the program reached the waiting status, its context was stored in the memory and the CPU was given some other program to execute. 

- This processing continued till all the processes at hand were completed. 

- Later,developments such as VMT or virtual machine technology and virtual memory greatly increased the efficiency of the multi – programming systems. 

- With these two technologies the programs were able to make use of the OS and the memory resources just as they were being used by the currently executing programs. 

- However, there is one drawback with multi–programming which is that is does not guarantees that all programs will be executed in a timely manner. 

- But then also it was of a great help in processing multiple batches of programs.

What is Multi-threading?

 

- With multi–tasking a great improvement was seen in the throughput of the computer systems. 

- So programmers found themselves implementing programs in sets of cooperating processes.

- Here, all the processes were assigned different tasks like one would take input, other one would process it and a third one would write the output to the display. 

- But for this, there was a requirement of tools that allowed an efficient exchange of the data.

- Threads were an outcome of the idea that the processes can be made to cooperate efficiently if their memory space is shared.

- Therefore, threads can be defined as the processing running in a memory context that is same for all. 

- These threads are said to be light – weight since there is no need for a change of memory context for switching between them. 

- The scheduling followed here is of the preemptively. 

What is a virtual memory?

- Virtual Memory is a memory management technique that is a compulsory requirement for the multi-tasking kernels. 

- With this technique, the architecture of a computer can be virtualized to different types of computer data storage such as disk drive storage and RAM i.e., the random access memory. 

- With this, programmers do not have to worry about designing applications that will suit this kind of storage. 

- The programs can be designed keeping in consideration only one kind of memory i.e., the virtual memory. 

- This memory behaves just like the usual memory but more than that. 

- It offers a direct as well as contiguous memory space for various operations. - Some of us might think that the programming the software might get difficult with the virtual memory. But this is not so.

- Instead the task becomes easy because the fragmentation of the main physical memory is hidden. 

- For achieving this, the burden of the management of the memory hierarchy is delegated to the kernel.

- This has another added advantage which is that the need for handling of overlays in an explicit way via program is eliminated.

- The need for the relocation of a program code or accessing the memory is obviated via relative addressing. 

- This lets the process to be executed in its own dedicated space. 

- The concept of the virtual memory in a more generalized form is called the memory virtualization.

- The modern computer architecture cannot do without the virtual memory. 

- The only requirement for implementing the virtual memory is the hardware support that is provided through the memory management unit that is in-built in CPU. 

- For increasing the performance of these virtual memory implementations hardware support can be employed by the virtual machines and emulators. 

- Computer systems with old operating systems such as DOS in mainframes  do not possess any functionality of the virtual memory. 

- The first computer that featured the virtual memory was the Apple Lisa that was designed in the year of 1980. 

- It appears that with the use of virtual memory as if every program has a sole access to it. 

- However, there were some older operating systems that had single address space Oss. 

- These operating systems used to process tasks in a single space. 

- This space is consisted of the virtual memory. 

- Very consistent response times are a requirement of the special purpose computer systems such as the embedded systems. 

- These systems do not prefer to use the virtual memory as it may decrease the determinism. 

- The unpredictable traps producing unwanted jitter while carrying out the I/O operations might be triggered by the virtual memory systems. 

- This happens because the cost of the embedded hardware is kept low. 

- The operations are included in the software rather than including them in the hardware. 

- This technique is termed as the bit banging. 

- The older programs needed to have logic for the management of both primary and secondary memory. 

- One such logic was that of the overlaying. 

- Therefore, virtual memory was introduced as a method for extending the primary memory and make this extension easy for the programmers.  

- In order to allow multi–tasking and multi–programming, the memory in the early systems was divided between many programs. 

Implementation of the virtual memory saw many problems. One among those problems was of the dynamic address translation that was difficult to be implemented and  quite expensive also. 

Explain the concepts of threads and processes in operating system?

Threads and processes are an important part of the operating systems that have features of multi–tasking and parallel programming. These come under the sole concept of ‘scheduling’. Let us try to understand these concepts with the help of an analogy.

- Consider the process to be a house and threads are its occupants. 

- Then, process is like a container having many attributes. 

- These attributes can be compared to that of a house such as number of rooms, floor space and so on. 

- Despite having so many attributes, this house is a passive thing which means it can’t perform anything on its own. 

- The active elements in this situation are the occupants of the home i.e., the threads. 

- The various attributes of the house are actually used by them. 

- Since you too live in a house you must have got an idea how it actually works and behaves. 

- You do whatever you like in the house if only you are there. 

- What if another person starts living with you? You just can’t do anything you want to do. 

- You cannot use the washroom without making sure that the other person is not there. 

- This can be related to multi – threading. 

- Just as a part of estate is occupied by the house, an amount of memory is occupied by the process. 

- Just as the occupants are allowed to freely access anything in the house, similarly the occupied memory is utilized by the threads that are a part of that process i.e., the access to memory is common. 

- If one process allocates some memory, it can be accessed by all other threads also. 

- If such a thing is happening, it has to be made sure that from all the threads, the access to the memory is synchronized. 

- If it cannot be synchronized, then it becomes clear that the memory has been allocated specifically to a thread. 

- But in actual, things are a lot more complicated because at some point of time everything has to be shared. 

- If one thread wants to use some resource that is already under use by some other thread, than that thread has to follow the concept of mutual exclusion. 

- An object known as the mutex is used by the thread for achieving exclusive access to that resource. 

- Mutex can be compared to a door lock. 

- Once a thread locks this, no other thread can use that resource until the mutex is again unlocked by that thread. 

- Mutex is one resource that a thread uses. 

- Now, suppose there are many threads waiting to use the resource when mutex is unlocked, the question that arises now is that who will be next one to use the resource. 

- This problem can be solved by either deciding on the basis of length of wait or on basis of priority. 

- Suppose there is a location that can be accessed by more than one threads simultaneously.

- You want to have only a limited number of threads using that memory location at any given point of time. 

- This problem cannot be solved by mutex but with another resource called semaphore. 

- Semaphore with a count of 1 is the resource that can only be used by one thread at a time. 

- In semaphore of greater count more threads can access it simultaneously.  

- It just depends up on how you characterize or set the lock.

What are Real-time operating systems?

- The RTOS or a real time operating system was developed with the intention of serving the application requests that occur in real time. 

- This type of operating system is capable of processing the data as and when it comes in to the system. 

- This it does without making any buffering delays. 

- The time requirements are processed in 10ths of seconds or even on much smaller scale. 

- A key characteristic feature of the real operating system is that the amount of time they take for accepting and processing a given task remains consistent. 

- The variability is so less that it can be ignored totally.

Real time operating systems also there are two types as stated below:

1. The soft real –time operating system: It produces more jitter.
2. The hard real – time operating system: It produces less jitter when compared to the previous one.

- The real time operating systems are driven by the goal of giving guaranteed hard or soft performance rather than just producing a high throughput. 

- Another distinction between these two operating systems is that the soft real time operating system can generally meet deadline whereas the hard real time operating system meets a deadline deterministic ally.

- For the scheduling purpose, some advance algorithms are used by these operating systems. 

- Flexibility in scheduling has many advantages to offer such as the cso (computer system orchestration) of the process priorities becomes wider.

- But a typical real time OS dedicates itself to a small number of applications at a time. 

- There are 2 key factors in any real –time OS namely:

1. Minimal interrupt latency and
2. Minimal thread switching latency.

- Two types of design philosophies are followed in designing the real  time Oss:

1. Time sharing design: As per this design, the tasks are switched based up on a clocked interrupt and events at regular intervals. This is also termed as the round robin scheduling.
2. Event – driven design: As per this design, the switching occurs only when some other event demands higher priority. This is why it is also termed as priority scheduling or preemptive priority.

- In the former designs, the tasks are switched more frequently than what is strictly required but it proves to be good at providing a smooth multi – tasking experience. 

- This gives the user an illusion that he/ she is solely using the machine. 

- The earlier designs of CPU forced us to have several cycles for switching a task and while switching it could not perform any other task. 

- This was the reason why the early operating systems avoided unnecessary switching in order to save the CPU time. 

- Typically, in any design there are 3 states of a task:

1. Running or executing on CPU
2. Ready to be executed
3. Waiting or blocked for some event

- Many of the tasks are kept in the second and third states because at a time the CPU can perform only one task. 

- The number of tasks waiting to be executed in the ready queue may vary depending on the running applications and the scheduler type being used by the CPU. 

- On multi – tasking systems that are non – preemptive, one task might have to give up its CPU time to let the other tasks to be executed. 

- This leads to a situation called the resource starvation i.e., the number of tasks to be executed is more and the resources are less.