Process & Thread Management
- 1. Process & It’s States Process : Defination: A process is basically a program in execution. The execution of a process must progress in a sequential fashion. Or A process is defined as an entity which represents the basic unit of work to be implemented in the system. Process States: Basically, A process necessarily goes through minimum 4 states. 1. New state 2. Ready state 3. Run state and 4. terminate or Exit state.
- 2. Process & It’s States Conti…. Following diagram shows the different states of process.
- 3. Process & It’s States Conti… 1. New State- A process is said to be in new state when a program present in the secondary memory is initiated for execution. 2. Ready State- A process moves from new state to ready state after it is loaded into the main memory and is ready for execution. In ready state, the process waits for its execution by the processor. In multiprogramming environment, many processes may be present in the ready state. 3. Run State- A process moves from ready state to run state after it is assigned the CPU for execution. 4. Terminate State- A process moves from run state to terminate state after its execution is completed. After entering the terminate state, context (PCB) of the process is deleted by the operating system.
- 4. Process & It’s States Conti… 5. Block Or Wait State- A process moves from run state to block or wait state if it requires an I/O operation or some blocked resource during its execution. After the I/O operation gets completed or resource becomes available, the process moves to the ready state. 6. Suspend Ready State- A process moves from ready state to suspend ready state if a process with higher priority has to be executed but the main memory is full. Moving a process with lower priority from ready state to suspend ready state creates a room for higher priority process in the ready state. The process remains in the suspend ready state until the main memory becomes available. When main memory becomes available, the process is brought back to the ready state. 6. Suspend Wait State- A process moves from wait state to suspend wait state if a process with higher priority has to be executed but the main memory is full.
- 5. Process & It’s States Conti… Moving a process with lower priority from wait state to suspend wait state creates a room for higher priority process in the ready state. After the resource becomes available, the process is moved to the suspend ready state. After main memory becomes available, the process is moved to the ready state. State Present in Memory New state Secondary Memory Ready state Main Memory Run state Main Memory Wait state Main Memory Suspend wait state Secondary Memory Suspend ready state Secondary Memory Terminate state –
- 6. Process & It’s States Conti…. Example: we write our computer programs in a text file and when we execute this program, it becomes a process which performs all the tasks mentioned in the program. When a program is loaded into the memory and it becomes a process, it can be divided into four sections ─ stack, heap, text and data. The following image shows a simplified layout of a process inside main memory − 1. Stack The process Stack contains the temporary data such as method/function parameters, return address and local variables. 2. Heap This is dynamically allocated memory to a process during its run time. 3. Text This includes the current activity represented by the value of Program Counter and the contents of the processor's registers. 4.Data This section contains the global and static variables.
- 7. Process & It’s States Conti… Operations on the Process 1. Creation Once the process is created, it will be ready and come into the ready queue (main memory) and will be ready for the execution. 2. Scheduling Out of the many processes present in the ready queue, the Operating system chooses one process and start executing it. Selecting the process which is to be executed next, is known as scheduling. 3. Execution Once the process is scheduled for the execution, the processor starts executing it. Process may come to the blocked or wait state during the execution then in that case the processor starts executing the other processes. 4. Deletion/killing Once the purpose of the process gets over then the OS will kill the process. The Context of the process (PCB) will be deleted and the process gets terminated by the Operating system.
- 8. Thread in Operating System What is a Thread? A thread is a path of execution within a process. A process can contain multiple threads. Or A thread is the subset of a process and is also known as the lightweight process. Or A thread is a single sequential flow of execution of tasks of a process. A process can have more than one thread, and these threads are managed independently by the scheduler. All the threads within one process are interrelated to each other. Threads have some common information, such as data segment, code segment, files, etc., that is shared to their peer threads. But contains its own registers, stack, and counter.
- 9. Thread in Operating System
- 10. Thread in Operating System How does thread work? As we have discussed that a thread is a subprocess or an execution unit within a process. A process can contain a single thread to multiple threads. A thread works as follows: When a process starts, OS assigns the memory and resources to it. Each thread within a process shares the memory and resources of that process only. Threads are mainly used to improve the processing of an application. In reality, only a single thread is executed at a time, but due to fast context switching between threads gives an illusion that threads are running parallelly. If a single thread executes in a process, it is known as a single-threaded And if multiple threads execute simultaneously, then it is known as multithreading. Need of Threads A thread is also called a lightweight process. Threads provide a way to improve application performance through parallelism. Threads represent a software approach to improving performance of operating system by reducing the overhead thread is equivalent to a classical process.
- 11. Thread in Operating System Why Multithreading? The idea is to achieve parallelism by dividing a process into multiple threads. For example, in a browser, multiple tabs can be different threads. MS Word uses multiple threads: one thread to format the text, another thread to process inputs, etc. Types of Threads 1. User Level Thread As the name suggests, the user-level threads are only managed by users, and the kernel does not have its information. These are faster, easy to create and manage. The kernel takes all these threads as a single process and handles them as one process only. The user-level threads are implemented by user-level libraries, not by the system calls. 2. Kernel-Level Thread The kernel-level threads are handled by the Operating system and managed by its kernel. These threads are slower than user-level threads because context information is managed by the kernel. To create and implement a kernel-level thread, we need to make a system call.
- 12. Thread in Operating System
- 13. Thread in Operating System Comparison Basis Process Thread Definition A process is a program under execution i.e an active program. A thread is a lightweight process that can be managed independently by a scheduler. Context switching time Processes require more time for context switching as they are more heavy. Threads require less time for context switching as they are lighter than processes. Memory Sharing Processes are totally independent and don’t share memory. A thread may share some memory with its peer threads. Communication Communication between processes requires more time than between threads. Communication between threads requires less time than between processes . Blocked If a process gets blocked, remaining processes can continue execution. If a user level thread gets blocked, all of its peer threads also get blocked.
- 14. Thread in Operating System Comparison Basis Process Thread Resource Consumption Processes require more resources than threads. Threads generally need less resources than processes. Dependency Individual processes are independent of each other. Threads are parts of a process and so are dependent. Data and Code sharing Processes have independent data and code segments. A thread shares the data segment, code segment, files etc. with its peer threads. Treatment by OS All the different processes are treated separately by the operating system. All user level peer threads are treated as a single task by the operating system. Time for creation Processes require more time for creation. Threads require less time for creation. Time for termination Processes require more time for termination. Threads require less time for termination.