Os Unit 2 Review(4).docx

Operating Systems Review (Unit 2: Process and Threads) Points to Remember: 1. Process : It is a program in execution 2. Components of the process are:  Object program  The input data  Resources (E.g., files, etc.)  PCB – Process Control Block, containing the state of the process in execution. 3. Difference between program and process: Program Process Dynamic entity that contains Object  Static entity made up of source program, the input data, Resources and program language statements PCB. 4. PCB: OS creates a PCB (process control block table) for each process to manage that process.

5. Process States and valid transitions between them

o New State ( means a process that just been created) o Ready State (means a process that is prepared to execute)  Running State (means a process is currently being executed)

 Waiting state ( means a process is waiting for some event to occur)  Terminated (means a process is just been terminated) 6. Cooperating Processes:  A process is said to be a cooperating process if it can affect or be affected by other processes in the system.  Advantages: o Information Sharing. o Computation Speedup 7. Types of inter process communication:  Shared Memory: Processes can exchange information by reading and writing data to the shared region.

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Message passing: Processes communicate by passing messages. SEND and RECEIVE primitives are used. 8. Message passing Types:  Blocking Send, Blocking Receive: Both receiver and sender are blocked until the message is delivered.  Non-Blocking Send, Blocking Receive: Sender can continue the execution after sending a message; the receiver is blocked until message arrives.  Non-Blocking Send, Non-Blocking Receive: Neither party waits. 9. Remote Procedure Call: To execute a procedure at a remote site and ship the results back. 10. Thread:  Thread is a light weighted process.  A thread is a basic unit of CPU utilization.  Example 1: A Web browser might have one thread display images or text while another thread retrieves data from the network.  Example 2 : A word processor may have a thread for displaying graphics, another thread for responding to keystrokes from the user, and a third thread for performing spelling and grammar checking in the background.  Advantages: Responsiveness, Resource sharing, Economy, Scalability. 11. Difference between Threads and processes: Processes  Heavy weighted  Processes don’t share an address space  When a process is created memory is allocated to store instructions and data.

Threads  Light Weighted  Threads share an address space  When a thread is created only registers and stack are allocated.

12. Multi-threading Models: • Many to One Model: - Many user-level threads mapped to single kernel thread. - Examples: Solaris Green Threads - Advantages: Easy to implement. - Disadvantages: Parallelism is not possible • One-to-One Model: - Each user-level thread maps to kernel thread. - Examples: Windows 95/98/NT/2000, Linux - Advantages: Parallelism is achieved. - Disadvantages: Costly • Many-To-Many Model - It multiplexes any number of user threads onto an equal or smaller number of kernel threads. - Example: HP-UX

13. Difference between user level and Kernel level Threads: Kernel Level Threads User Level Threads  Threads implemented inside the OS  It is implemented in user-level libraries  Thread operations are system calls  All operations are normal procedure calls  Advantage: It is good for those  Advantage: Simple, Fast and Efficient applications that are frequently block than kernel threads  Disadvantage: They are slow and  Disadvantages: Lack of coordination inefficient. between threads 14. Schedulers:  Long-term scheduler (or job scheduler) : It selects which processes should be brought into the ready queue.  Short-term scheduler (or CPU scheduler) : It selects which process should be executed next and allocates to CPU .

15. Scheduling Criteria a. Processor Utilization b. Throughput c. Turnaround Time (tat) d. Waiting Time (wt) e. Response Time (rt) 16. Scheduling Algorithms  FIFO (or FCFS)  Shortest-Job-First (SJF)  Priority-based  Round-robin  Multilevel Queue  Multilevel feedback queue

Refer Class notes for Scheduling problems 17. Preemptive and non-preemptive scheduling:  Preemptive – if a new process with CPU burst less than remaining time of current process, preempt the current process.  Non-preemptive scheduling: Once CPU assigned, process not preempted until its CPU burst completes.